diff --git a/application/api/answer/routes/messages.py b/application/api/answer/routes/messages.py
deleted file mode 100644
index b0f4e159..00000000
--- a/application/api/answer/routes/messages.py
+++ /dev/null
@@ -1,135 +0,0 @@
-"""GET /api/messages/<message_id>/events — chat-stream reconnect endpoint.
-
-Authenticates the caller, verifies ``message_id`` belongs to the user,
-then hands off to ``build_message_event_stream`` for snapshot+tail.
-"""
-
-from __future__ import annotations
-
-import logging
-import re
-from typing import Iterator, Optional
-
-from flask import Blueprint, Response, jsonify, make_response, request, stream_with_context
-from sqlalchemy import text
-
-from application.core.settings import settings
-from application.storage.db.session import db_readonly
-from application.streaming.event_replay import (
-    DEFAULT_KEEPALIVE_SECONDS,
-    DEFAULT_POLL_TIMEOUT_SECONDS,
-    build_message_event_stream,
-)
-
-logger = logging.getLogger(__name__)
-
-messages_bp = Blueprint("message_stream", __name__)
-
-# A message_id is the canonical UUID hex format. Reject anything else
-# before the SQL layer so a malformed cookie can't surface as a 500.
-_MESSAGE_ID_RE = re.compile(
-    r"^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-"
-    r"[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
-)
-# ``sequence_no`` is a non-negative decimal integer. Anything else is
-# corrupt client state — fall through to a fresh-replay cursor and let
-# the snapshot reader catch the client up.
-_SEQUENCE_NO_RE = re.compile(r"^\d+$")
-
-
-def _normalise_last_event_id(raw: Optional[str]) -> Optional[int]:
-    if raw is None:
-        return None
-    raw = raw.strip()
-    if not raw or not _SEQUENCE_NO_RE.match(raw):
-        return None
-    return int(raw)
-
-
-def _user_owns_message(message_id: str, user_id: str) -> bool:
-    """Return True iff ``message_id`` belongs to ``user_id``."""
-    try:
-        with db_readonly() as conn:
-            row = conn.execute(
-                text(
-                    """
-                    SELECT 1 FROM conversation_messages
-                    WHERE id = CAST(:id AS uuid)
-                      AND user_id = :u
-                    LIMIT 1
-                    """
-                ),
-                {"id": message_id, "u": user_id},
-            ).first()
-        return row is not None
-    except Exception:
-        logger.exception(
-            "Ownership lookup failed for message_id=%s user_id=%s",
-            message_id,
-            user_id,
-        )
-        return False
-
-
-@messages_bp.route("/api/messages/<message_id>/events", methods=["GET"])
-def stream_message_events(message_id: str) -> Response:
-    decoded = getattr(request, "decoded_token", None)
-    user_id = decoded.get("sub") if isinstance(decoded, dict) else None
-    if not user_id:
-        return make_response(
-            jsonify({"success": False, "message": "Authentication required"}),
-            401,
-        )
-
-    if not _MESSAGE_ID_RE.match(message_id):
-        return make_response(
-            jsonify({"success": False, "message": "Invalid message id"}),
-            400,
-        )
-
-    if not _user_owns_message(message_id, user_id):
-        # Don't disclose whether the row exists — a malicious caller
-        # gets the same 404 whether the id is bogus, taken by another
-        # user, or simply gone.
-        return make_response(
-            jsonify({"success": False, "message": "Not found"}),
-            404,
-        )
-
-    raw_cursor = request.headers.get("Last-Event-ID") or request.args.get(
-        "last_event_id"
-    )
-    last_event_id = _normalise_last_event_id(raw_cursor)
-    keepalive_seconds = float(
-        getattr(settings, "SSE_KEEPALIVE_SECONDS", DEFAULT_KEEPALIVE_SECONDS)
-    )
-
-    @stream_with_context
-    def generate() -> Iterator[str]:
-        try:
-            yield from build_message_event_stream(
-                message_id,
-                last_event_id=last_event_id,
-                keepalive_seconds=keepalive_seconds,
-                poll_timeout_seconds=DEFAULT_POLL_TIMEOUT_SECONDS,
-            )
-        except GeneratorExit:
-            return
-        except Exception:
-            logger.exception(
-                "Reconnect stream crashed for message_id=%s user_id=%s",
-                message_id,
-                user_id,
-            )
-
-    response = Response(generate(), mimetype="text/event-stream")
-    response.headers["Cache-Control"] = "no-store"
-    response.headers["X-Accel-Buffering"] = "no"
-    response.headers["Connection"] = "keep-alive"
-    logger.info(
-        "message.event.connect message_id=%s user_id=%s last_event_id=%s",
-        message_id,
-        user_id,
-        last_event_id if last_event_id is not None else "-",
-    )
-    return response
diff --git a/application/api/async_sse.py b/application/api/async_sse.py
new file mode 100644
index 00000000..7c4efe32
--- /dev/null
+++ b/application/api/async_sse.py
@@ -0,0 +1,243 @@
+"""Native-async (ASGI) SSE reader routes, mounted ahead of the Flask app.
+
+These Starlette routes serve the chat-stream *reconnect* path on the event
+loop, so a long-lived, mostly-idle tail costs a coroutine instead of one of
+the 32 a2wsgi threadpool slots (see ``application/asgi.py``). They are the
+sole reconnect reader — the old Flask blueprint has been removed. The heavy
+*producer* (``POST /api/answer/stream`` → agent → LLM) stays on the sync
+path untouched.
+
+Auth, message-id validation, ``Last-Event-ID`` parsing and ownership are
+done here; the snapshot/tail wire format is shared with the producer's
+journal via ``build_message_event_stream_async`` → ``format_sse_event``.
+"""
+
+from __future__ import annotations
+
+import logging
+import re
+from typing import Optional
+
+import anyio
+from sqlalchemy import text
+from starlette.requests import Request
+from starlette.responses import JSONResponse, StreamingResponse
+from starlette.routing import Route
+
+from application.auth import handle_auth
+from application.core.settings import settings
+from application.events.keys import connection_counter_key
+from application.storage.db.session import db_readonly
+from application.streaming.async_event_replay import (
+    build_message_event_stream_async,
+)
+from application.streaming.async_redis import get_async_redis_instance
+from application.streaming.event_replay import (
+    DEFAULT_KEEPALIVE_SECONDS,
+    DEFAULT_POLL_TIMEOUT_SECONDS,
+)
+
+logger = logging.getLogger(__name__)
+
+# Per-user concurrent-connection counter TTL (seconds) — orphaned counts
+# from a hard crash self-heal after this window, mirroring the /api/events
+# notification stream. The reconnect reader shares the same counter key, so
+# the cap bounds a user's *total* live SSE footprint.
+_COUNTER_TTL_SECONDS = 3600
+
+# A message_id is the canonical UUID hex format. Reject anything else before
+# the SQL layer so a malformed cookie can't surface as a 500.
+_MESSAGE_ID_RE = re.compile(
+    r"^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-"
+    r"[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
+)
+# ``sequence_no`` is a non-negative decimal integer. Anything else is corrupt
+# client state — fall through to a fresh-replay cursor.
+_SEQUENCE_NO_RE = re.compile(r"^\d+$")
+
+
+def _normalise_last_event_id(raw: Optional[str]) -> Optional[int]:
+    """Parse a ``Last-Event-ID`` cursor; ``None`` for missing/invalid."""
+    if raw is None:
+        return None
+    raw = raw.strip()
+    if not raw or not _SEQUENCE_NO_RE.match(raw):
+        return None
+    return int(raw)
+
+
+def _user_owns_message(message_id: str, user_id: str) -> bool:
+    """Return True iff ``message_id`` belongs to ``user_id``."""
+    try:
+        with db_readonly() as conn:
+            row = conn.execute(
+                text(
+                    """
+                    SELECT 1 FROM conversation_messages
+                    WHERE id = CAST(:id AS uuid)
+                      AND user_id = :u
+                    LIMIT 1
+                    """
+                ),
+                {"id": message_id, "u": user_id},
+            ).first()
+        return row is not None
+    except Exception:
+        logger.exception(
+            "Ownership lookup failed for message_id=%s user_id=%s",
+            message_id,
+            user_id,
+        )
+        return False
+
+_SSE_HEADERS = {
+    "Cache-Control": "no-store",
+    "X-Accel-Buffering": "no",
+    "Connection": "keep-alive",
+    # Marks the response as served by the event-loop reader rather than the
+    # WSGI-threaded Flask fallback. Purely diagnostic — the frontend reads
+    # the body via fetch+getReader and ignores response headers.
+    "X-SSE-Transport": "async",
+}
+
+
+def _json(message: str, status_code: int) -> JSONResponse:
+    return JSONResponse(
+        {"success": False, "message": message}, status_code=status_code
+    )
+
+
+async def _acquire_stream_slot(user_id: str):
+    """Reserve a per-user connection slot; returns ``(redis, key)`` to release.
+
+    Mirrors the ``/api/events`` cap (INCR + safety TTL, reject when the
+    post-increment count exceeds ``SSE_MAX_CONCURRENT_PER_USER``). Returns
+    ``(None, None)`` when the cap is disabled or Redis is unavailable
+    (fail-open, like the notification stream). Raises ``_CapExceeded`` when
+    the user is over the cap so the caller can 429.
+    """
+    cap = int(getattr(settings, "SSE_MAX_CONCURRENT_PER_USER", 0))
+    if cap <= 0:
+        return None, None
+    redis = await get_async_redis_instance()
+    if redis is None:
+        return None, None
+    key = connection_counter_key(user_id)
+    try:
+        current = int(await redis.incr(key))
+    except Exception:
+        logger.debug("async SSE counter INCR failed for user=%s", user_id)
+        return None, None
+    # EXPIRE failure must not bypass the cap, so it's best-effort after INCR.
+    try:
+        await redis.expire(key, _COUNTER_TTL_SECONDS)
+    except Exception:
+        logger.debug("async SSE counter EXPIRE failed for user=%s", user_id)
+    if current > cap:
+        await _release_stream_slot(redis, key)
+        raise _CapExceeded()
+    return redis, key
+
+
+async def _release_stream_slot(redis, key) -> None:
+    if redis is None or key is None:
+        return
+    try:
+        await redis.decr(key)
+    except Exception:
+        logger.debug("async SSE counter DECR failed for key=%s", key)
+
+
+class _CapExceeded(Exception):
+    """Raised when a user is over their concurrent-stream cap."""
+
+
+async def _counted_stream(inner, redis, key):
+    """Wrap the reader so the per-user slot is released when it ends.
+
+    The slot is reserved before the response starts (so over-cap surfaces as
+    HTTP 429, not mid-stream); the release runs in ``finally`` on terminal
+    close, client disconnect, or error. Shielded so a disconnect-cancellation
+    can't skip the DECR and leak the count.
+    """
+    try:
+        async for line in inner:
+            yield line
+    finally:
+        with anyio.CancelScope(shield=True):
+            await _release_stream_slot(redis, key)
+
+
+async def stream_message_events(request: Request) -> JSONResponse | StreamingResponse:
+    """GET /api/messages/{message_id}/events — async reconnect tail.
+
+    Mirrors the Flask handler's gates (auth → id format → ownership →
+    cursor → per-user connection cap) then streams snapshot+tail off the
+    event loop.
+    """
+    # ``handle_auth`` only reads ``request.headers.get("Authorization")``;
+    # Starlette's headers are case-insensitive, so the Flask helper works
+    # verbatim. With AUTH_TYPE unset it returns ``{"sub": "local"}``.
+    decoded = handle_auth(request)
+    if isinstance(decoded, dict) and "error" in decoded:
+        return _json("Authentication error: invalid token", 401)
+    user_id = decoded.get("sub") if isinstance(decoded, dict) else None
+    if not user_id:
+        return _json("Authentication required", 401)
+
+    message_id = request.path_params["message_id"]
+    if not _MESSAGE_ID_RE.match(message_id):
+        return _json("Invalid message id", 400)
+
+    # Ownership check is a sync DB read — push it off the loop.
+    owns = await anyio.to_thread.run_sync(_user_owns_message, message_id, user_id)
+    if not owns:
+        # Same opaque 404 as the Flask route — don't disclose existence.
+        return _json("Not found", 404)
+
+    # Per-user concurrent-connection cap — reserve before the response opens
+    # so an over-cap caller gets a clean 429 instead of a mid-stream cutoff.
+    try:
+        redis, counter_key = await _acquire_stream_slot(user_id)
+    except _CapExceeded:
+        logger.warning("sse.reconnect.rejected user_id=%s (over cap)", user_id)
+        return _json("Too many concurrent SSE connections", 429)
+
+    raw_cursor = request.headers.get("Last-Event-ID") or request.query_params.get(
+        "last_event_id"
+    )
+    last_event_id = _normalise_last_event_id(raw_cursor)
+    keepalive_seconds = float(
+        getattr(settings, "SSE_KEEPALIVE_SECONDS", DEFAULT_KEEPALIVE_SECONDS)
+    )
+
+    logger.info(
+        "message.event.connect.async message_id=%s user_id=%s last_event_id=%s",
+        message_id,
+        user_id,
+        last_event_id if last_event_id is not None else "-",
+    )
+
+    stream = build_message_event_stream_async(
+        message_id,
+        last_event_id=last_event_id,
+        user_id=user_id,
+        keepalive_seconds=keepalive_seconds,
+        poll_timeout_seconds=DEFAULT_POLL_TIMEOUT_SECONDS,
+    )
+    return StreamingResponse(
+        _counted_stream(stream, redis, counter_key),
+        media_type="text/event-stream",
+        headers=_SSE_HEADERS,
+    )
+
+
+# Mounted in ``application/asgi.py`` ahead of the Flask catch-all. Keep
+# each route's path identical to the Flask blueprint it shadows.
+async_sse_routes = [
+    Route(
+        "/api/messages/{message_id}/events",
+        stream_message_events,
+        methods=["GET"],
+    ),
+]
diff --git a/application/app.py b/application/app.py
index 0d3f37bc..324d778f 100644
--- a/application/app.py
+++ b/application/app.py
@@ -16,7 +16,6 @@ setup_logging()
 
