diff --git a/docsgpt/agents/tool_executor.py b/docsgpt/agents/tool_executor.py
index 94186b79..7bbde1f4 100644
--- a/docsgpt/agents/tool_executor.py
+++ b/docsgpt/agents/tool_executor.py
@@ -14,6 +14,7 @@ from docsgpt.agents.default_tools import (
     synthesized_default_tools,
 )
 from docsgpt import tracing
+from docsgpt.agents.tool_pins import iter_parameters, llm_fills, resolve_arguments
 from docsgpt.agents.tools.tool_action_parser import ToolActionParser
 from docsgpt.agents.tools.tool_manager import ToolManager
 from docsgpt.guardrails.types import Stage as GuardrailStage, resolve_tool_result
@@ -761,17 +762,22 @@ class ToolExecutor:
             )
         return result
 
-    def _build_tool_parameters(self, action: Dict) -> Dict:
+    def _build_tool_parameters(self, action: Dict, hidden: Optional[set] = None) -> Dict:
+        """The JSON schema the model sees for ``action``.
+
+        Parameters the model does not fill (fixed values) and ``hidden`` ones
+        (values the connection fixes) are left out, so the model is never
+        asked for them.
+        """
         params = {"type": "object", "properties": {}, "required": []}
-        for param_type in ["query_params", "headers", "body", "parameters"]:
-            if param_type in action and action[param_type].get("properties"):
-                for k, v in action[param_type]["properties"].items():
-                    if v.get("filled_by_llm", True):
-                        params["properties"][k] = {
-                            key: value for key, value in v.items() if key not in ("filled_by_llm", "value", "required")
-                        }
-                        if v.get("required", False):
-                            params["required"].append(k)
+        for _section, k, v in iter_parameters(action):
+            if not llm_fills(v) or (hidden and k in hidden):
+                continue
+            params["properties"][k] = {
+                key: value for key, value in v.items() if key not in ("filled_by_llm", "value", "required")
+            }
+            if v.get("required", False):
+                params["required"].append(k)
         return params
 
     def _guardrail_tool_result(self, result: Any, tool_name: str, action_name: str) -> Any:
@@ -1393,23 +1399,10 @@ class ToolExecutor:
 
             tool_call_data["access"] = action_access(tool_data.get("name"), action_data)
 
-        query_params, headers, body, parameters = {}, {}, {}, {}
-        param_types = {
-            "query_params": query_params,
-            "headers": headers,
-            "body": body,
-            "parameters": parameters,
-        }
-
-        for param_type, target_dict in param_types.items():
-            if param_type in action_data and action_data[param_type].get("properties"):
-                for param, details in action_data[param_type]["properties"].items():
-                    if param not in call_args and "value" in details and details["value"]:
-                        target_dict[param] = details["value"]
-        for param, value in call_args.items():
-            for param_type, target_dict in param_types.items():
-                if param_type in action_data and param in action_data[param_type].get("properties", {}):
-                    target_dict[param] = value
+        # Fixed values win over whatever the model sent for the same key.
+        sections = resolve_arguments(action_data, call_args)
+        query_params, headers = sections["query_params"], sections["headers"]
+        body, parameters = sections["body"], sections["parameters"]
 
         # Load tool (with caching)
         from docsgpt.connectors.service import ConnectionUnavailable
diff --git a/docsgpt/agents/tool_pins.py b/docsgpt/agents/tool_pins.py
new file mode 100644
index 00000000..f473f175
--- /dev/null
+++ b/docsgpt/agents/tool_pins.py
@@ -0,0 +1,290 @@
+"""Fixed ("pinned") tool parameters.
+
+Every parameter of a stored tool action carries ``filled_by_llm`` and
+``value``. A parameter with ``filled_by_llm`` false is hidden from the model;
+when it also has a value, that value is *pinned*: it is sent on every call and
+the model can neither see nor override it. An empty string means "no value":
+the parameter is left out of the call (optional OpenAPI parameters are stored
+that way), while ``0`` and ``false`` are real values.
