diff --git a/channel/web/api/openai_compat.py b/channel/web/api/openai_compat.py
index 9c88e95d..cc4339ec 100644
--- a/channel/web/api/openai_compat.py
+++ b/channel/web/api/openai_compat.py
@@ -345,7 +345,8 @@ def _sse_frame(payload: dict) -> str:
 
 
 def _session_lock(session_id: str, agent_id: str | None = None) -> threading.Lock:
-    return _SESSION_LOCKS[hash((agent_id or "", session_id)) % len(_SESSION_LOCKS)]
+    owner = agent_id or _agent_registry().default_agent_id
+    return _SESSION_LOCKS[hash((owner, session_id)) % len(_SESSION_LOCKS)]
 
 
 def _request_cancel_scope(
diff --git a/tests/test_openai_default_session_lock.py b/tests/test_openai_default_session_lock.py
new file mode 100644
index 00000000..be336afd
--- /dev/null
+++ b/tests/test_openai_default_session_lock.py
@@ -0,0 +1,104 @@
+"""Equivalent default-Agent routes serialize one persistent conversation."""
+
+import json
+import threading
+from types import SimpleNamespace
+
+import pytest
+import web
+
+from agent.registry import AgentProfile, AgentRegistry
+from channel.web.api import openai_compat as api
+
+
+@pytest.fixture
+def console(tmp_path, monkeypatch):
+    session = "openai:conversation:shared"
+    stripes = len(api._SESSION_LOCKS)
+    # Avoid incidental stripe collisions, which would hide the bug or turn
+    # the independent-Agent control into an unrelated scheduling wait.
+    primary = next(f"primary-{i}" for i in range(256)
+                   if hash((f"primary-{i}", session)) % stripes != hash(("", session)) % stripes)
+    other = next(f"other-{i}" for i in range(256)
+                 if hash((f"other-{i}", session)) % stripes != hash((primary, session)) % stripes)
+    registry = AgentRegistry([
+        AgentProfile(primary, "Primary", str(tmp_path / "primary")),
+        AgentProfile(other, "Other", str(tmp_path / "other")),
+    ], default_agent_id=primary)
+    monkeypatch.setattr(api, "_agent_registry", lambda: registry)
+    monkeypatch.setattr(api, "_request_cancel_scope", lambda request, session_id=None, agent_id=None:
+                        (request, f"{agent_id or primary}:{session_id}"))
+    monkeypatch.setattr(api, "conf", lambda: {
+        "external_api_enabled": True, "external_api_token": "fixture-token", "model": "fixture-model",
+    })
+    state = SimpleNamespace(active=0, maximum=0, first_started=threading.Event(),
+                            second_started=threading.Event(), release_first=threading.Event())
+    guard = threading.Lock()
+
+    def run(query, session_id, send_chunk, **kwargs):
+        with guard:
+            state.active += 1
+            state.maximum = max(state.maximum, state.active)
+        try:
+            if query == "first":
+                state.first_started.set()
+                assert state.release_first.wait(5), "test coordinator did not release its first request"
+            else:
+                state.second_started.set()
+            send_chunk({"chunk_type": "content", "delta": query})
+        finally:
+            with guard:
+                state.active -= 1
+
+    monkeypatch.setattr(api, "_run_chat_service", run)
+    app = web.application(("/v1/chat/completions", "OpenAIChatCompletionsHandler"), vars(api), autoreload=False)
+    return app, primary, other, state
+
+
+def _overlapping_requests(console, first_stream, second_agent):
+    app, primary, other, state = console
+    responses, errors = [], []
+
+    def request(query, stream, agent_id):
+        body = {"conversation_id": "shared", "messages": [{"role": "user", "content": query}], "stream": stream}
+        if agent_id is not None:
+            body["agent_id"] = agent_id
+        try:
+            response = app.request("/v1/chat/completions", method="POST", data=json.dumps(body),
+                                   headers={"Authorization": "Bearer fixture-token"})
+            responses.append(response)
+        except BaseException as error:
+            errors.append(error)
+
+    threads = [threading.Thread(target=request, args=("first", first_stream, None), daemon=True),
+               threading.Thread(target=request, args=("second", False, second_agent), daemon=True)]
+    try:
+        threads[0].start()
+        assert state.first_started.wait(2)
+        threads[1].start()
+        concurrent = state.second_started.wait(1)
+    finally:
+        state.release_first.set()
+        for thread in threads:
+            if thread.ident is not None:
+                thread.join(timeout=3)
+    assert not any(t.is_alive() for t in threads)
+    assert not errors, errors
+    assert len(responses) == 2
+    assert all(response.status == "200 OK" for response in responses)
+    return concurrent, state.maximum
+
+
+@pytest.mark.parametrize("first_stream", [False, True])
+def test_omitted_and_explicit_default_agent_cannot_overlap_one_conversation(console, first_stream):
+    _, primary, _, _ = console
+    concurrent, maximum = _overlapping_requests(console, first_stream, primary)
+    assert not concurrent, "two default-Agent routes entered the same conversation at once"
+    assert maximum == 1
+
+
+def test_different_agents_keep_independent_conversation_execution(console):
+    _, _, other, _ = console
+    concurrent, maximum = _overlapping_requests(console, False, other)
+    assert concurrent
+    assert maximum == 2