 from application.api import api  # noqa: E402
 from application.api.answer import answer  # noqa: E402
-from application.api.answer.routes.messages import messages_bp  # noqa: E402
 from application.api.devices import devices_bp  # noqa: E402
 from application.api.events.routes import events  # noqa: E402
 from application.api.internal.routes import internal  # noqa: E402
@@ -59,7 +58,6 @@ app = Flask(__name__)
 app.register_blueprint(user)
 app.register_blueprint(answer)
 app.register_blueprint(events)
-app.register_blueprint(messages_bp)
 app.register_blueprint(internal)
 app.register_blueprint(connector)
 app.register_blueprint(devices_bp)
diff --git a/application/asgi.py b/application/asgi.py
index d107c021..1ffb1d6d 100644
--- a/application/asgi.py
+++ b/application/asgi.py
@@ -8,6 +8,7 @@ from starlette.middleware import Middleware
 from starlette.middleware.cors import CORSMiddleware
 from starlette.routing import Mount
 
+from application.api.async_sse import async_sse_routes
 from application.app import app as flask_app
 from application.mcp_server import mcp
 
@@ -18,6 +19,12 @@ mcp_app = mcp.http_app(path="/")
 asgi_app = Starlette(
     routes=[
         Mount("/mcp", app=mcp_app),
+        # Native-async SSE readers intercept their exact paths before the
+        # Flask catch-all, so a mostly-idle reconnect tail rides the event
+        # loop instead of pinning a WSGI threadpool slot. Order matters:
+        # Starlette matches routes top-to-bottom, so these must precede the
+        # Mount("/") that hands everything else to Flask.
+        *async_sse_routes,
         Mount("/", app=WSGIMiddleware(flask_app, workers=_WSGI_THREADPOOL)),
     ],
     middleware=[
diff --git a/application/storage/db/repositories/message_events.py b/application/storage/db/repositories/message_events.py
index c35e47c6..7bca3aad 100644
--- a/application/storage/db/repositories/message_events.py
+++ b/application/storage/db/repositories/message_events.py
@@ -123,6 +123,7 @@ class MessageEventsRepository:
         self,
         message_id: str,
         last_sequence_no: Optional[int] = None,
+        user_id: Optional[str] = None,
     ) -> list[dict]:
         """Return events with ``sequence_no > last_sequence_no``.
 
@@ -133,23 +134,38 @@ class MessageEventsRepository:
         data the planner may pick a bitmap+sort. Either way the result
         is sorted on ``sequence_no``.
 
+        When ``user_id`` is given the scan joins ``conversation_messages``
+        and filters on ``cm.user_id`` — a non-owner gets an empty result.
+        This lets the reconnect reader re-assert ownership at the data
+        layer rather than trusting only the route gate.
+
         Returns a ``list`` (not a generator) so the underlying
         ``Result`` is fully drained before the caller can issue
         another query on the same connection.
         """
         cursor = -1 if last_sequence_no is None else int(last_sequence_no)
-        rows = self._conn.execute(
-            text(
-                """
+        params = {"message_id": str(message_id), "cursor": cursor}
+        if user_id is None:
+            sql = """
                 SELECT message_id, sequence_no, event_type, payload, created_at
                 FROM message_events
                 WHERE message_id = CAST(:message_id AS uuid)
                   AND sequence_no > :cursor
                 ORDER BY sequence_no ASC
                 """
-            ),
-            {"message_id": str(message_id), "cursor": cursor},
-        ).fetchall()
+        else:
+            params["u"] = user_id
+            sql = """
+                SELECT me.message_id, me.sequence_no, me.event_type,
+                       me.payload, me.created_at
+                FROM message_events me
+                JOIN conversation_messages cm ON cm.id = me.message_id
+                WHERE me.message_id = CAST(:message_id AS uuid)
+                  AND cm.user_id = :u
+                  AND me.sequence_no > :cursor
+                ORDER BY me.sequence_no ASC
+                """
+        rows = self._conn.execute(text(sql), params).fetchall()
         return [row_to_dict(row) for row in rows]
 