+
+The run-time rules live in :func:`resolve_arguments`; the helpers below
+validate and apply pin changes coming from the API.
+"""
+
+from __future__ import annotations
+
+import copy
+from typing import Any, Iterable, Mapping, Optional
+
+#: The places an action keeps parameter schemas: ``parameters`` for ordinary
+#: tools and MCP, the other three for ``api_tool`` requests.
+PARAM_SECTIONS = ("query_params", "headers", "body", "parameters")
+
+_NUMERIC = {"integer": int, "number": float}
+
+
+def _properties(action: Mapping, section: str) -> dict:
+    block = action.get(section)
+    if not isinstance(block, dict):
+        return {}
+    properties = block.get("properties")
+    return properties if isinstance(properties, dict) else {}
+
+
+def iter_parameters(action: Mapping) -> Iterable[tuple[str, str, dict]]:
+    """``(section, name, details)`` for every parameter schema of ``action``."""
+    for section in PARAM_SECTIONS:
+        for name, details in _properties(action, section).items():
+            if isinstance(details, dict):
+                yield section, name, details
+
+
+def llm_fills(details: Mapping) -> bool:
+    """Whether the model fills this parameter (the default when unset)."""
+    return bool(details.get("filled_by_llm", True))
+
+
+def has_value(details: Mapping) -> bool:
+    """Whether a parameter carries a value: anything but a missing, None or empty string."""
+    value = details.get("value")
+    return value is not None and value != ""
+
+
+def is_pinned(details: Mapping) -> bool:
+    """Whether the parameter is hidden from the model and always sent with its value."""
+    return not llm_fills(details) and has_value(details)
+
+
+def pinned_names(action: Mapping) -> list[str]:
+    """Names of the parameters of ``action`` that carry a pinned value."""
+    return [name for _section, name, details in iter_parameters(action) if is_pinned(details)]
+
+
+def resolve_arguments(
+    action: Mapping,
+    llm_arguments: Mapping[str, Any],
+    connection_pins: Optional[Mapping[str, Any]] = None,
+) -> dict[str, dict]:
+    """The arguments a call runs with, per parameter section.
+
+    Only parameters the action declares are passed. For each one:
+
+    * a value the connection fixes (``connection_pins``, e.g. Telegram's
+      default chat) always wins;
+    * a parameter hidden from the model takes its stored value, or is left
+      out when it has none; whatever the model sent for it is ignored, so a
+      model that names the key anyway (a mistake, or a prompt injection)
+      changes nothing;
+    * any other parameter takes the model's value, falling back to its
+      stored default.
+
+    Args:
+        action: The stored action (``parameters`` / ``query_params`` /
+            ``headers`` / ``body`` schemas with ``filled_by_llm`` and ``value``).
+        llm_arguments: The arguments the model sent.
+        connection_pins: Parameter name to a value the connection fixes.
+
+    Returns:
+        Section name to ``{parameter: value}`` for every section in
+        :data:`PARAM_SECTIONS` (empty dicts included).
+    """
+    connection_pins = connection_pins or {}
+    resolved: dict[str, dict] = {section: {} for section in PARAM_SECTIONS}
+    for section, name, details in iter_parameters(action):
+        target = resolved[section]
+        if name in connection_pins:
+            target[name] = connection_pins[name]
+        elif not llm_fills(details):
+            if has_value(details):
+                target[name] = details["value"]
+        elif name in llm_arguments:
+            target[name] = llm_arguments[name]
+        elif has_value(details):
+            target[name] = details["value"]
+    return resolved
+
+
+def coerce_value(details: Mapping, value: Any) -> Any:
+    """Check a value for a pin against the parameter's declared type.
+
+    Strings typed into a form are converted for ``integer``, ``number`` and
+    ``boolean`` parameters.
+
+    Raises:
+        ValueError: The value does not fit the type, or is empty.
