diff --git a/gateway/platforms/api_server.py b/gateway/platforms/api_server.py
index 3c89ada157..796cc53a4f 100644
--- a/gateway/platforms/api_server.py
+++ b/gateway/platforms/api_server.py
@@ -3530,6 +3530,12 @@ class APIServerAdapter(OpenAICompatRoutesMixin, BasePlatformAdapter):
         headers = {
             "Content-Type": "text/event-stream", "Cache-Control": "no-cache",
             "X-Accel-Buffering": "no", **self._session_headers(session_id, gateway_session_key)}
+        # CORS middleware can't inject headers into StreamResponse after
+        # prepare() flushes them, so resolve CORS headers up front (#72892).
+        origin = request.headers.get("Origin", "")
+        cors = self._cors_headers_for_origin(origin) if origin else None
+        if cors:
+            headers.update(cors)
         response = web.StreamResponse(status=200, headers=headers)
         await response.prepare(request)
         try:
diff --git a/tests/gateway/test_api_server.py b/tests/gateway/test_api_server.py
index 9b190e89b5..ec007ce696 100644
--- a/tests/gateway/test_api_server.py
+++ b/tests/gateway/test_api_server.py
@@ -2263,6 +2263,61 @@ class TestCORS:
             assert "Authorization" in resp.headers.get("Access-Control-Allow-Headers", "")
 
 
+    @pytest.mark.asyncio
+    async def test_cors_headers_present_on_session_chat_stream(self):
+        """SSE streaming endpoint must include CORS headers on the response.
+
+        The CORS middleware cannot inject headers into a ``StreamResponse``
+        after ``prepare()`` flushes them, so the handler must resolve CORS
+        headers up front — same pattern as ``/v1/chat/completions`` and
+        ``/v1/responses`` streaming paths (#72892).
+        """
+        adapter = _make_adapter(cors_origins=["http://localhost:3000"])
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with (
+                patch.object(adapter, "_get_existing_session_or_404", return_value=({"id": "s1"}, None)),
+                patch.object(adapter, "_conversation_history_for_session", return_value=[]),
+                patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run,
+            ):
+                mock_run.return_value = (
+                    {"final_response": "ok", "messages": [], "api_calls": 1},
+                    {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2},
+                )
+                resp = await cli.post(
+                    "/api/sessions/s1/chat/stream",
+                    json={"message": "hi"},
+                    headers={"Origin": "http://localhost:3000"},
+                )
+                assert resp.status == 200
+                await resp.text()  # consume SSE stream fully
+        assert resp.headers.get("Access-Control-Allow-Origin") == "http://localhost:3000"
+        assert "POST" in resp.headers.get("Access-Control-Allow-Methods", "")
+
+    @pytest.mark.asyncio
+    async def test_cors_headers_absent_on_session_chat_stream_no_origin(self):
+        """No CORS headers on the streaming endpoint when no Origin is sent."""
+        adapter = _make_adapter()
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with (
+                patch.object(adapter, "_get_existing_session_or_404", return_value=({"id": "s1"}, None)),
+                patch.object(adapter, "_conversation_history_for_session", return_value=[]),
+                patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run,
+            ):
+                mock_run.return_value = (
+                    {"final_response": "ok", "messages": [], "api_calls": 1},
+                    {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2},
+                )
+                resp = await cli.post(
+                    "/api/sessions/s1/chat/stream",
+                    json={"message": "hi"},
+                )
+                assert resp.status == 200
+                await resp.text()
+        assert resp.headers.get("Access-Control-Allow-Origin") is None
+
+
 # ---------------------------------------------------------------------------
 # Conversation parameter
 # ---------------------------------------------------------------------------