     def cleanup_older_than(self, ttl_days: int) -> int:
diff --git a/application/streaming/async_broadcast_channel.py b/application/streaming/async_broadcast_channel.py
new file mode 100644
index 00000000..ae708897
--- /dev/null
+++ b/application/streaming/async_broadcast_channel.py
@@ -0,0 +1,119 @@
+"""Async Redis pub/sub Topic for the native-async SSE reader.
+
+Event-loop twin of :class:`application.streaming.broadcast_channel.Topic`.
+Same contract — ``subscribe`` yields ``None`` on poll timeout (so the
+caller can emit keepalives / run the watchdog) and ``bytes`` per delivered
+message, fires ``on_subscribe`` once after Redis acks SUBSCRIBE, and tears
+the pubsub down cleanly on client disconnect — but awaitable so an idle
+stream costs a coroutine instead of a WSGI thread.
+
+Publishing stays on the sync side (the producer writes via
+``broadcast_channel.Topic.publish``); this is read-only fan-out.
+"""
+
+from __future__ import annotations
+
+import inspect
+import logging
+from typing import AsyncIterator, Awaitable, Callable, Optional, Union
+
+import anyio
+
+from application.streaming.async_redis import get_async_redis_instance
+
+logger = logging.getLogger(__name__)
+
+OnSubscribe = Callable[[], Union[None, Awaitable[None]]]
+
+
+class AsyncTopic:
+    """An async pub/sub channel identified by a string name."""
+
+    def __init__(self, name: str) -> None:
+        self.name = name
+
+    async def subscribe(
+        self,
+        on_subscribe: Optional[OnSubscribe] = None,
+        poll_timeout: float = 1.0,
+    ) -> AsyncIterator[Optional[bytes]]:
+        """Subscribe to the topic; yield raw payloads or ``None`` on tick.
+
+        ``on_subscribe`` runs (and is awaited if it returns a coroutine)
+        after Redis acks SUBSCRIBE — use it to seed snapshot state that
+        must be ordered after the subscriber is live but before the first
+        live message is processed. If Redis is unavailable, returns
+        immediately without yielding so the caller can fall back to a
+        direct snapshot read. Cleanly unsubscribes on close / disconnect.
+        """
+        redis = await get_async_redis_instance()
+        if redis is None:
+            logger.debug(
+                "Async Redis unavailable; subscribe to %s yielded nothing",
+                self.name,
+            )
+            return
+        pubsub = redis.pubsub()
+        on_subscribe_fired = False
+        try:
+            try:
+                await pubsub.subscribe(self.name)
+            except Exception:
+                # Transient subscribe failure is treated like "Redis
+                # unavailable": yield nothing, let the caller fall back to
+                # its own snapshot read. The finally block still tears the
+                # pubsub down cleanly.
+                logger.exception("async pubsub.subscribe failed for %s", self.name)
+                return
+            while True:
+                try:
+                    msg = await pubsub.get_message(timeout=poll_timeout)
+                except Exception:
+                    logger.exception(
+                        "async pubsub.get_message failed for %s", self.name
+                    )
+                    return
+                if msg is None:
+                    yield None
+                    continue
+                msg_type = msg.get("type")
+                if msg_type == "subscribe":
+                    if not on_subscribe_fired and on_subscribe is not None:
+                        try:
+                            result = on_subscribe()
+                            if inspect.isawaitable(result):
+                                await result
+                        except Exception:
+                            logger.exception(
+                                "on_subscribe callback failed for %s", self.name
+                            )
+                    on_subscribe_fired = True
+                    continue
+                if msg_type != "message":
+                    continue
+                data = msg.get("data")
+                if data is None:
+                    continue
+                yield data if isinstance(data, bytes) else str(data).encode("utf-8")
+        finally:
+            # Client disconnect cancels this generator at the ``await
+            # get_message`` above; without shielding, the cancellation could
+            # re-fire mid-teardown and skip ``aclose()``, leaking the pooled
+            # connection back to nothing. Shield so unsubscribe + aclose
+            # always complete and the connection returns to the pool.
+            with anyio.CancelScope(shield=True):
+                if on_subscribe_fired:
+                    try:
+                        await pubsub.unsubscribe(self.name)
+                    except Exception:
+                        logger.debug(
+                            "async pubsub unsubscribe error for %s",
+                            self.name,
+                            exc_info=True,
+                        )
+                try:
+                    await pubsub.aclose()
+                except Exception:
+                    logger.debug(
+                        "async pubsub close error for %s", self.name, exc_info=True
+                    )
diff --git a/application/streaming/async_event_replay.py b/application/streaming/async_event_replay.py
new file mode 100644
index 00000000..1be3d789
--- /dev/null
+++ b/application/streaming/async_event_replay.py
@@ -0,0 +1,232 @@
+"""Native-async snapshot+tail iterator for chat-stream reconnect.
+
+The sole reconnect reader: a ``: connected`` prelude, a snapshot flush
+inside the SUBSCRIBE-ack callback, a dedup'd live tail, keepalive +
+producer-liveness watchdog, and close-on-terminal — all as an async
+generator driven off the event loop instead of a WSGI thread.
+
+Wire format, dedup floor, and terminal detection come from
+``event_replay`` (``read_snapshot_lines``, ``format_sse_event``,
+``_decode_pubsub_message``, ``_payload_is_terminal``,
+``_check_producer_liveness``), the same primitives the producer's journal
+writes through — so the reader and writer cannot drift on wire shape. The
+only sync I/O (snapshot read, watchdog DB probe) is pushed to a worker
+thread via ``anyio.to_thread`` so it never blocks the loop.
+"""
+
+from __future__ import annotations
+
+import logging
+import time
+from typing import AsyncIterator, Optional
+
+import anyio
+
+from application.streaming.async_broadcast_channel import AsyncTopic
+from application.streaming.event_replay import (
+    DEFAULT_KEEPALIVE_SECONDS,
+    DEFAULT_POLL_TIMEOUT_SECONDS,
+    DEFAULT_PRODUCER_IDLE_SECONDS,
+    DEFAULT_WATCHDOG_INTERVAL_SECONDS,
+    _check_producer_liveness,
+    _decode_pubsub_message,
+    _payload_is_terminal,
+    format_sse_event,
+    read_snapshot_lines,
+)
+from application.streaming.keys import message_topic_name
+
+logger = logging.getLogger(__name__)
+
+# The snapshot read and watchdog probe are the reader's only DB I/O; they run
+# in worker threads and each borrows a connection from the app-wide SQLAlchemy
+# pool (pool_size=10 + max_overflow=20 = 30). The reader scales to many
+# concurrent event-loop streams, so without a bound a burst of aligned
+# watchdog/snapshot ticks could exhaust the pool and starve every other route.
+# Cap the reader's concurrent DB-thread usage well below the pool so it can
+# never monopolise it; excess ticks queue briefly (watchdog cadence is 5s, so
+# a short queue delay is harmless).
+_MAX_CONCURRENT_DB_READS = 8
+_db_read_limiter: Optional[anyio.CapacityLimiter] = None
+
+
+def _get_db_read_limiter() -> anyio.CapacityLimiter:
+    """Lazily build the shared limiter on the (single) event loop.
+
+    Created on first use rather than at import so it binds to the running
+    loop; creation is synchronous, so the single-worker loop has no race.
+    """
+    global _db_read_limiter
+    if _db_read_limiter is None:
+        _db_read_limiter = anyio.CapacityLimiter(_MAX_CONCURRENT_DB_READS)
+    return _db_read_limiter
+
+
+async def build_message_event_stream_async(
+    message_id: str,
+    last_event_id: Optional[int] = None,
+    *,
+    user_id: Optional[str] = None,
+    keepalive_seconds: float = DEFAULT_KEEPALIVE_SECONDS,
+    poll_timeout_seconds: float = DEFAULT_POLL_TIMEOUT_SECONDS,
+    watchdog_interval_seconds: float = DEFAULT_WATCHDOG_INTERVAL_SECONDS,
+    producer_idle_seconds: float = DEFAULT_PRODUCER_IDLE_SECONDS,
+) -> AsyncIterator[str]:
+    """Yield SSE-formatted lines for one ``message_id`` reconnect stream.
+
+    First frame is ``: connected``; subsequent frames are snapshot rows,
+    live-tail events, or ``: keepalive`` comments. Runs until the client
+    disconnects or a terminal event is delivered.
+    """
+    yield ": connected\n\n"
+
+    replay_buffer: list[str] = []
+    max_replayed_seq: Optional[int] = last_event_id
+    replay_done = False
+    replay_failed = False
+    terminal_in_snapshot = False
+
+    async def _load_snapshot() -> None:
+        nonlocal max_replayed_seq, replay_failed, terminal_in_snapshot
+        try:
+            lines, max_seq, terminal = await anyio.to_thread.run_sync(
+                read_snapshot_lines,
+                message_id,
+                last_event_id,
+                user_id,
+                limiter=_get_db_read_limiter(),
+            )
+        except Exception:
+            logger.exception(
+                "Snapshot read failed for message_id=%s last_event_id=%s",
+                message_id,
+                last_event_id,
+            )
+            replay_failed = True
+            return
+        replay_buffer.extend(lines)
+        max_replayed_seq = max_seq
+        terminal_in_snapshot = terminal
+
+    async def _on_subscribe() -> None:
+        # SUBSCRIBE acked — Postgres reads from this point capture every
+        # committed row; pub/sub messages published after this are queued
+        # at the connection level until the loop polls again.
+        nonlocal replay_done
+        try:
+            await _load_snapshot()
+        finally:
+            replay_done = True
+
+    topic = AsyncTopic(message_topic_name(message_id))
+    last_keepalive = time.monotonic()
+    last_watchdog_check = float("-inf")
+    watchdog_synthetic_seq = -1
+
+    try:
+        async for payload in topic.subscribe(
+            on_subscribe=_on_subscribe,
+            poll_timeout=poll_timeout_seconds,
+        ):
+            # Flush snapshot exactly once after the SUBSCRIBE callback ran.
+            if replay_done and replay_buffer:
+                for line in replay_buffer:
+                    yield line
+                replay_buffer.clear()
+                if terminal_in_snapshot:
+                    # Original stream already finished; tailing would just
+                    # emit keepalives forever.
+                    return
+
+            if replay_failed:
+                yield format_sse_event(
+                    {
+                        "type": "error",
+                        "error": "Stream replay failed; please refresh to load the latest state.",
+                        "code": "snapshot_failed",
+                        "message_id": message_id,
+                    },
+                    sequence_no=-1,
+                )
+                return
+
+            now = time.monotonic()
+            if payload is None:
+                # Idle tick — gate the watchdog on ``replay_done`` so we
+                # don't race the snapshot read on the first iteration.
+                if (
+                    replay_done
+                    and watchdog_interval_seconds >= 0
+                    and now - last_watchdog_check >= watchdog_interval_seconds
+                ):
+                    last_watchdog_check = now
+                    terminal_payload = await anyio.to_thread.run_sync(
+                        _check_producer_liveness,
+                        message_id,
+                        user_id,
+                        producer_idle_seconds,
+                        limiter=_get_db_read_limiter(),
+                    )
+                    if terminal_payload is not None:
+                        yield format_sse_event(
+                            terminal_payload,
+                            sequence_no=watchdog_synthetic_seq,
+                        )
+                        return
+                if now - last_keepalive >= keepalive_seconds:
+                    yield ": keepalive\n\n"
+                    last_keepalive = now
+                continue
+
+            envelope = _decode_pubsub_message(payload)
+            if envelope is None:
+                continue
+            seq = envelope.get("sequence_no")
+            inner = envelope.get("payload")
+            if (
+                not isinstance(seq, int)
+                or isinstance(seq, bool)
+                or not isinstance(inner, dict)
+            ):
+                continue
+            if max_replayed_seq is not None and seq <= max_replayed_seq:
+                # Snapshot already covered this id — drop the duplicate.
+                continue
+            yield format_sse_event(inner, seq)
+            max_replayed_seq = seq
+            last_keepalive = now
+            if _payload_is_terminal(inner, envelope.get("event_type")):
+                return
+
+        # Subscribe exited without yielding (Redis unavailable / subscribe
+        # raised). The snapshot half is still in Postgres — read it
+        # directly so a Redis-only outage doesn't cost the client their
+        # backlog. Gate on ``replay_done`` so we don't double-read.
+        if not replay_done:
+            await _load_snapshot()
+            replay_done = True
+        for line in replay_buffer:
+            yield line
+        replay_buffer.clear()
+        if replay_failed:
+            yield format_sse_event(
+                {
+                    "type": "error",
+                    "error": "Stream replay failed; please refresh to load the latest state.",
+                    "code": "snapshot_failed",
+                    "message_id": message_id,
+                },
+                sequence_no=-1,
+            )
+            return
+        if terminal_in_snapshot:
+            return
+    except Exception:
+        # GeneratorExit / CancelledError are BaseException subclasses, so a
+        # client disconnect bypasses this handler and propagates to close
+        # the inner AsyncTopic generator (tearing its pubsub down in that
+        # generator's finally). Only genuine bugs land here.
+        logger.exception(
+            "Async reconnect stream crashed for message_id=%s", message_id
+        )
+        return
diff --git a/application/streaming/async_redis.py b/application/streaming/async_redis.py
new file mode 100644
index 00000000..791afec6
--- /dev/null
+++ b/application/streaming/async_redis.py
@@ -0,0 +1,47 @@
+"""Lazy async Redis client for the native-async SSE reader.
+
+Async twin of :func:`application.cache.get_redis_instance`. The
+Starlette-mounted reader (``application.api.async_sse``) tails pub/sub on
+the event loop, so it needs a ``redis.asyncio`` client rather than the
+sync one used by the producer side. The app runs a single ASGI worker /
+event loop, so a module-level singleton is sufficient and avoids
+reconnecting per request.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Optional
+
+import redis.asyncio as aioredis
+
+from application.core.settings import settings
+
+logger = logging.getLogger(__name__)
+
+_async_redis: Optional[aioredis.Redis] = None
+_creation_failed = False
+
+
+async def get_async_redis_instance() -> Optional[aioredis.Redis]:
+    """Return a process-wide async Redis client, or ``None`` if unavailable.
+
+    ``from_url`` builds the client without opening a socket (connection is
+    lazy), so a transient broker outage surfaces later on the first command
+    rather than here. Mirrors the sync client's ``socket_connect_timeout``
+    and ``health_check_interval`` so a half-open TCP can't wedge the tail
+    loop past its keepalive cadence.
+    """
+    global _async_redis, _creation_failed
+    if _async_redis is None and not _creation_failed:
+        try:
+            _async_redis = aioredis.Redis.from_url(
+                settings.CACHE_REDIS_URL,
+                socket_connect_timeout=2,
+                health_check_interval=10,
+            )
+        except ValueError as e:
+            logger.error("Invalid Redis URL for async client: %s", e)
+            _creation_failed = True
+            _async_redis = None
+    return _async_redis
diff --git a/application/streaming/event_replay.py b/application/streaming/event_replay.py
index d2bbc402..6be45100 100644
--- a/application/streaming/event_replay.py
+++ b/application/streaming/event_replay.py
@@ -1,8 +1,13 @@
-"""Snapshot+tail iterator for chat-stream reconnect-after-disconnect.
+"""Shared snapshot/replay primitives for chat-stream reconnect.
 
-Subscribe to ``channel:{message_id}``, snapshot ``message_events``
-rows past ``last_event_id`` inside the SUBSCRIBE-ack callback, flush
-snapshot, then tail live pub/sub (dedup'd by ``sequence_no``). See
+The reconnect reader itself is the native-async generator in
+``async_event_replay.build_message_event_stream_async``; this module holds
+the pieces both it and the producer's journal depend on: the SSE wire
+format (``format_sse_event``), the ``message_events`` snapshot read
+(``read_snapshot_lines``), the producer-liveness watchdog probe
+(``_check_producer_liveness``), and the pub/sub envelope encode/decode.
+Keeping them here lets the async reader and the sync journal agree on the
+exact wire shape and dedup/terminal rules. See
 ``docs/runbooks/sse-notifications.md``.
 """
 
@@ -11,8 +16,7 @@ from __future__ import annotations
 import json
 import logging
 import re
-import time
-from typing import Iterator, Optional
+from typing import Optional
 
 from sqlalchemy import text as sql_text
 
@@ -20,8 +24,6 @@ from application.storage.db.repositories.message_events import (
     MessageEventsRepository,
 )
 from application.storage.db.session import db_readonly
-from application.streaming.broadcast_channel import Topic
-from application.streaming.keys import message_topic_name
 
 logger = logging.getLogger(__name__)
 
@@ -81,11 +83,16 @@ def format_sse_event(payload: dict, sequence_no: int) -> str:
 
 
 def _check_producer_liveness(
-    message_id: str, idle_seconds: float
+    message_id: str, user_id: Optional[str], idle_seconds: float
 ) -> Optional[dict]:
     """Inspect ``conversation_messages`` and return a terminal SSE
     payload when the producer is no longer alive, else ``None``.
 
+    When ``user_id`` is given the lookup is scoped to ``AND user_id = :u``
+    (defence in depth: this long-lived re-read re-asserts the ownership the
+    route gated on, so a stream cannot keep tailing a row it no longer
+    owns). A non-matching row reads as missing → a terminal ``error``.
+
     Three terminal cases collapse into a single DB round-trip:
 
     - ``status='complete'`` — the live finalize ran but its journal
@@ -100,11 +107,17 @@ def _check_producer_liveness(
       Synthesise ``error`` so the client doesn't hang on keepalives
       until the proxy idle-timeout kicks in.
     """
+    owner_clause = " AND user_id = :u" if user_id is not None else ""
+    params = {"id": message_id, "idle_secs": float(idle_seconds)}
+    if user_id is not None:
+        params["u"] = user_id
     try:
         with db_readonly() as conn:
             row = conn.execute(
                 sql_text(
-                    """
+                    # ``owner_clause`` is a fixed literal (no user input in the
+                    # SQL string); ``user_id`` is bound via ``:u``.
+                    f"""
                     SELECT
                         status,
                         message_metadata->>'error' AS err,
@@ -118,10 +131,10 @@ def _check_producer_liveness(
                         ) < now() - make_interval(secs => :idle_secs)
                             AS is_stale
                     FROM conversation_messages
-                    WHERE id = CAST(:id AS uuid)
+                    WHERE id = CAST(:id AS uuid){owner_clause}
                     """
                 ),
-                {"id": message_id, "idle_secs": float(idle_seconds)},
+                params,
             ).first()
     except Exception:
         logger.exception(
@@ -161,236 +174,49 @@ def _check_producer_liveness(
     return None
 
 
-def build_message_event_stream(
-    message_id: str,
-    last_event_id: Optional[int] = None,
-    *,
-    keepalive_seconds: float = DEFAULT_KEEPALIVE_SECONDS,
-    poll_timeout_seconds: float = DEFAULT_POLL_TIMEOUT_SECONDS,
-    watchdog_interval_seconds: float = DEFAULT_WATCHDOG_INTERVAL_SECONDS,
-    producer_idle_seconds: float = DEFAULT_PRODUCER_IDLE_SECONDS,
-) -> Iterator[str]:
-    """Yield SSE-formatted lines for one ``message_id`` reconnect stream.
+def read_snapshot_lines(
+    message_id: str, last_event_id: Optional[int], user_id: Optional[str] = None
+) -> tuple[list[str], Optional[int], bool]:
+    """Read journal rows after ``last_event_id`` as SSE-formatted lines.
 