+    """
+    if value is None or value == "":
+        raise ValueError("A fixed value cannot be empty")
+    kind = details.get("type")
+    if isinstance(kind, list):
+        kind = next((k for k in kind if k != "null"), None)
+    if kind in _NUMERIC:
+        if isinstance(value, bool):
+            raise ValueError(f"Expected a {kind}")
+        try:
+            number = _NUMERIC[kind](value)
+        except (TypeError, ValueError):
+            raise ValueError(f"Expected a {kind}") from None
+        if kind == "integer" and isinstance(value, float) and not value.is_integer():
+            raise ValueError("Expected an integer")
+        return number
+    if kind == "boolean":
+        if isinstance(value, bool):
+            return value
+        if isinstance(value, str) and value.strip().lower() in ("true", "false"):
+            return value.strip().lower() == "true"
+        raise ValueError("Expected true or false")
+    if kind == "array":
+        if not isinstance(value, list):
+            raise ValueError("Expected a list")
+        return value
+    if kind == "object":
+        if not isinstance(value, dict):
+            raise ValueError("Expected an object")
+        return value
+    if kind in (None, "string"):
+        if not isinstance(value, (str, int, float, bool)):
+            raise ValueError("Expected text")
+        return str(value) if kind == "string" else value
+    return value
+
+
+def set_pins(action: Mapping, pins: Mapping[str, Any]) -> dict:
+    """Return ``action`` with ``pins`` applied.
+
+    Args:
+        action: A stored action.
+        pins: Parameter name to the value to always use, or None to let the
+            model decide again.
+
+    Raises:
+        ValueError: A parameter does not exist on the action, or a value does
+            not fit its type.
+    """
+    updated = copy.deepcopy(dict(action))
+    known = {name: details for _section, name, details in iter_parameters(updated)}
+    for name, value in pins.items():
+        details = known.get(name)
+        if details is None:
+            raise ValueError(f"Unknown parameter: {name}")
+        if value is None:
+            details["filled_by_llm"] = True
+            details["value"] = ""
+        else:
+            details["value"] = coerce_value(details, value)
+            details["filled_by_llm"] = False
+    return updated
+
+
+class PinChangeRefused(PermissionError):
+    """Someone other than the tool's owner tried to change a fixed value."""
+
+
+_ACTION_KEYS = ("active", "require_approval", "description")
+_PARAM_KEYS = ("filled_by_llm", "value", "description")
+
+
+def _pin_state(details: Mapping) -> tuple:
+    return (llm_fills(details), details.get("value") if has_value(details) else None)
+
+
+def merge_submitted_actions(stored: list, submitted: Any, *, may_change_pins: bool) -> list:
+    """Validate a full list of actions sent by a client against the stored ones.
+
+    The stored actions are the schema: a client can switch actions on and off,
+    change approval and descriptions and (the owner only) fixed values, but
+    cannot add actions or parameters, or change a parameter's type.
+
+    Args:
+        stored: The tool's stored actions.
+        submitted: What the client sent (untrusted).
+        may_change_pins: False for a team editor: any change to
+            ``filled_by_llm`` or ``value`` is refused.
+
+    Returns:
+        The actions to store: every stored action, updated from the
+        submission.
+
+    Raises:
+        ValueError: The submission is malformed or names an unknown action or
+            parameter, or a fixed value does not fit its parameter.
+        PinChangeRefused: ``may_change_pins`` is False and a fixed value changed.
