diff --git a/internal/eventbus/bus_impl.go b/internal/eventbus/bus_impl.go
index 64ae246f..bbbb53a9 100644
--- a/internal/eventbus/bus_impl.go
+++ b/internal/eventbus/bus_impl.go
@@ -63,6 +63,9 @@ func (b *busImpl) Publish(event DomainEvent) {
 	if b.draining.Load() {
 		return
 	}
+	// Publish-time observer: warns on non-UUID AgentID drift without blocking.
+	// See validate_agent_id.go for rationale (PR #826 regression safety net).
+	validateAgentID(event)
 	select {
 	case b.queue <- event:
 	default:
diff --git a/internal/eventbus/validate_agent_id.go b/internal/eventbus/validate_agent_id.go
new file mode 100644
index 00000000..2ed21f08
--- /dev/null
+++ b/internal/eventbus/validate_agent_id.go
@@ -0,0 +1,33 @@
+package eventbus
+
+import (
+	"log/slog"
+
+	"github.com/google/uuid"
+)
+
+// validateAgentID is a publish-time observer that logs a warning when a DomainEvent
+// carries a non-UUID AgentID. It does NOT block the publish — observability only.
+//
+// Motivation: PR #826 fixed multiple call sites that passed agent_key strings
+// (e.g. "goctech-leader") into DomainEvent.AgentID, silently corrupting downstream
+// consumers. This helper acts as a safety net to catch any future drift BEFORE
+// the event reaches a consumer that parses the field as a UUID.
+//
+// Log field name: `non_uuid_agent_id` (intentionally distinct from the standard
+// `agent_id` field used elsewhere) to avoid collision with observability tooling
+// that parses `agent_id` as a UUID. See red-team finding H6.
+//
+// See docs/agent-identity-conventions.md (Phase 6) for the convention.
+func validateAgentID(event DomainEvent) {
+	if event.AgentID == "" {
+		return // legitimate team-owned, tenant-scoped, or anonymous event
+	}
+	if _, err := uuid.Parse(event.AgentID); err != nil {
+		slog.Warn("eventbus.non_uuid_agent_id",
+			"event_type", event.Type,
+			"non_uuid_agent_id", event.AgentID,
+			"source_id", event.SourceID,
+		)
+	}
+}
diff --git a/internal/eventbus/validate_agent_id_test.go b/internal/eventbus/validate_agent_id_test.go
new file mode 100644
index 00000000..732d9477
--- /dev/null
+++ b/internal/eventbus/validate_agent_id_test.go
@@ -0,0 +1,132 @@
+package eventbus
+
+import (
+	"bytes"
+	"context"
+	"log/slog"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// captureSlog swaps the default slog logger for one that writes to a buffer
+// and returns a restore function plus a pointer to the buffer.
+func captureSlog(t *testing.T) (*bytes.Buffer, func()) {
+	t.Helper()
+	var buf bytes.Buffer
+	handler := slog.NewTextHandler(&buf, &slog.HandlerOptions{Level: slog.LevelDebug})
+	prev := slog.Default()
+	slog.SetDefault(slog.New(handler))
+	return &buf, func() { slog.SetDefault(prev) }
+}
+
+func TestValidateAgentID_EmptyAgentID_NoWarn(t *testing.T) {
+	buf, restore := captureSlog(t)
+	defer restore()
+
+	validateAgentID(DomainEvent{
+		Type:     EventRunCompleted,
+		SourceID: "r1",
+		AgentID:  "",
+	})
+
+	if strings.Contains(buf.String(), "non_uuid_agent_id") {
+		t.Errorf("empty AgentID should not trigger warning, got log:\n%s", buf.String())
+	}
+}
+
+func TestValidateAgentID_ValidUUID_NoWarn(t *testing.T) {
+	buf, restore := captureSlog(t)
+	defer restore()
+
+	validateAgentID(DomainEvent{
+		Type:     EventEpisodicCreated,
+		SourceID: "s1",
+		AgentID:  uuid.New().String(),
+	})
+
+	if strings.Contains(buf.String(), "non_uuid_agent_id") {
+		t.Errorf("valid UUID should not trigger warning, got log:\n%s", buf.String())
+	}
+}
+
+func TestValidateAgentID_NonUUIDWarns(t *testing.T) {
+	buf, restore := captureSlog(t)
+	defer restore()
+
+	validateAgentID(DomainEvent{
+		Type:     EventEpisodicCreated,
+		SourceID: "s1",
+		AgentID:  "goctech-leader", // agent_key, not UUID
+	})
+
+	out := buf.String()
+	if !strings.Contains(out, "non_uuid_agent_id") {
+		t.Errorf("non-UUID AgentID should emit warning with distinct field, got log:\n%s", out)
+	}
+	if !strings.Contains(out, "goctech-leader") {
+		t.Errorf("warning should include the offending value, got log:\n%s", out)
+	}
+	if !strings.Contains(out, "eventbus.non_uuid_agent_id") {
+		t.Errorf("warning message should be eventbus.non_uuid_agent_id, got log:\n%s", out)
+	}
+}
+
+func TestValidateAgentID_DistinctFieldName_NoCollision(t *testing.T) {
+	// H6 mitigation: the field name must NOT be `agent_id` (which downstream
+	// observability tooling parses as UUID) — it must be `non_uuid_agent_id`.
+	buf, restore := captureSlog(t)
+	defer restore()
+
+	validateAgentID(DomainEvent{
+		Type:     EventRunCompleted,
+		SourceID: "r1",
+		AgentID:  "bad-key",
+	})
+
+	out := buf.String()
+	if !strings.Contains(out, "non_uuid_agent_id=") {
+		t.Errorf("expected field name `non_uuid_agent_id=`, got log:\n%s", out)
+	}
+}
+
+// TestBusPublish_InvokesValidator verifies the validator is wired into Publish,
+// so every publish site is covered without per-caller instrumentation.
+func TestBusPublish_InvokesValidator(t *testing.T) {
+	buf, restore := captureSlog(t)
+	defer restore()
+
+	bus := newTestBus()
+	ctx := t.Context()
+	bus.Start(ctx)
+	defer func() { _ = bus.Drain(500 * time.Millisecond) }()
+
+	var received atomic.Int32
+	bus.Subscribe(EventRunCompleted, func(_ context.Context, _ DomainEvent) error {
+		received.Add(1)
+		return nil
+	})
+
+	// Publish with non-UUID AgentID — should still dispatch (observability only, not blocking)
+	bus.Publish(DomainEvent{
+		Type:     EventRunCompleted,
+		SourceID: "validator-test",
+		AgentID:  "invalid-agent-key",
+	})
+
+	// Wait up to 200ms for the worker to dispatch.
+	deadline := time.Now().Add(200 * time.Millisecond)
+	for received.Load() == 0 && time.Now().Before(deadline) {
+		time.Sleep(2 * time.Millisecond)
+	}
+
+	if received.Load() != 1 {
+		t.Errorf("expected event to still be dispatched (non-blocking), got received=%d", received.Load())
+	}
+	if !strings.Contains(buf.String(), "non_uuid_agent_id") {
+		t.Errorf("expected bus.Publish to invoke validateAgentID, got log:\n%s", buf.String())
+	}
+}