-    First frame is ``: connected``; subsequent frames are snapshot rows,
-    live-tail events, or ``: keepalive`` comments. Runs until the client
-    disconnects.
+    Returns ``(lines, max_sequence_no, terminal)``: ``max_sequence_no`` is
+    seeded with ``last_event_id`` and advanced past every row read,
+    ``terminal`` is True if any row carried a terminal ``end``/``error``.
+    Raises on DB error so the caller can drive its replay-failed path.
+
+    Used by ``async_event_replay.build_message_event_stream_async`` (the
+    reconnect reader); it shares ``format_sse_event`` / ``_payload_is_terminal``
+    with the producer's journal writer so reader and writer never drift on
+    wire shape or terminal semantics.
     """
-    yield ": connected\n\n"
-
-    # Replay buffer — populated inside ``_on_subscribe`` (or the
-    # Redis-unavailable fallback below), drained on the first iteration
-    # of the subscribe loop after the callback runs.
-    replay_buffer: list[str] = []
-    # Dedup floor: seeded with the client's cursor so an empty snapshot
-    # still rejects re-published live events with seq <= last_event_id.
-    # Advanced by snapshot rows AND by yielded live events, so any
-    # republish past the snapshot ceiling is also dropped.
-    max_replayed_seq: Optional[int] = last_event_id
-    replay_done = False
-    replay_failed = False
-    # Set when a snapshot row carries a terminal ``end`` / ``error``
-    # event. After flushing the buffer the generator returns; if we
-    # kept tailing we'd loop on keepalives forever for a stream that
-    # already finished.
-    terminal_in_snapshot = False
-
-    def _read_snapshot_into_buffer() -> None:
-        nonlocal max_replayed_seq, replay_failed, terminal_in_snapshot
-        try:
-            with db_readonly() as conn:
-                rows = MessageEventsRepository(conn).read_after(
-                    message_id, last_sequence_no=last_event_id
-                )
-            for row in rows:
-                seq = int(row["sequence_no"])
-                payload = row.get("payload")
-                if not isinstance(payload, dict):
-                    # ``record_event`` rejects non-dict payloads at the
-                    # write gate, so this can only be a legacy row from
-                    # before that contract or a direct SQL insert. The
-                    # original synthetic fallback (``{"type": event_type}``)
-                    # used to ship a malformed envelope here — drop the
-                    # row instead so a corrupt journal entry doesn't
-                    # poison a reconnect.
-                    logger.warning(
-                        "Skipping non-dict payload from message_events: "
-                        "message_id=%s seq=%s type=%s",
-                        message_id,
-                        seq,
-                        row.get("event_type"),
-                    )
-                    continue
-                replay_buffer.append(format_sse_event(payload, seq))
-                if max_replayed_seq is None or seq > max_replayed_seq:
-                    max_replayed_seq = seq
-                if _payload_is_terminal(payload, row.get("event_type")):
-                    terminal_in_snapshot = True
-        except Exception:
-            logger.exception(
-                "Snapshot read failed for message_id=%s last_event_id=%s",
+    lines: list[str] = []
+    max_seq = last_event_id
+    terminal = False
+    with db_readonly() as conn:
+        rows = MessageEventsRepository(conn).read_after(
+            message_id, last_sequence_no=last_event_id, user_id=user_id
+        )
+    for row in rows:
+        seq = int(row["sequence_no"])
+        payload = row.get("payload")
+        if not isinstance(payload, dict):
+            # ``record_event`` rejects non-dict payloads at the write gate,
+            # so this is a legacy/direct-SQL row — drop it rather than ship
+            # a malformed envelope that would poison a reconnect.
+            logger.warning(
+                "Skipping non-dict payload from message_events: "
+                "message_id=%s seq=%s type=%s",
                 message_id,
-                last_event_id,
+                seq,
+                row.get("event_type"),
             )
-            replay_failed = True
-
-    def _on_subscribe() -> None:
-        # SUBSCRIBE has been acked — Postgres reads from this point
-        # capture every row that's been committed. Pub/sub messages
-        # published after this point are queued at the connection level
-        # until the outer loop calls ``get_message`` again.
-        nonlocal replay_done
-        try:
-            _read_snapshot_into_buffer()
-        finally:
-            # Flip even on failure so the outer loop continues to live
-            # tail and the client doesn't hang waiting for a snapshot
-            # flush that will never come.
-            replay_done = True
-
-    topic = Topic(message_topic_name(message_id))
-    last_keepalive = time.monotonic()
-    # Rate-limit the watchdog's DB hit. ``-inf`` makes the first idle
-    # tick after replay_done fire immediately so a snapshot-already-
-    # terminal-in-DB case is surfaced before any keepalive cadence.
-    # Subsequent checks are gated by ``watchdog_interval_seconds``.
-    last_watchdog_check = float("-inf")
-    # Synthetic terminal events emitted by the watchdog use the same
-    # ``sequence_no=-1`` convention as the snapshot-failure path so the
-    # frontend's strict ``\d+`` cursor regex rejects them as a
-    # ``Last-Event-ID`` for any future reconnect. The chosen
-    # discriminator ensures a manual page refresh after a watchdog-fired
-    # error doesn't loop on the same synthetic id.
-    watchdog_synthetic_seq = -1
-
-    try:
-        for payload in topic.subscribe(
-            on_subscribe=_on_subscribe,
-            poll_timeout=poll_timeout_seconds,
-        ):
-            # Flush snapshot exactly once after the SUBSCRIBE callback
-            # has run and produced a buffer.
-            if replay_done and replay_buffer:
-                for line in replay_buffer:
-                    yield line
-                replay_buffer.clear()
-                if terminal_in_snapshot:
-                    # The original stream already finished; tailing
-                    # would just emit keepalives forever and pin both a
-                    # client reconnect promise and a server WSGI thread.
-                    return
-
-            if replay_failed:
-                # Snapshot read failed (DB blip / transient timeout). Emit a
-                # terminal ``error`` event and return — the client only
-                # reconnects after the original stream has already moved on,
-                # so without a snapshot there's nothing live left to tail and
-                # holding the connection open would just emit keepalives
-                # until the proxy idle-timeout fires. ``code`` preserves the
-                # snapshot-vs-agent-loop distinction so a future client can
-                # opt into a refetch instead of a hard failure.
-                yield format_sse_event(
-                    {
-                        "type": "error",
-                        "error": "Stream replay failed; please refresh to load the latest state.",
-                        "code": "snapshot_failed",
-                        "message_id": message_id,
-                    },
-                    sequence_no=-1,
-                )
-                return
-
-            now = time.monotonic()
-            if payload is None:
-                # Idle tick — check both keepalive and watchdog. The
-                # watchdog only kicks in once the snapshot half has been
-                # flushed (``replay_done``) so we don't race the
-                # snapshot read on the first iteration.
-                if (
-                    replay_done
-                    and watchdog_interval_seconds >= 0
-                    and now - last_watchdog_check >= watchdog_interval_seconds
-                ):
-                    last_watchdog_check = now
-                    terminal_payload = _check_producer_liveness(
-                        message_id, producer_idle_seconds
-                    )
-                    if terminal_payload is not None:
-                        yield format_sse_event(
-                            terminal_payload,
-                            sequence_no=watchdog_synthetic_seq,
-                        )
-                        return
-                if now - last_keepalive >= keepalive_seconds:
-                    yield ": keepalive\n\n"
-                    last_keepalive = now
-                continue
-
-            envelope = _decode_pubsub_message(payload)
-            if envelope is None:
-                continue
-            seq = envelope.get("sequence_no")
-            inner = envelope.get("payload")
-            if (
-                not isinstance(seq, int)
-                or isinstance(seq, bool)
-                or not isinstance(inner, dict)
-            ):
-                continue
-            if max_replayed_seq is not None and seq <= max_replayed_seq:
-                # Snapshot already covered this id — drop the duplicate.
-                continue
-            yield format_sse_event(inner, seq)
-            # Advance the dedup floor on the live path too, so a stale
-            # republish of an already-yielded seq (process restart, retry
-            # tool, etc.) is dropped on a later iteration.
-            max_replayed_seq = seq
-            last_keepalive = now
-            if _payload_is_terminal(inner, envelope.get("event_type")):
-                # Live tail just delivered the terminal event — close
-                # out the reconnect stream so the client's drain
-                # promise resolves and the WSGI thread is freed.
-                return
-
-        # Subscribe exited without ever yielding (Redis unavailable,
-        # ``pubsub.subscribe`` raised, or the inner loop died between
-        # SUBSCRIBE-ack and the first poll). The snapshot half is in
-        # Postgres and is still serviceable — read it directly so a
-        # Redis-only outage doesn't cost the client their reconnect
-        # backlog. Gate the read on ``replay_done`` rather than
-        # ``subscribe_started``: if ``_on_subscribe`` already populated
-        # the buffer, re-reading would append the same rows twice and
-        # double the answer chunks on the client (the per-message
-        # reconnect dispatcher does not dedup by ``id``).
-        if not replay_done:
-            _read_snapshot_into_buffer()
-            replay_done = True
-        for line in replay_buffer:
-            yield line
-        replay_buffer.clear()
-        if replay_failed:
-            # Mirror the live-tail branch: emit a terminal ``error`` so
-            # the frontend's existing end/error handling drives the UI
-            # to a failed state instead of relying on the proxy timeout.
-            yield format_sse_event(
-                {
-                    "type": "error",
-                    "error": "Stream replay failed; please refresh to load the latest state.",
-                    "code": "snapshot_failed",
-                    "message_id": message_id,
-                },
-                sequence_no=-1,
-            )
-            return
-        # Same close-on-terminal contract as the live-tail branch.
-        # Without it a Redis-down + already-completed-stream client
-        # would also hang on a never-ending generator.
-        if terminal_in_snapshot:
-            return
-    except GeneratorExit:
-        # Client disconnect — let the underlying ``Topic.subscribe``
-        # ``finally`` block tear down its pubsub cleanly.
-        return
+            continue
+        lines.append(format_sse_event(payload, seq))
+        if max_seq is None or seq > max_seq:
+            max_seq = seq
+        if _payload_is_terminal(payload, row.get("event_type")):
+            terminal = True
+    return lines, max_seq, terminal
 
 
 def _decode_pubsub_message(raw) -> Optional[dict]:
diff --git a/tests/api/answer/test_snapshot_tail_integration.py b/tests/api/answer/test_snapshot_tail_integration.py
index 4d6005df..9cbdd09b 100644
--- a/tests/api/answer/test_snapshot_tail_integration.py
+++ b/tests/api/answer/test_snapshot_tail_integration.py
@@ -1,14 +1,17 @@
 """Integration tests for the end-to-end snapshot+tail handoff.
 
-Exercises the publisher → journal → reconnect endpoint round-trip
-without mocking the journal layer, so a regression in any of:
+Exercises the publisher → journal → reconnect-reader round-trip without
+mocking the journal layer, so a regression in any of:
 - complete_stream's _emit closure
 - record_event's commit-per-call contract
-- build_message_event_stream's snapshot-from-DB path
-- the reconnect route's auth + ownership gates
+- the shared snapshot read (``event_replay.read_snapshot_lines``)
+- the reconnect route's ownership SQL (``async_sse._user_owns_message``)
 - message_events repo SQL
-
 would surface here as a failed integration assertion.
+
+The live tail itself (pub/sub) and the full async HTTP route are covered by
+``scripts/e2e_async_sse.py`` against a real Redis + uvicorn. These tests pin
+the DB-backed halves against a transactional ``pg_conn`` fixture.
 """
 
 from __future__ import annotations
@@ -56,55 +59,47 @@ def _seed_message(conn, user_id: str | None = None):
     return user_id, str(msg_id)
 
 
+def _emitted_ids(lines: list[str]) -> list[int]:
+    """Extract the ``id:`` sequence numbers from formatted SSE frames."""
+    return sorted(
+        int(line.split("\n", 1)[0].split(": ", 1)[1])
+        for line in lines
+        if line.startswith("id: ")
+    )
+
+
 @pytest.mark.integration
 class TestSnapshotPlusTailRoundTrip:
     def test_record_event_then_snapshot_returns_what_was_journaled(
         self, pg_conn,
     ):
-        """End-to-end of the journal half: ``record_event`` writes through
-        a real ``MessageEventsRepository``, ``build_message_event_stream``
-        reads the snapshot back via the same repo + a stub Topic.
+        """End-to-end of the journal half: ``record_event`` writes through a
+        real ``MessageEventsRepository``, ``read_snapshot_lines`` reads the
+        snapshot back via the same repo and formats it for the wire — the
+        exact primitive the async reader replays through.
         """
-        from application.streaming import event_replay
+        from application.streaming.event_replay import read_snapshot_lines
         from application.streaming.message_journal import record_event
 
         _, message_id = _seed_message(pg_conn)
 
         with _patch_journal_session(pg_conn):
-            # Three events stamp the journal.
             record_event(message_id, 0, "answer", {"type": "answer", "answer": "A"})
             record_event(message_id, 1, "answer", {"type": "answer", "answer": "B"})
             record_event(message_id, 2, "end", {"type": "end"})
 
-            # Reconnect path: subscribe yields nothing (Redis-down
-            # branch); the post-loop fallback runs the snapshot read
-            # synchronously and yields the journal contents.
-            def _empty_subscribe(self, on_subscribe=None, poll_timeout=1.0):
-                return
-                yield  # pragma: no cover
+            lines, max_seq, terminal = read_snapshot_lines(message_id, None)
 