+    """
+    if not isinstance(submitted, list):
+        raise ValueError("actions must be a list")
+    by_name = {a.get("name"): a for a in stored if isinstance(a, dict) and a.get("name")}
+    seen: set = set()
+    updates: dict[str, dict] = {}
+    for entry in submitted:
+        if not isinstance(entry, dict) or not isinstance(entry.get("name"), str):
+            raise ValueError("Each action needs a name")
+        name = entry["name"]
+        if name not in by_name:
+            raise ValueError(f"Unknown action: {name}")
+        if name in seen:
+            raise ValueError(f"Duplicate action: {name}")
+        seen.add(name)
+        updates[name] = entry
+
+    merged = []
+    for action in stored:
+        if not isinstance(action, dict) or action.get("name") not in updates:
+            merged.append(action)
+            continue
+        entry = updates[action["name"]]
+        result = copy.deepcopy(action)
+        for key in _ACTION_KEYS:
+            if key in entry:
+                value = entry[key]
+                if key == "description" and not isinstance(value, str):
+                    raise ValueError("description must be text")
+                result[key] = bool(value) if key != "description" else value
+        for section in PARAM_SECTIONS:
+            sent = _properties(entry, section)
+            if not sent:
+                continue
+            properties = _properties(result, section)
+            for param, sent_details in sent.items():
+                details = properties.get(param)
+                if details is None or not isinstance(details, dict):
+                    raise ValueError(f"Unknown parameter {param} on {action['name']}")
+                if not isinstance(sent_details, dict):
+                    raise ValueError(f"Parameter {param} must be an object")
+                before = _pin_state(details)
+                for key in _PARAM_KEYS:
+                    if key in sent_details:
+                        details[key] = sent_details[key]
+                details["filled_by_llm"] = llm_fills(details)
+                if _pin_state(details) == before:
+                    continue
+                if not may_change_pins:
+                    raise PinChangeRefused("Only the tool's owner can change fixed values")
+                if is_pinned(details):
+                    details["value"] = coerce_value(details, details["value"])
+        merged.append(result)
+    return merged
+
+
+def carry_pins_between(previous: Optional[list], fresh: list) -> list:
+    """:func:`carry_pins` for whole action lists, matching actions by name."""
+    old = {a.get("name"): a for a in previous or [] if isinstance(a, dict)}
+    return [carry_pins(old.get(a.get("name")), a) if isinstance(a, dict) else a for a in fresh]
+
+
+def carry_pins(previous: Optional[Mapping], fresh: Mapping) -> dict:
+    """Copy fixed values from an action's old copy onto its re-discovered one.
+
+    Parameters that still exist keep ``filled_by_llm`` and ``value``; removed
+    ones are gone with the new schema.
+    """
+    result = copy.deepcopy(dict(fresh))
+    if not previous:
+        return result
+    old = {(section, name): details for section, name, details in iter_parameters(previous)}
+    for section, name, details in iter_parameters(result):
+        before = old.get((section, name))
+        if before is None:
+            continue
+        details["filled_by_llm"] = llm_fills(before)
+        details["value"] = before.get("value", "")
+    return result
diff --git a/tests/agents/test_tool_pins.py b/tests/agents/test_tool_pins.py
new file mode 100644
index 00000000..383262da
--- /dev/null
+++ b/tests/agents/test_tool_pins.py
@@ -0,0 +1,240 @@
+"""Fixed ("pinned") tool parameters: the model never sees or overrides them."""
+
+from unittest.mock import Mock
+
+import pytest
+
+from docsgpt.agents.tool_executor import ToolExecutor
+
+
+def _action(**properties):
+    return {
+        "name": "telegram_send_message",
+        "description": "Send a message",
+        "active": True,
+        "parameters": {"type": "object", "properties": properties},
+    }
+
+
+def _llm(description="", **extra):
+    return {"type": "string", "description": description, "filled_by_llm": True, "value": "", **extra}
+
+
+def _pinned(value, type_="string"):
+    return {"type": type_, "description": "", "filled_by_llm": False, "value": value}
+
+
+def _run(executor, tools_dict, call_args, monkeypatch):
+    monkeypatch.setattr(
+        "docsgpt.agents.tool_executor.ToolActionParser",
+        lambda _cls, **kw: Mock(parse_args=Mock(return_value=("t1", "telegram_send_message", call_args))),
+    )
+    call = Mock()
+    call.name = "telegram_send_message"
+    call.id = "c1"
+    call.arguments = "{}"
+    gen = executor.execute(tools_dict, call, "MockLLM")
+    while True:
+        try:
+            next(gen)
+        except StopIteration as stop:
+            return stop.value
+
+
+def _tools(action):
+    return {
+        "t1": {
+            "id": "00000000-0000-0000-0000-000000000001",
+            "name": "telegram",
+            "config": {},
+            "actions": [action],
+        }
+    }
+
+
+@pytest.mark.unit
+class TestPinnedValuesAtRunTime:
+    def test_llm_cannot_override_a_pinned_value(self, mock_tool_manager, monkeypatch):
+        """A model that sends the pinned key anyway (a mistake, or a prompt
+        injection naming another chat) still sends to the pinned chat."""
