diff --git a/backend/app/gateway/services.py b/backend/app/gateway/services.py
index 2c5c01e61..e9b5e212a 100644
--- a/backend/app/gateway/services.py
+++ b/backend/app/gateway/services.py
@@ -315,6 +315,21 @@ async def start_run(
                 detail=f"Model {model_name!r} is not in the configured model allowlist",
             )
 
+    # Stateless run endpoints carry thread_id in the request *body*, so the
+    # @require_permission(owner_check=True) decorator -- which resolves ownership
+    # from the path param -- cannot protect them. Enforce thread ownership here,
+    # before any run is created, so one user cannot start runs on (or read /wait
+    # checkpoint state from) another user's thread. Missing rows (auto-created
+    # temp threads) and NULL-owner rows (shared / pre-auth data) stay accessible
+    # via check_access; only a thread already owned by another user is rejected
+    # with 404, matching thread_runs.py's anti-enumeration behaviour. Internal
+    # channel runs act on behalf of IM users they do not own (see
+    # inject_authenticated_user_context), so the internal system role is exempt.
+    user = getattr(request.state, "user", None)
+    if user is not None and getattr(user, "system_role", None) != INTERNAL_SYSTEM_ROLE:
+        if not await run_ctx.thread_store.check_access(thread_id, str(user.id)):
+            raise HTTPException(status_code=404, detail=f"Thread {thread_id} not found")
+
     try:
         record = await run_mgr.create_or_reject(
             thread_id,
diff --git a/backend/tests/test_stateless_runs_owner_isolation.py b/backend/tests/test_stateless_runs_owner_isolation.py
new file mode 100644
index 000000000..60a20d17c
--- /dev/null
+++ b/backend/tests/test_stateless_runs_owner_isolation.py
@@ -0,0 +1,173 @@
+"""Cross-user isolation for the stateless ``POST /api/runs/stream`` and ``/wait`` endpoints.
+
+These endpoints receive ``thread_id`` in the request body, so the
+``@require_permission(owner_check=True)`` decorator — which reads the
+``thread_id`` *path* parameter — cannot protect them. The owner check
+lives inside ``services.start_run()`` instead; this suite pins it at the
+HTTP layer so the gap cannot silently reopen.
+
+Strategy
+--------
+``app.state.run_manager.create_or_reject`` raises ``ConflictError``, so a
+request that *passes* the owner check deterministically short-circuits
+with 409 before any agent code runs. The two outcomes:
+
+- 404 + ``create_or_reject`` never awaited -> blocked by the owner check
+- 409 + ``create_or_reject`` awaited       -> passed the owner check
+
+The thread store is a real ``MemoryThreadMetaStore`` (not a mock) so the
+``check_access`` semantics under test — missing row allows, ``user_id``
+NULL allows, foreign owner denies — are exercised through real code.
+"""
+
+from __future__ import annotations
+
+import asyncio
+from contextlib import contextmanager
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
+
+import pytest
+from _router_auth_helpers import make_authed_test_app
+from fastapi.testclient import TestClient
+from langgraph.store.memory import InMemoryStore
+
+from app.gateway.auth.models import User
+from app.gateway.routers import runs
+from deerflow.config.app_config import AppConfig, reset_app_config, set_app_config
+from deerflow.persistence.thread_meta.memory import MemoryThreadMetaStore
+from deerflow.runtime import ConflictError
+
+USER_A = User(email="owner-a@example.com", password_hash="x", system_role="user", id=uuid4())
+USER_B = User(email="intruder-b@example.com", password_hash="x", system_role="user", id=uuid4())
+INTERNAL_USER = SimpleNamespace(id="default", system_role="internal")
+
+THREAD_A = "thread-owned-by-a"
+THREAD_SHARED = "thread-shared-null-owner"
+
+
+@pytest.fixture(autouse=True)
+def _stub_app_config():
+    """Inject a minimal AppConfig so the allowed path (which builds a
+    RunContext via ``get_config()``) never reads config.yaml from disk."""