-            with patch.object(
-                event_replay.Topic,
-                "subscribe",
-                _empty_subscribe,
-                create=False,
-            ):
-                gen = event_replay.build_message_event_stream(
-                    message_id,
-                    last_event_id=None,
-                    keepalive_seconds=0.05,
-                    poll_timeout_seconds=0.01,
-                )
-                out = list(gen)
-
-        # Prelude + 3 snapshot frames.
-        assert out[0] == ": connected\n\n"
-        assert "id: 0" in out[1] and '"answer": "A"' in out[1]
-        assert "id: 1" in out[2] and '"answer": "B"' in out[2]
-        assert "id: 2" in out[3] and '"type": "end"' in out[3]
+        assert len(lines) == 3
+        assert "id: 0" in lines[0] and '"answer": "A"' in lines[0]
+        assert "id: 1" in lines[1] and '"answer": "B"' in lines[1]
+        assert "id: 2" in lines[2] and '"type": "end"' in lines[2]
+        assert max_seq == 2
+        # A terminal event in the snapshot tells the reader to close.
+        assert terminal is True
 
     def test_snapshot_resumes_past_last_event_id(self, pg_conn):
-        from application.streaming import event_replay
+        from application.streaming.event_replay import read_snapshot_lines
         from application.streaming.message_journal import record_event
 
         _, message_id = _seed_message(pg_conn)
@@ -115,119 +110,81 @@ class TestSnapshotPlusTailRoundTrip:
                     message_id, seq, "answer", {"type": "answer", "answer": str(seq)}
                 )
 
-            def _empty_subscribe(self, on_subscribe=None, poll_timeout=1.0):
-                return
-                yield  # pragma: no cover
+            # Client says it has seen up through seq=2; expect 3 + 4.
+            lines, max_seq, terminal = read_snapshot_lines(message_id, 2)
 
-            with patch.object(
-                event_replay.Topic,
-                "subscribe",
-                _empty_subscribe,
-                create=False,
-            ):
-                # Client says it has seen up through seq=2; expect 3 + 4.
-                out = list(
-                    event_replay.build_message_event_stream(
-                        message_id,
-                        last_event_id=2,
-                        keepalive_seconds=0.05,
-                        poll_timeout_seconds=0.01,
-                    )
-                )
+        assert _emitted_ids(lines) == [3, 4]
+        assert max_seq == 4
+        assert terminal is False
 
-        ids_seen = [line for line in out if line.startswith("id: ")]
-        # Multi-line records: extract the id integers we delivered.
-        emitted = sorted(
-            int(line.split(": ", 1)[1].split("\n")[0])
-            for line in out
-            if line.startswith("id: ")
-        )
-        # Filter for non-negative (the snapshot-failure synthetic uses -1).
-        emitted = [e for e in emitted if e >= 0]
-        assert emitted == [3, 4]
-        assert ids_seen  # sanity
-
-    def test_reconnect_route_round_trip(self, pg_conn, flask_app):
-        """``/api/messages/<id>/events`` returns the journaled events
-        for an authenticated owner.
+    def test_ownership_sql_accepts_owner_and_rejects_others(self, pg_conn):
+        """The async route's ownership gate runs real SQL against
+        ``conversation_messages`` — the owner passes, everyone else 404s.
         """
-        from flask import Flask, request
+        from application.api import async_sse
 
-        from application.api.answer.routes.messages import messages_bp
+        user_id, message_id = _seed_message(pg_conn)
+
+        @contextmanager
+        def _yield():
+            yield pg_conn
+
+        with patch("application.api.async_sse.db_readonly", _yield):
+            assert async_sse._user_owns_message(message_id, user_id) is True
+            assert async_sse._user_owns_message(message_id, "different-user") is False
+            # A well-formed but unknown id is also not owned.
+            assert async_sse._user_owns_message(str(_uuid.uuid4()), user_id) is False
+
+    def test_snapshot_read_is_user_scoped(self, pg_conn):
+        """``read_snapshot_lines(..., user_id=)`` re-asserts ownership at the
+        data layer: the owner gets the journal rows, a non-owner gets none.
+        """
+        from application.streaming.event_replay import read_snapshot_lines
         from application.streaming.message_journal import record_event
 
-        # Build a fresh Flask app routing to the reconnect blueprint
-        # plus a tiny auth shim that injects the test user.
         user_id, message_id = _seed_message(pg_conn)
-        app = Flask(__name__)
-        app.register_blueprint(messages_bp)
-        app.config["TESTING"] = True
-
-        @app.before_request
-        def _shim_auth():
-            request.decoded_token = {"sub": user_id}
 
         with _patch_journal_session(pg_conn):
             record_event(message_id, 0, "answer", {"type": "answer", "answer": "x"})
             record_event(message_id, 1, "end", {"type": "end"})
 
-            from application.streaming import event_replay
+            owner_lines, _, owner_terminal = read_snapshot_lines(
+                message_id, None, user_id
+            )
+            other_lines, _, other_terminal = read_snapshot_lines(
+                message_id, None, "different-user"
+            )
+            # Unscoped (user_id=None) still returns everything.
+            unscoped_lines, _, _ = read_snapshot_lines(message_id, None)
 
-            def _empty_subscribe(self, on_subscribe=None, poll_timeout=1.0):
-                return
-                yield  # pragma: no cover
+        assert _emitted_ids(owner_lines) == [0, 1] and owner_terminal is True
+        assert other_lines == [] and other_terminal is False
+        assert _emitted_ids(unscoped_lines) == [0, 1]
 
-            with patch.object(
-                event_replay.Topic,
-                "subscribe",
-                _empty_subscribe,
-                create=False,
-            ), patch(
-                "application.api.answer.routes.messages.db_readonly"
-            ) as ro:
-                ro.return_value.__enter__.return_value = pg_conn
-
-                with app.test_client() as c:
-                    r = c.get(f"/api/messages/{message_id}/events")
-                    assert r.status_code == 200
-                    body = b""
-                    for chunk in r.iter_encoded():
-                        body += chunk
-                        if body.count(b"\n\n") >= 4:
-                            break
-                    r.close()
-                # Both journaled events present in the response.
-                text = body.decode("utf-8")
-                assert ": connected" in text
-                assert '"answer": "x"' in text
-                assert '"type": "end"' in text
-                # The seq lines are correct.
-                assert "id: 0" in text and "id: 1" in text
-
-    def test_reconnect_rejects_non_owner(self, pg_conn, flask_app):
-        from flask import Flask, request
-
-        from application.api.answer.routes.messages import messages_bp
+    def test_watchdog_is_user_scoped(self, pg_conn):
+        """``_check_producer_liveness(..., user_id)`` only sees the caller's
+        own row; a non-owner reads as missing (terminal), not as the row's
+        real status.
+        """
+        from application.streaming.event_replay import _check_producer_liveness
 
+        # Seed a row and flip it to a terminal status the watchdog reports.
         user_id, message_id = _seed_message(pg_conn)
-        app = Flask(__name__)
-        app.register_blueprint(messages_bp)
+        pg_conn.execute(
+            sql_text(
+                "UPDATE conversation_messages SET status='complete' WHERE id = :id"
+            ),
+            {"id": message_id},
+        )
 
-        @app.before_request
-        def _shim_auth():
-            request.decoded_token = {"sub": "different-user"}
+        @contextmanager
+        def _yield():
+            yield pg_conn
 
-        # Make the ownership check use the test connection.
-        with patch(
-            "application.api.answer.routes.messages.db_readonly"
-        ) as ro:
-            from contextlib import contextmanager as _cm
+        with patch("application.streaming.event_replay.db_readonly", _yield):
+            owner = _check_producer_liveness(message_id, user_id, 90.0)
+            other = _check_producer_liveness(message_id, "different-user", 90.0)
 