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm(), chat_id=_pinned("111"))
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi", "chat_id": "666"}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi", chat_id="111")
+
+    @pytest.mark.parametrize("value", [0, False])
+    def test_falsy_pins_are_honoured(self, mock_tool_manager, monkeypatch, value):
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm(), limit=_pinned(value, "integer"))
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi", "limit": 50}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi", limit=value)
+
+    def test_hidden_parameter_without_a_value_is_omitted_even_if_the_llm_sends_it(
+        self, mock_tool_manager, monkeypatch,
+    ):
+        """An empty fixed value means "leave it out" (an OpenAPI optional
+        parameter); the model was never shown it, so its value is dropped."""
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm(), chat_id=_pinned(""))
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi", "chat_id": "666"}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi")
+
+    def test_llm_filled_parameter_keeps_its_default_when_omitted(self, mock_tool_manager, monkeypatch):
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm(), chat_id=_llm(value="42"))
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi"}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi", chat_id="42")
+
+    def test_llm_filled_parameter_takes_the_llm_value(self, mock_tool_manager, monkeypatch):
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm(), chat_id=_llm(value="42"))
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi", "chat_id": "7"}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi", chat_id="7")
+
+    def test_unknown_llm_arguments_are_dropped(self, mock_tool_manager, monkeypatch):
+        tool = mock_tool_manager.load_tool.return_value
+        action = _action(text=_llm())
+        _run(ToolExecutor(user="u"), _tools(action), {"text": "hi", "token": "x"}, monkeypatch)
+        tool.execute_action.assert_called_once_with("telegram_send_message", text="hi")
+
+    def test_api_tool_pinned_header_is_not_overridden(self, mock_tool_manager, monkeypatch):
+        executor = ToolExecutor(user="u")
+        monkeypatch.setattr(
+            "docsgpt.agents.tool_executor.ToolActionParser",
+            lambda _cls, **kw: Mock(parse_args=Mock(return_value=("t1", "get_item", {"id": "1", "X-Tenant": "b"}))),
+        )
+        tools_dict = {
+            "t1": {
+                "id": "00000000-0000-0000-0000-000000000001",
+                "name": "api_tool",
+                "config": {
+                    "actions": {
+                        "get_item": {
+                            "name": "get_item",
+                            "url": "https://api.example.com/items",
+                            "method": "GET",
+                            "active": True,
+                            "headers": {"properties": {"X-Tenant": _pinned("a")}},
+                            "query_params": {"properties": {"id": _llm()}},
+                            "body": {"properties": {}},
+                        }
+                    }
+                },
+            }
+        }
+        call = Mock()
+        call.name = "get_item"
+        call.id = "c1"
+        gen = executor.execute(tools_dict, call, "MockLLM")
+        while True:
+            try:
+                next(gen)
+            except StopIteration:
+                break
+        _, kwargs = mock_tool_manager.load_tool.call_args
+        assert kwargs["tool_config"]["headers"] == {"X-Tenant": "a"}
+        assert kwargs["tool_config"]["query_params"] == {"id": "1"}
+
+
+@pytest.mark.unit
+class TestPinnedValuesInTheSchema:
+    def test_pinned_parameter_is_not_shown_to_the_llm(self):
+        executor = ToolExecutor(user="u")
+        action = _action(text=_llm(), chat_id=_pinned("111"), limit=_pinned(0, "integer"))
+        functions = executor.prepare_tools_for_llm(_tools(action))
+        params = functions[0]["function"]["parameters"]
+        assert set(params["properties"]) == {"text"}
+
+    def test_pinned_parameter_is_dropped_from_the_required_list(self):
+        executor = ToolExecutor(user="u")
+        action = _action(text=_llm(required=True), chat_id={**_pinned("111"), "required": True})
+        params = executor.prepare_tools_for_llm(_tools(action))[0]["function"]["parameters"]
+        assert params["required"] == ["text"]
+
+
+@pytest.mark.unit
+class TestPinHelpers:
+    def test_set_pins_fixes_and_releases_parameters(self):
+        from docsgpt.agents.tool_pins import set_pins
+
+        action = _action(text=_llm(), chat_id=_llm())
+        pinned = set_pins(action, {"chat_id": "123"})
+        assert pinned["parameters"]["properties"]["chat_id"] == {
+            "type": "string", "description": "", "filled_by_llm": False, "value": "123",
+        }
+        released = set_pins(pinned, {"chat_id": None})
+        assert released["parameters"]["properties"]["chat_id"]["filled_by_llm"] is True
+        assert released["parameters"]["properties"]["chat_id"]["value"] == ""
+        # The input is not modified.