+    set_app_config(AppConfig.model_validate({"sandbox": {"use": "deerflow.sandbox.local:LocalSandboxProvider"}}))
+    yield
+    reset_app_config()
+
+
+def _make_thread_store() -> MemoryThreadMetaStore:
+    store = MemoryThreadMetaStore(InMemoryStore())
+
+    async def _seed():
+        await store.create(THREAD_A, user_id=str(USER_A.id))
+        await store.create(THREAD_SHARED, user_id=None)
+
+    asyncio.run(_seed())
+    return store
+
+
+@contextmanager
+def _client(user):
+    """Yield a ``TestClient`` authenticated as ``user`` plus the stubbed
+    ``create_or_reject`` mock, closing the client (and its anyio portal /
+    background threads) on exit.
+
+    ``create_or_reject`` raises ``ConflictError`` so a request that passes the
+    owner check short-circuits to 409 before any agent code runs.
+    """
+    app = make_authed_test_app(user_factory=lambda: user)
+    app.include_router(runs.router)
+    app.state.thread_store = _make_thread_store()
+    app.state.stream_bridge = MagicMock()
+    app.state.checkpointer = MagicMock()
+    app.state.store = MagicMock()
+    app.state.run_events_config = None
+    app.state.run_event_store = MagicMock()
+    run_manager = MagicMock()
+    run_manager.create_or_reject = AsyncMock(side_effect=ConflictError("sentinel: owner check passed"))
+    app.state.run_manager = run_manager
+    with TestClient(app) as client:
+        yield client, run_manager.create_or_reject
+
+
+def _body(thread_id: str | None = None) -> dict:
+    if thread_id is None:
+        return {}
+    return {"config": {"configurable": {"thread_id": thread_id}}}
+
+
+# ---------------------------------------------------------------------------
+# Denied: another user's thread
+# ---------------------------------------------------------------------------
+
+
+def test_stream_cross_user_returns_404():
+    """User B cannot start a run on user A's thread via /api/runs/stream."""
+    with _client(USER_B) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body(THREAD_A))
+    assert response.status_code == 404
+    assert response.json()["detail"] == f"Thread {THREAD_A} not found"
+    create_or_reject.assert_not_awaited()
+
+
+def test_wait_cross_user_returns_404_without_channel_values():
+    """User B cannot read user A's checkpoint state via /api/runs/wait."""
+    with _client(USER_B) as (client, create_or_reject):
+        response = client.post("/api/runs/wait", json=_body(THREAD_A))
+    assert response.status_code == 404
+    assert response.json() == {"detail": f"Thread {THREAD_A} not found"}
+    create_or_reject.assert_not_awaited()
+
+
+# ---------------------------------------------------------------------------
+# Allowed: owner, fresh/untracked/shared threads, internal role
+# ---------------------------------------------------------------------------
+
+
+def test_stream_owner_passes_owner_check():
+    """User A reaches run creation on their own thread (409 sentinel)."""
+    with _client(USER_A) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body(THREAD_A))
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()
+
+
+def test_wait_owner_passes_owner_check():
+    with _client(USER_A) as (client, create_or_reject):
+        response = client.post("/api/runs/wait", json=_body(THREAD_A))
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()
+
+
+def test_stream_without_thread_id_passes_owner_check():
+    """Stateless run with no thread_id auto-creates a thread — never blocked."""
+    with _client(USER_B) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body())
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()
+
+
+def test_stream_untracked_thread_passes_owner_check():
+    """A thread_id with no thread_meta row (untracked legacy) stays accessible."""
+    with _client(USER_B) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body("never-created-thread"))
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()
+
+
+def test_stream_shared_thread_passes_owner_check():
+    """A thread_meta row with user_id NULL (shared / pre-auth data) stays accessible."""
+    with _client(USER_B) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body(THREAD_SHARED))
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()
+
+
+def test_stream_internal_role_bypasses_owner_check():
+    """IM channels run with the internal system role on behalf of platform
+    users whose threads they do not own — the owner check must not break them."""
+    with _client(INTERNAL_USER) as (client, create_or_reject):
+        response = client.post("/api/runs/stream", json=_body(THREAD_A))
+    assert response.status_code == 409
+    create_or_reject.assert_awaited()