-            @_cm
-            def _yield():
-                yield pg_conn
-
-            ro.side_effect = lambda: _yield()
-            with app.test_client() as c:
-                r = c.get(f"/api/messages/{message_id}/events")
-            assert r.status_code == 404
+        # Owner sees the real terminal state; non-owner sees "missing".
+        assert owner == {"type": "end"}
+        assert other is not None and other.get("code") == "message_missing"
diff --git a/tests/api/test_async_sse_routes.py b/tests/api/test_async_sse_routes.py
new file mode 100644
index 00000000..4726a811
--- /dev/null
+++ b/tests/api/test_async_sse_routes.py
@@ -0,0 +1,252 @@
+"""Tests for ``application/api/async_sse.py``.
+
+Native-async reconnect endpoint: GET /api/messages/<id>/events. Auth gate,
+ownership gate, malformed-id rejection, Last-Event-ID normalisation, and the
+SSE response shape (headers + ``: connected`` prelude). The route is a
+Starlette endpoint, so it's driven through Starlette's TestClient over a
+minimal app built from ``async_sse_routes``.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, patch
+
+import pytest
+from starlette.applications import Starlette
+from starlette.testclient import TestClient
+
+from application.api.async_sse import (
+    _MESSAGE_ID_RE,
+    _normalise_last_event_id,
+    async_sse_routes,
+)
+from application.core.settings import settings
+
+VALID_UUID = "67d65e8f-e7fb-4df1-9e6e-99ea6c830206"
+
+_AUTH = "application.api.async_sse.handle_auth"
+_OWNS = "application.api.async_sse._user_owns_message"
+_STREAM = "application.api.async_sse.build_message_event_stream_async"
+_AREDIS = "application.api.async_sse.get_async_redis_instance"
+
+
+def _client() -> TestClient:
+    return TestClient(Starlette(routes=async_sse_routes))
+
+
+@pytest.fixture(autouse=True)
+def _no_redis_by_default():
+    """Disable the per-user cap (no Redis) so gate tests stay hermetic.
+
+    Cap-specific tests override this with their own mock Redis.
+    """
+    with patch(_AREDIS, AsyncMock(return_value=None)):
+        yield
+
+
+def _mock_redis(incr_value: int) -> AsyncMock:
+    redis = AsyncMock()
+    redis.incr = AsyncMock(return_value=incr_value)
+    redis.expire = AsyncMock(return_value=True)
+    redis.decr = AsyncMock(return_value=incr_value - 1)
+    return redis
+
+
+def _fake_stream(record: dict | None = None):
+    """Async builder stub: records the cursor, yields the prelude, returns."""
+
+    async def _gen(message_id, last_event_id=None, **kwargs):
+        if record is not None:
+            record["message_id"] = message_id
+            record["last_event_id"] = last_event_id
+        yield ": connected\n\n"
+
+    return _gen
+
+
+# ── pure helpers ─────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestNormaliseLastEventId:
+    def test_none_passthrough(self):
+        assert _normalise_last_event_id(None) is None
+
+    def test_empty_string(self):
+        assert _normalise_last_event_id("") is None
+
+    def test_whitespace_only(self):
+        assert _normalise_last_event_id("   ") is None
+
+    def test_valid_int(self):
+        assert _normalise_last_event_id("42") == 42
+
+    def test_stripped_whitespace(self):
+        assert _normalise_last_event_id("  7  ") == 7
+
+    def test_zero_is_valid(self):
+        assert _normalise_last_event_id("0") == 0
+
+    def test_negative_rejected(self):
+        # We expose only non-negative cursors; -1 is reserved for the
+        # snapshot-failure synthetic terminal event.
+        assert _normalise_last_event_id("-1") is None
+
+    def test_non_numeric_rejected(self):
+        for bad in ("foo", "1.5", "1e3", "abc-123", "null"):
+            assert _normalise_last_event_id(bad) is None, bad
+
+
+@pytest.mark.unit
+class TestMessageIdRegex:
+    def test_canonical_uuid_accepted(self):
+        assert _MESSAGE_ID_RE.match(VALID_UUID)
+
+    def test_uppercase_uuid_accepted(self):
+        assert _MESSAGE_ID_RE.match(VALID_UUID.upper())
+
+    def test_no_dashes_rejected(self):
+        assert not _MESSAGE_ID_RE.match(VALID_UUID.replace("-", ""))
+
+    def test_legacy_mongo_id_rejected(self):
+        assert not _MESSAGE_ID_RE.match("507f1f77bcf86cd799439011")
+
+
+# ── auth gate ──────────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestAuthGate:
+    def test_401_when_handle_auth_returns_none(self):
+        with patch(_AUTH, return_value=None):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 401
+
+    def test_401_when_decoded_token_missing_sub(self):
+        with patch(_AUTH, return_value={"email": "x@y"}):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 401
+
+    def test_401_when_handle_auth_returns_error(self):
+        with patch(_AUTH, return_value={"error": "invalid_token"}):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 401
+
+
+# ── message-id validation ───────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestMessageIdValidation:
+    def test_400_on_malformed_id(self):
+        with patch(_AUTH, return_value={"sub": "alice"}):
+            r = _client().get("/api/messages/not-a-uuid/events")
+        assert r.status_code == 400
+
+
+# ── ownership gate ──────────────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestOwnershipGate:
+    def test_404_when_user_does_not_own_message(self):
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=False
+        ):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 404
+
+    def test_200_when_user_owns_message(self):
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream()):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 200
+        assert r.headers.get("content-type", "").startswith("text/event-stream")
+        assert r.headers.get("Cache-Control") == "no-store"
+        assert r.headers.get("X-Accel-Buffering") == "no"
+        assert r.headers.get("X-SSE-Transport") == "async"
+        assert ": connected" in r.text
+
+
+# ── Last-Event-ID parsing ───────────────────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestLastEventIdParsing:
+    def test_header_passes_through_to_builder(self):
+        captured: dict = {}
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream(captured)):
+            _client().get(
+                f"/api/messages/{VALID_UUID}/events",
+                headers={"Last-Event-ID": "12"},
+            )
+        assert captured["message_id"] == VALID_UUID
+        assert captured["last_event_id"] == 12
+
+    def test_query_param_fallback(self):
+        captured: dict = {}
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream(captured)):
+            _client().get(f"/api/messages/{VALID_UUID}/events?last_event_id=5")
+        assert captured["last_event_id"] == 5
+
+    def test_invalid_cursor_normalised_to_none(self):
+        captured: dict = {}
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream(captured)):
+            _client().get(
+                f"/api/messages/{VALID_UUID}/events",
+                headers={"Last-Event-ID": "definitely-not-a-number"},
+            )
+        assert captured["last_event_id"] is None
+
+
+# ── per-user concurrent-connection cap ──────────────────────────────────────
+
+
+@pytest.mark.unit
+class TestConnectionCap:
+    def _cap(self) -> int:
+        return int(settings.SSE_MAX_CONCURRENT_PER_USER) or 8
+
+    def test_429_when_over_cap(self):
+        cap = self._cap()
+        redis = _mock_redis(incr_value=cap + 1)  # post-incr count exceeds cap
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream()), patch(
+            _AREDIS, AsyncMock(return_value=redis)
+        ):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 429
+        # The increment is rolled back so a rejected attempt doesn't wedge
+        # the counter at the cap forever.
+        redis.decr.assert_awaited_once()
+
+    def test_200_and_slot_released_when_under_cap(self):
+        redis = _mock_redis(incr_value=1)
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream()), patch(
+            _AREDIS, AsyncMock(return_value=redis)
+        ):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 200
+        assert ": connected" in r.text
+        redis.incr.assert_awaited_once()
+        # Slot released when the stream finishes (terminal/close).
+        redis.decr.assert_awaited_once()
+
+    def test_cap_skipped_when_redis_unavailable(self):
+        # The autouse fixture already makes get_async_redis_instance -> None;
+        # the stream is served (fail-open, like /api/events).
+        with patch(_AUTH, return_value={"sub": "alice"}), patch(
+            _OWNS, return_value=True
+        ), patch(_STREAM, _fake_stream()):
+            r = _client().get(f"/api/messages/{VALID_UUID}/events")
+        assert r.status_code == 200
diff --git a/tests/api/test_message_stream_routes.py b/tests/api/test_message_stream_routes.py
deleted file mode 100644
index 7da6bc30..00000000
--- a/tests/api/test_message_stream_routes.py
+++ /dev/null
@@ -1,220 +0,0 @@
-"""Tests for ``application/api/answer/routes/messages.py``.
-
-Reconnect endpoint: GET /api/messages/<id>/events. Auth gate, ownership
-gate, malformed-id rejection, Last-Event-ID normalisation, and a smoke
-test that the SSE response shape matches the user-events endpoint.
-"""
-
-from __future__ import annotations
-
-from unittest.mock import patch
-
-from flask import Flask, request
-
-from application.api.answer.routes.messages import (
-    _MESSAGE_ID_RE,
-    _normalise_last_event_id,
-    messages_bp,
-)
-
-
-def _make_app(decoded_token=None):
-    app = Flask(__name__)
-    app.register_blueprint(messages_bp)
-    app.config["TESTING"] = True
-
-    @app.before_request
-    def _shim_auth():
-        request.decoded_token = decoded_token
-
-    return app
-
-
-VALID_UUID = "67d65e8f-e7fb-4df1-9e6e-99ea6c830206"
-
-
-class TestNormaliseLastEventId:
-    def test_none_passthrough(self):
-        assert _normalise_last_event_id(None) is None
-
-    def test_empty_string(self):
-        assert _normalise_last_event_id("") is None
-
-    def test_whitespace_only(self):
-        assert _normalise_last_event_id("   ") is None
-
-    def test_valid_int(self):
-        assert _normalise_last_event_id("42") == 42
-
-    def test_stripped_whitespace(self):
-        assert _normalise_last_event_id("  7  ") == 7
-
-    def test_zero_is_valid(self):
-        assert _normalise_last_event_id("0") == 0
-
-    def test_negative_rejected(self):
-        # We expose only non-negative cursors; -1 is reserved for the
-        # snapshot-failure synthetic terminal event and shouldn't
-        # round-trip back.
-        assert _normalise_last_event_id("-1") is None
-
-    def test_non_numeric_rejected(self):
-        for bad in ("foo", "1.5", "1e3", "abc-123", "null"):
-            assert _normalise_last_event_id(bad) is None, bad
-
-
-class TestMessageIdRegex:
-    def test_canonical_uuid_accepted(self):
-        assert _MESSAGE_ID_RE.match(VALID_UUID)
-
-    def test_uppercase_uuid_accepted(self):
-        assert _MESSAGE_ID_RE.match(VALID_UUID.upper())
-
-    def test_no_dashes_rejected(self):
-        assert not _MESSAGE_ID_RE.match(VALID_UUID.replace("-", ""))
-
-    def test_legacy_mongo_id_rejected(self):
-        # 24-char hex with no dashes — a Mongo objectid-shaped string
-        # that happened to leak through somewhere.
-        assert not _MESSAGE_ID_RE.match("507f1f77bcf86cd799439011")
-
-
-class TestAuthGate:
-    def test_401_when_no_decoded_token(self):
-        app = _make_app(decoded_token=None)
-        with app.test_client() as c:
-            r = c.get(f"/api/messages/{VALID_UUID}/events")
-        assert r.status_code == 401
-
-    def test_401_when_decoded_token_missing_sub(self):
-        app = _make_app(decoded_token={"email": "x@y"})
-        with app.test_client() as c:
-            r = c.get(f"/api/messages/{VALID_UUID}/events")
-        assert r.status_code == 401
-
-
-class TestMessageIdValidation:
-    def test_400_on_malformed_id(self):
-        app = _make_app(decoded_token={"sub": "alice"})
-        with app.test_client() as c:
-            r = c.get("/api/messages/not-a-uuid/events")
-        assert r.status_code == 400
-
-
-class TestOwnershipGate:
-    def test_404_when_user_does_not_own_message(self):
-        from application.api.answer.routes import messages as messages_module
-
-        app = _make_app(decoded_token={"sub": "alice"})
-        with patch.object(
-            messages_module, "_user_owns_message", return_value=False
-        ):
-            with app.test_client() as c:
-                r = c.get(f"/api/messages/{VALID_UUID}/events")
-        assert r.status_code == 404
-
-    def test_200_when_user_owns_message(self):
-        from application.api.answer.routes import messages as messages_module
-
-        app = _make_app(decoded_token={"sub": "alice"})
-
-        # Have build_message_event_stream yield just the prelude then
-        # exit so the test can drain the response without blocking on
-        # a live pubsub subscription.
-        def _fake_builder(message_id, last_event_id=None, **kwargs):
-            yield ": connected\n\n"
-
-        with patch.object(
-            messages_module, "_user_owns_message", return_value=True
-        ), patch.object(
-            messages_module, "build_message_event_stream", _fake_builder
-        ):
-            with app.test_client() as c:
-                r = c.get(f"/api/messages/{VALID_UUID}/events")
-                assert r.status_code == 200
-                assert r.mimetype == "text/event-stream"
-                assert r.headers.get("Cache-Control") == "no-store"
-                assert r.headers.get("X-Accel-Buffering") == "no"
-                body = b""
-                for chunk in r.iter_encoded():
-                    body += chunk
-                    if b": connected" in body:
-                        break
-                r.close()
-                assert b": connected" in body
-
-
-class TestLastEventIdParsing:
-    def test_header_passes_through_to_builder(self):
-        from application.api.answer.routes import messages as messages_module
-
-        captured = {}
-
-        def _fake_builder(message_id, last_event_id=None, **kwargs):
-            captured["message_id"] = message_id
-            captured["last_event_id"] = last_event_id
-            yield ": connected\n\n"
-
-        app = _make_app(decoded_token={"sub": "alice"})
-        with patch.object(
-            messages_module, "_user_owns_message", return_value=True
-        ), patch.object(
-            messages_module, "build_message_event_stream", _fake_builder
-        ):
-            with app.test_client() as c:
-                r = c.get(
-                    f"/api/messages/{VALID_UUID}/events",
-                    headers={"Last-Event-ID": "12"},
-                )
-                # Drain a tick.
-                next(iter(r.iter_encoded()), None)
-                r.close()
-        assert captured["message_id"] == VALID_UUID
-        assert captured["last_event_id"] == 12
-
-    def test_query_param_fallback(self):
-        from application.api.answer.routes import messages as messages_module
-
-        captured = {}
-
-        def _fake_builder(message_id, last_event_id=None, **kwargs):
-            captured["last_event_id"] = last_event_id
-            yield ": connected\n\n"
-
-        app = _make_app(decoded_token={"sub": "alice"})
-        with patch.object(
-            messages_module, "_user_owns_message", return_value=True
-        ), patch.object(
-            messages_module, "build_message_event_stream", _fake_builder
-        ):
-            with app.test_client() as c:
-                r = c.get(
-                    f"/api/messages/{VALID_UUID}/events?last_event_id=5"
-                )
-                next(iter(r.iter_encoded()), None)
-                r.close()
-        assert captured["last_event_id"] == 5
-
-    def test_invalid_cursor_normalised_to_none(self):
-        from application.api.answer.routes import messages as messages_module
-
-        captured = {}
-
-        def _fake_builder(message_id, last_event_id=None, **kwargs):
-            captured["last_event_id"] = last_event_id
-            yield ": connected\n\n"
-
-        app = _make_app(decoded_token={"sub": "alice"})
-        with patch.object(
-            messages_module, "_user_owns_message", return_value=True
-        ), patch.object(
-            messages_module, "build_message_event_stream", _fake_builder
-        ):
-            with app.test_client() as c:
-                r = c.get(
-                    f"/api/messages/{VALID_UUID}/events",
-                    headers={"Last-Event-ID": "definitely-not-a-number"},
-                )
-                next(iter(r.iter_encoded()), None)
-                r.close()
-        assert captured["last_event_id"] is None
diff --git a/tests/test_event_replay.py b/tests/test_event_replay.py
index db00a7b2..5b2c3ae4 100644
--- a/tests/test_event_replay.py
+++ b/tests/test_event_replay.py
@@ -1,37 +1,48 @@
-"""Unit tests for ``application/streaming/event_replay.py``.
+"""Unit tests for the chat-stream reconnect snapshot+tail boundary.
 