+        assert action["parameters"]["properties"]["chat_id"]["filled_by_llm"] is True
+
+    def test_set_pins_rejects_unknown_parameters(self):
+        from docsgpt.agents.tool_pins import set_pins
+
+        with pytest.raises(ValueError):
+            set_pins(_action(text=_llm()), {"token": "x"})
+
+    @pytest.mark.parametrize(
+        "type_, value, expected",
+        [
+            ("integer", "5", 5),
+            ("integer", 0, 0),
+            ("number", "2.5", 2.5),
+            ("boolean", "false", False),
+            ("boolean", True, True),
+            ("string", 12, "12"),
+        ],
+    )
+    def test_coerce_value(self, type_, value, expected):
+        from docsgpt.agents.tool_pins import coerce_value
+
+        assert coerce_value({"type": type_}, value) == expected
+
+    @pytest.mark.parametrize(
+        "type_, value",
+        [("integer", "five"), ("integer", 2.5), ("boolean", "maybe"), ("string", ""), ("string", None),
+         ("string", {"a": 1}), ("array", "x")],
+    )
+    def test_coerce_value_rejects(self, type_, value):
+        from docsgpt.agents.tool_pins import coerce_value
+
+        with pytest.raises(ValueError):
+            coerce_value({"type": type_}, value)
+
+    def test_merge_keeps_schema_and_ignores_type_changes(self):
+        from docsgpt.agents.tool_pins import merge_submitted_actions
+
+        stored = [_action(text=_llm(), chat_id=_llm())]
+        merged = merge_submitted_actions(stored, [{
+            "name": "telegram_send_message",
+            "parameters": {"properties": {"chat_id": {"type": "object", "filled_by_llm": False, "value": "9"}}},
+        }], may_change_pins=True)
+        chat_id = merged[0]["parameters"]["properties"]["chat_id"]
+        assert chat_id["type"] == "string"
+        assert chat_id["value"] == "9" and chat_id["filled_by_llm"] is False
+
+    def test_merge_refuses_pin_changes_without_permission(self):
+        from docsgpt.agents.tool_pins import PinChangeRefused, merge_submitted_actions
+
+        stored = [_action(text=_llm(), chat_id=_pinned("111"))]
+        with pytest.raises(PinChangeRefused):
+            merge_submitted_actions(stored, [{
+                "name": "telegram_send_message",
+                "parameters": {"properties": {"chat_id": {"filled_by_llm": True}}},
+            }], may_change_pins=False)
+        # Resending the stored values unchanged is fine.
+        merged = merge_submitted_actions(stored, [{
+            "name": "telegram_send_message",
+            "active": False,
+            "parameters": {"properties": {"chat_id": {"filled_by_llm": False, "value": "111"}}},
+        }], may_change_pins=False)
+        assert merged[0]["active"] is False
+
+    def test_carry_pins(self):
+        from docsgpt.agents.tool_pins import carry_pins
+
+        old = _action(q=_llm(), team=_pinned("ENG"))
+        fresh = _action(q={"type": "string", "filled_by_llm": True, "value": ""}, team={"type": "string"})
+        carried = carry_pins(old, fresh)
+        assert carried["parameters"]["properties"]["team"]["value"] == "ENG"
+        assert carried["parameters"]["properties"]["team"]["filled_by_llm"] is False