-The replay generator is the snapshot+tail boundary the chat-stream
-reconnect path lives on. The boundary correctness invariants worth
-locking down:
+``event_replay`` holds the shared leaf primitives (SSE wire format, pub/sub
+envelope encode/decode, snapshot read, watchdog probe); the reader itself is
+the async generator ``async_event_replay.build_message_event_stream_async``.
+Boundary correctness invariants worth locking down:
 
-- Snapshot replay yields rows in ``sequence_no`` order with the SSE
-  ``id:`` header set to that sequence_no.
+- Snapshot replay yields rows in ``sequence_no`` order with the SSE ``id:``
+  header set to that sequence_no.
 - Live tail dedupes pub/sub messages whose ``sequence_no`` is already
   covered by the snapshot.
 - A backlog read failure inside ``on_subscribe`` doesn't wedge the
   generator — it surfaces a terminal ``error`` event (``code:
-  snapshot_failed``) and returns, freeing the WSGI thread instead of
-  pinning it on keepalives the client can't hear as terminal.
+  snapshot_failed``) and returns.
 - Keepalive comments fire after the configured silence window.
 - ``encode_pubsub_message`` round-trips with ``_decode_pubsub_message``.
+
+The snapshot read and watchdog probe run via ``anyio.to_thread`` inside the
+async generator but are the same ``event_replay`` functions, so tests patch
+them at ``application.streaming.event_replay.*`` exactly as before.
 """
 
 from __future__ import annotations
 
+import inspect
 import json
-from typing import Iterator
+from typing import AsyncIterator
 from unittest.mock import MagicMock, patch
 
 import pytest
 
+from application.streaming.async_event_replay import (
+    build_message_event_stream_async,
+)
 from application.streaming.event_replay import (
     _SSE_LINE_SPLIT_PATTERN,
     _decode_pubsub_message,
-    build_message_event_stream,
     encode_pubsub_message,
     format_sse_event,
 )
 
+_ASYNC_TOPIC = "application.streaming.async_event_replay.AsyncTopic.subscribe"
+_READONLY = "application.streaming.event_replay.db_readonly"
+_REPO = "application.streaming.event_replay.MessageEventsRepository"
+
 
 # ── format_sse_event ────────────────────────────────────────────────────
 
@@ -91,19 +102,20 @@ class TestPubsubEnvelope:
         assert envelope == {"sequence_no": 1, "payload": {}}
 
 
-# ── build_message_event_stream ──────────────────────────────────────────
+# ── build_message_event_stream_async ────────────────────────────────────
 
 
-def _fake_topic_subscribe(messages: list, *, fire_callback: bool = True):
-    """Build a ``Topic.subscribe`` mock that fires ``on_subscribe`` then
-    yields the supplied bytes in order, then yields ``None`` ticks
-    indefinitely so the generator can keep running until the test
-    closes it.
+def _fake_subscribe(messages: list, *, fire_callback: bool = True):
+    """Build an ``AsyncTopic.subscribe`` mock that fires ``on_subscribe``
+    then yields the supplied bytes in order, then yields ``None`` ticks
+    indefinitely so the generator keeps running until the test closes it.
     """
 
-    def _impl(self, on_subscribe=None, poll_timeout=1.0):
+    async def _impl(self, on_subscribe=None, poll_timeout=1.0):
         if fire_callback and on_subscribe is not None:
-            on_subscribe()
+            res = on_subscribe()
+            if inspect.isawaitable(res):
+                await res
         for m in messages:
             yield m
         while True:
@@ -112,43 +124,57 @@ def _fake_topic_subscribe(messages: list, *, fire_callback: bool = True):
     return _impl
 
 
-def _drain(gen: Iterator[str], *, max_items: int = 50) -> list[str]:
-    out = []
+def _subscribe_returns_immediately(*, fire_callback: bool):
+    """``AsyncTopic.subscribe`` that exits without yielding (Redis-down).
+
+    With ``fire_callback`` it runs ``on_subscribe`` first (the
+    SUBSCRIBE-ack-then-get_message-dies race); without it, nothing runs
+    (subscribe itself failed), exercising the post-loop fallback read.
+    """
+
+    async def _impl(self, on_subscribe=None, poll_timeout=1.0):
+        if fire_callback and on_subscribe is not None:
+            res = on_subscribe()
+            if inspect.isawaitable(res):
+                await res
+        return
+        yield  # pragma: no cover — make the function an async generator
+
+    return _impl
+
+
+async def _drain(agen: AsyncIterator[str], *, max_items: int = 50) -> list[str]:
+    out: list[str] = []
     try:
         for _ in range(max_items):
-            out.append(next(gen))
-    except StopIteration:
+            out.append(await agen.__anext__())
+    except StopAsyncIteration:
         pass
     finally:
-        gen.close()
+        await agen.aclose()
     return out
 
 
 @pytest.mark.unit
+@pytest.mark.asyncio
 class TestBuildMessageEventStream:
-    def test_yields_connected_prelude_first(self):
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([]),
-            create=False,
+    async def test_yields_connected_prelude_first(self):
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            first = next(gen)
-            gen.close()
+            first = await gen.__anext__()
+            await gen.aclose()
             assert first == ": connected\n\n"
 
-    def test_snapshot_replays_in_sequence_order(self):
+    async def test_snapshot_replays_in_sequence_order(self):
         rows = [
             {
                 "sequence_no": 0,
@@ -161,39 +187,27 @@ class TestBuildMessageEventStream:
                 "payload": {"type": "answer", "answer": "B"},
             },
         ]
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = rows
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=4)
+            out = await _drain(gen, max_items=4)
 
-        # Expect: prelude, two snapshot frames, then keepalive ticks (None
-        # not yielded as a string — generator yields keepalive comments
-        # instead). With poll_timeout=1.0 and short test window we may
-        # only see the prelude + snapshot before close.
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
         assert "id: 1" in out[2]
-        # The repo was queried with the right cursor.
         mock_repo_cls.return_value.read_after.assert_called_once_with(
-            "msg-1", last_sequence_no=None
+            "msg-1", last_sequence_no=None, user_id=None
         )
 
-    def test_live_tail_dedupes_against_snapshot(self):
+    async def test_live_tail_dedupes_against_snapshot(self):
         snapshot_rows = [
             {
                 "sequence_no": 5,
@@ -208,25 +222,18 @@ class TestBuildMessageEventStream:
             "msg-1", 6, "answer", {"type": "answer", "answer": "fresh"}
         ).encode("utf-8")
 
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live_envelope, live_new_envelope]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live_envelope, live_new_envelope])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = snapshot_rows
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=4)
+            out = await _drain(gen, max_items=4)
 
         assert out[0] == ": connected\n\n"
         assert "id: 5" in out[1]
@@ -234,51 +241,36 @@ class TestBuildMessageEventStream:
         # The duplicate live event (seq=5) is dropped.
         assert "id: 6" in out[2]
         assert '"answer": "fresh"' in out[2]
-        # No third frame past the fresh one (only None ticks).
 
-    def test_live_tail_passes_through_when_seq_strictly_greater_than_replay(self):
+    async def test_live_tail_passes_through_when_seq_strictly_greater_than_replay(self):
         """No snapshot rows; every live event is fresh and yielded."""
         live = encode_pubsub_message(
             "msg-1", 0, "answer", {"type": "answer", "answer": "x"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=3)
+            out = await _drain(gen, max_items=3)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
         assert '"answer": "x"' in out[1]
 
-    def test_snapshot_read_failure_surfaces_synthetic_event(self):
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([]),
-            create=False,
+    async def test_snapshot_read_failure_surfaces_synthetic_event(self):
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.side_effect = RuntimeError("boom")
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
@@ -286,125 +278,101 @@ class TestBuildMessageEventStream:
             )
             # Ask for more than the expected output so we'd notice a
             # regression where the generator keeps emitting keepalives.
-            out = _drain(gen, max_items=10)
+            out = await _drain(gen, max_items=10)
 
         assert out[0] == ": connected\n\n"
-        # Terminal ``error`` event with the snapshot-specific ``code`` so
-        # the frontend's existing end/error contract closes the stream.
         assert '"type": "error"' in out[1]
         assert '"code": "snapshot_failed"' in out[1]
         assert "id: -1" in out[1]
-        # Generator must return after the synthetic — otherwise the
-        # client hangs on keepalives waiting for a terminal event that
-        # would never arrive.
+        # Generator must return after the synthetic.
         assert len(out) == 2
 
-    def test_malformed_pubsub_message_dropped_silently(self):
+    async def test_malformed_pubsub_message_dropped_silently(self):
         bad = b"not-json"
         good = encode_pubsub_message(
             "msg-1", 0, "answer", {"type": "answer", "answer": "ok"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([bad, good]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([bad, good])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=3)
+            out = await _drain(gen, max_items=3)
 
         assert out[0] == ": connected\n\n"
         # Bad message dropped; good one yielded.
         assert "id: 0" in out[1]
         assert '"answer": "ok"' in out[1]
 
-    def test_pubsub_envelope_with_non_int_sequence_dropped(self):
+    async def test_pubsub_envelope_with_non_int_sequence_dropped(self):
         bad = json.dumps(
             {"sequence_no": "not-int", "payload": {"type": "x"}}
         ).encode("utf-8")
         good = encode_pubsub_message(
             "msg-1", 0, "answer", {"type": "answer", "answer": "ok"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([bad, good]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([bad, good])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=3)
+            out = await _drain(gen, max_items=3)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
 
 
 @pytest.mark.unit
+@pytest.mark.asyncio
 class TestDedupFloorSeededFromCursor:
     """Regressions for the dedup-floor bugs the round-1 review flagged.
 
-    With ``max_replayed_seq`` initialised to ``last_event_id``, an
-    empty snapshot still rejects republished live events the client
-    has already seen. Advancing on yield protects against republish
-    past the snapshot ceiling.
+    With ``max_replayed_seq`` initialised to ``last_event_id``, an empty
+    snapshot still rejects republished live events the client has already
+    seen. Advancing on yield protects against republish past the snapshot
+    ceiling.
     """
 
-    def test_empty_snapshot_dedups_against_last_event_id(self):
-        # No snapshot rows; live event with seq=3 (already seen by
-        # client at last_event_id=5) must be dropped.
+    async def test_empty_snapshot_dedups_against_last_event_id(self):
+        # No snapshot rows; live event with seq=3 (already seen by client
+        # at last_event_id=5) must be dropped.
         live_dup = encode_pubsub_message(
             "msg-1", 3, "answer", {"type": "answer", "answer": "stale"}
         ).encode("utf-8")
         live_fresh = encode_pubsub_message(
             "msg-1", 6, "answer", {"type": "answer", "answer": "fresh"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live_dup, live_fresh]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live_dup, live_fresh])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=5,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=3)
+            out = await _drain(gen, max_items=3)
 
         assert out[0] == ": connected\n\n"
         # Stale live event dropped; only the fresh one yielded.
         assert "id: 6" in out[1]
         assert '"answer": "fresh"' in out[1]
 
-    def test_yielded_live_event_advances_dedup_floor(self):
+    async def test_yielded_live_event_advances_dedup_floor(self):
         """A republish of an already-yielded seq must be dropped."""
         live_first = encode_pubsub_message(
             "msg-1", 0, "answer", {"type": "answer", "answer": "first"}
@@ -415,25 +383,18 @@ class TestDedupFloorSeededFromCursor:
         live_third = encode_pubsub_message(
             "msg-1", 1, "answer", {"type": "answer", "answer": "third"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live_first, live_dup, live_third]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live_first, live_dup, live_third])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = _drain(gen, max_items=4)
+            out = await _drain(gen, max_items=4)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
@@ -444,13 +405,13 @@ class TestDedupFloorSeededFromCursor:
 
 
 @pytest.mark.unit
+@pytest.mark.asyncio
 class TestSnapshotWhenSubscribeUnavailable:
-    """Regression for C1: when Redis is down, ``Topic.subscribe`` exits
-    immediately without firing ``on_subscribe``. The snapshot is in
-    Postgres and must still be served.
+    """When Redis is down, ``AsyncTopic.subscribe`` exits immediately. The
+    snapshot is in Postgres and must still be served.
     """
 
-    def test_snapshot_served_when_subscribe_returns_immediately(self):
+    async def test_snapshot_served_when_subscribe_returns_immediately(self):
         rows = [
             {
                 "sequence_no": 0,
@@ -458,44 +419,30 @@ class TestSnapshotWhenSubscribeUnavailable:
                 "payload": {"type": "answer", "answer": "from snapshot"},
             },
         ]
-        # Topic.subscribe yields nothing (Redis-down behaviour).
-        def _empty_subscribe(self, on_subscribe=None, poll_timeout=1.0):
-            return
-            yield  # pragma: no cover  (make the function a generator)
-
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _empty_subscribe,
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _subscribe_returns_immediately(fire_callback=False)
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = rows
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
         assert out[0] == ": connected\n\n"
         # Snapshot row served via the post-subscribe fallback path.
         assert "id: 0" in out[1]
         assert '"answer": "from snapshot"' in out[1]
 
-    def test_callback_fired_then_subscribe_dies_does_not_duplicate(self):
-        """Regression: if ``on_subscribe`` ran and populated the buffer,
-        a subsequent inner-generator failure (e.g. transient Redis
-        ``get_message`` exception between SUBSCRIBE-ack and the first
-        poll) must not trigger a second snapshot read. Re-reading would
-        append the same rows twice and double the answer chunks on the
-        client (the per-message reconnect dispatcher does not dedup
-        by ``id``).
+    async def test_callback_fired_then_subscribe_dies_does_not_duplicate(self):
+        """If ``on_subscribe`` ran and populated the buffer, a subsequent
+        inner-generator failure must not trigger a second snapshot read.
+        Re-reading would append the same rows twice and double the answer
+        chunks on the client (the reconnect dispatcher does not dedup by
+        ``id``).
         """
         rows = [
             {
@@ -509,46 +456,26 @@ class TestSnapshotWhenSubscribeUnavailable:
                 "payload": {"type": "answer", "answer": "second"},
             },
         ]
-
-        # Mimic the broadcast_channel race: SUBSCRIBE acks, on_subscribe
-        # runs, then the inner ``get_message`` raises and the generator
-        # returns without ever yielding.
-        def _subscribe_dies_after_callback(
-            self, on_subscribe=None, poll_timeout=1.0
-        ):
-            if on_subscribe is not None:
-                on_subscribe()
-            return
-            yield  # pragma: no cover  (make the function a generator)
-
         repo_mock = MagicMock()
         repo_mock.read_after.return_value = rows
 
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository",
-            return_value=repo_mock,
+        with patch(_READONLY) as mock_readonly, patch(
+            _REPO, return_value=repo_mock
         ), patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _subscribe_dies_after_callback,
-            create=False,
+            _ASYNC_TOPIC, _subscribe_returns_immediately(fire_callback=True)
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
-        # The snapshot must have been read exactly once — re-reading
-        # would have re-appended the rows behind the originals.
+        # The snapshot must have been read exactly once.
         assert repo_mock.read_after.call_count == 1
         assert out[0] == ": connected\n\n"
-        # Each row appears exactly once, in order.
         assert "id: 0" in out[1]
         assert '"answer": "first"' in out[1]
         assert "id: 1" in out[2]
@@ -558,15 +485,14 @@ class TestSnapshotWhenSubscribeUnavailable:
 
 
 @pytest.mark.unit
+@pytest.mark.asyncio
 class TestTerminalEventClosesStream:
-    """Regression: ``/api/messages/<id>/events`` is a live tail that
-    keeps emitting keepalives. Without explicit close-on-terminal the
-    client's drain promise never resolves and the WSGI thread is
-    pinned waiting for events that won't come for an already-finished
-    stream.
+    """Without explicit close-on-terminal the client's drain promise never
+    resolves and the connection is pinned waiting for events that won't
+    come for an already-finished stream.
     """
 
-    def test_terminal_in_snapshot_closes_after_flush(self):
+    async def test_terminal_in_snapshot_closes_after_flush(self):
         rows = [
             {
                 "sequence_no": 0,
@@ -579,28 +505,18 @@ class TestTerminalEventClosesStream:
                 "payload": {"type": "end"},
             },
         ]
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = rows
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            # Drain to exhaustion. Without the close-on-terminal fix
-            # this would hang; with it, the generator returns after
-            # flushing the snapshot.
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
@@ -608,10 +524,10 @@ class TestTerminalEventClosesStream:
         # No keepalives or further frames after the terminal.
         assert all("keepalive" not in line for line in out[3:])
 
-    def test_terminal_falls_back_to_event_type_when_payload_lacks_type(self):
-        # Belt-and-suspenders: a journal write that records ``end`` only
-        # in the column (e.g. an abort handler that didn't seed
-        # ``payload.type``) must still terminate the replay.
+    async def test_terminal_falls_back_to_event_type_when_payload_lacks_type(self):
+        # A journal write that records ``end`` only in the column (e.g. an
+        # abort handler that didn't seed ``payload.type``) must still
+        # terminate the replay.
         rows = [
             {
                 "sequence_no": 0,
@@ -624,89 +540,68 @@ class TestTerminalEventClosesStream:
                 "payload": {},
             },
         ]
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = rows
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1]
         assert "id: 1" in out[2]
         assert all("keepalive" not in line for line in out[3:])
 
-    def test_terminal_in_live_tail_closes(self):
+    async def test_terminal_in_live_tail_closes(self):
         live_answer = encode_pubsub_message(
             "msg-1", 0, "answer", {"type": "answer", "answer": "x"}
         ).encode("utf-8")
         live_end = encode_pubsub_message(
             "msg-1", 1, "end", {"type": "end"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live_answer, live_end]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live_answer, live_end])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1] and '"answer": "x"' in out[1]
         assert "id: 1" in out[2] and '"type": "end"' in out[2]
         assert all("keepalive" not in line for line in out[3:])
 
-    def test_error_event_also_closes(self):
+    async def test_error_event_also_closes(self):
         """The agent's catch-all path emits ``error`` with no trailing
         ``end`` — treating ``error`` as terminal closes that path too.
         """
         live_err = encode_pubsub_message(
             "msg-1", 0, "error", {"type": "error", "error": "boom"}
         ).encode("utf-8")
-        with patch(
-            "application.streaming.event_replay.db_readonly"
-        ) as mock_readonly, patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        ) as mock_repo_cls, patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            _fake_topic_subscribe([live_err]),
-            create=False,
+        with patch(_READONLY) as mock_readonly, patch(_REPO) as mock_repo_cls, patch(
+            _ASYNC_TOPIC, _fake_subscribe([live_err])
         ):
             mock_readonly.return_value.__enter__.return_value = MagicMock()
             mock_repo_cls.return_value.read_after.return_value = []
-
-            gen = build_message_event_stream(
+            gen = build_message_event_stream_async(
                 "msg-1",
                 last_event_id=None,
                 keepalive_seconds=0.05,
                 poll_timeout_seconds=0.01,
             )
-            out = list(gen)
+            out = await _drain(gen, max_items=50)
 
         assert out[0] == ": connected\n\n"
         assert "id: 0" in out[1] and '"type": "error"' in out[1]
@@ -719,36 +614,34 @@ def test_sse_line_split_pattern_handles_all_terminators():
 
 
 @pytest.mark.unit
+@pytest.mark.asyncio
 class TestWatchdogClosesIdleReconnect:
-    """Without the watchdog a reconnect stream with a non-terminal
-    snapshot and a dead producer (worker crash between chunks and
-    finalize) would emit keepalives forever — the live tail blocks
-    on a pub/sub that nobody publishes to and the frontend's drain
-    promise never resolves. The watchdog periodically inspects
-    ``conversation_messages`` and closes the stream with a terminal
-    SSE event when the row has gone terminal in the DB or the
-    producer's heartbeat has gone stale.
+    """Without the watchdog a reconnect stream with a non-terminal snapshot
+    and a dead producer would emit keepalives forever. The watchdog
+    periodically inspects ``conversation_messages`` and closes the stream
+    with a terminal SSE event when the row has gone terminal in the DB or
+    the producer's heartbeat has gone stale.
     """
 
     @staticmethod
-    def _patch_subscribe_idle_forever():
-        """Build a fake ``Topic.subscribe`` that fires ``on_subscribe``
-        and then yields ``None`` ticks indefinitely (i.e. the producer
-        is gone). Returns the generator function and a list collecting
-        the ``on_subscribe`` calls so tests can assert ordering.
+    def _subscribe_idle_forever():
+        """``AsyncTopic.subscribe`` that fires ``on_subscribe`` then yields
+        ``None`` ticks indefinitely (i.e. the producer is gone).
         """
 
-        def _impl(self, on_subscribe=None, poll_timeout=1.0):
+        async def _impl(self, on_subscribe=None, poll_timeout=1.0):
             if on_subscribe is not None:
-                on_subscribe()
+                res = on_subscribe()
+                if inspect.isawaitable(res):
+                    await res
             while True:
                 yield None
 
         return _impl
 
     def _mock_liveness_row(self, status, err=None, is_stale=False):
-        """Build the ``conn.execute(...).first()`` return value the
-        watchdog SQL expects — ``(status, err, is_stale)``.
+        """Build the ``conn.execute(...).first()`` return value the watchdog
+        SQL expects — ``(status, err, is_stale)``.
         """
         return (status, err, is_stale)
 
@@ -762,25 +655,17 @@ class TestWatchdogClosesIdleReconnect:
     ):
         """Wire up the patches the watchdog tests share.
 
-        The snapshot read goes through the patched
-        ``MessageEventsRepository`` (returns empty), and the watchdog
-        liveness check goes through ``conn.execute(...).first()`` on the
-        same ``db_readonly``-yielded ``MagicMock`` connection.
+        The snapshot read goes through the patched ``MessageEventsRepository``
+        (returns empty), and the watchdog liveness check goes through
+        ``conn.execute(...).first()`` on the same ``db_readonly``-yielded
+        ``MagicMock`` connection.
         """
         mock_conn = MagicMock()
         mock_conn.execute.return_value.first.return_value = liveness_row
 
-        readonly_patch = patch(
-            "application.streaming.event_replay.db_readonly"
-        )
-        repo_patch = patch(
-            "application.streaming.event_replay.MessageEventsRepository"
-        )
-        subscribe_patch = patch(
-            "application.streaming.event_replay.Topic.subscribe",
-            self._patch_subscribe_idle_forever(),
-            create=False,
-        )
+        readonly_patch = patch(_READONLY)
+        repo_patch = patch(_REPO)
+        subscribe_patch = patch(_ASYNC_TOPIC, self._subscribe_idle_forever())
 
         mock_readonly = readonly_patch.start()
         mock_repo_cls = repo_patch.start()
@@ -789,7 +674,7 @@ class TestWatchdogClosesIdleReconnect:
         mock_readonly.return_value.__enter__.return_value = mock_conn
         mock_repo_cls.return_value.read_after.return_value = []
 
-        gen = build_message_event_stream(
+        gen = build_message_event_stream_async(
             "msg-1",
             last_event_id=None,
             keepalive_seconds=keepalive_seconds,
@@ -799,34 +684,32 @@ class TestWatchdogClosesIdleReconnect:
         )
         return gen, [readonly_patch, repo_patch, subscribe_patch]
 
-    def test_watchdog_emits_synthetic_end_when_status_complete(self):
-        """A row that flipped to ``complete`` after the snapshot read
-        (e.g. finalize ran on another worker, journal write lost) must
+    async def test_watchdog_emits_synthetic_end_when_status_complete(self):
+        """A row that flipped to ``complete`` after the snapshot read must
         be surfaced as ``end`` so the client closes cleanly.
         """
         gen, patches = self._build_gen_with_liveness(
             self._mock_liveness_row("complete")
         )
         try:
-            out = _drain(gen, max_items=5)
+            out = await _drain(gen, max_items=5)
         finally:
             for p in patches:
                 p.stop()
 
         assert out[0] == ": connected\n\n"
-        # Watchdog synthetic: id:-1, ``{"type": "end"}``
         terminal = [s for s in out if '"type": "end"' in s]
         assert len(terminal) == 1
         assert "id: -1" in terminal[0]
 
-    def test_watchdog_emits_synthetic_error_when_status_failed(self):
+    async def test_watchdog_emits_synthetic_error_when_status_failed(self):
         gen, patches = self._build_gen_with_liveness(
             self._mock_liveness_row(
                 "failed", err="RuntimeError: upstream blew up"
             )
         )
         try:
-            out = _drain(gen, max_items=5)
+            out = await _drain(gen, max_items=5)
         finally:
             for p in patches:
                 p.stop()
@@ -835,21 +718,15 @@ class TestWatchdogClosesIdleReconnect:
         terminal = [s for s in out if '"type": "error"' in s]
         assert len(terminal) == 1
         assert '"code": "producer_failed"' in terminal[0]
-        # The stashed error from metadata is surfaced verbatim so the
-        # UI can show the real reason instead of a generic message.
         assert "RuntimeError: upstream blew up" in terminal[0]
 
-    def test_watchdog_emits_synthetic_error_when_producer_stale(self):
-        """Non-terminal status + heartbeat older than the threshold ⇒
-        producing worker is presumed dead. Without this, the live tail
-        would hang on keepalives until the proxy idle-timeout fires.
-        """
+    async def test_watchdog_emits_synthetic_error_when_producer_stale(self):
         gen, patches = self._build_gen_with_liveness(
             self._mock_liveness_row("streaming", is_stale=True),
             producer_idle_seconds=1.0,
         )
         try:
-            out = _drain(gen, max_items=5)
+            out = await _drain(gen, max_items=5)
         finally:
             for p in patches:
                 p.stop()
@@ -859,37 +736,34 @@ class TestWatchdogClosesIdleReconnect:
         assert len(terminal) == 1
         assert '"code": "producer_stale"' in terminal[0]
 
-    def test_watchdog_does_not_fire_while_producer_alive(self):
+    async def test_watchdog_does_not_fire_while_producer_alive(self):
         """A non-terminal row with a fresh heartbeat is healthy; the
-        watchdog must keep silent (yield keepalives instead) so a
-        slow-but-alive stream isn't prematurely terminated.
+        watchdog must keep silent (yield keepalives instead).
         """
         gen, patches = self._build_gen_with_liveness(
             self._mock_liveness_row("streaming", is_stale=False),
             keepalive_seconds=0.01,
         )
         try:
-            out = _drain(gen, max_items=5)
+            out = await _drain(gen, max_items=5)
         finally:
             for p in patches:
                 p.stop()
 
         assert out[0] == ": connected\n\n"
-        # No synthetic terminal — the rest of the output is keepalives.
         assert all(
             '"type": "end"' not in s and '"type": "error"' not in s
             for s in out
         )
         assert any("keepalive" in s for s in out)
 
-    def test_watchdog_handles_missing_row_as_terminal(self):
-        """If the message row got deleted out from under us mid-tail,
-        the watchdog must close the stream rather than tail forever
-        on a row that no longer exists.
+    async def test_watchdog_handles_missing_row_as_terminal(self):
+        """If the message row got deleted out from under us mid-tail, the
+        watchdog must close the stream rather than tail forever.
         """
         gen, patches = self._build_gen_with_liveness(None)
         try:
-            out = _drain(gen, max_items=5)
+            out = await _drain(gen, max_items=5)
         finally:
             for p in patches:
                 p.stop()