fix: lazy-init SessionDB on adapter instance instead of per-request

Reuse a single SessionDB across requests by caching on self._session_db
with lazy initialization. Avoids creating a new SQLite connection per
request when X-Hermes-Session-Id is used. Updated tests to set
adapter._session_db directly instead of patching the constructor.

gateway/platforms/api_server.py

@@ -2,7 +2,7 @@
 OpenAI-compatible API server platform adapter.
 
 Exposes an HTTP server with endpoints:
-- POST /v1/chat/completions        — OpenAI Chat Completions format (stateless)
+- POST /v1/chat/completions        — OpenAI Chat Completions format (stateless; opt-in session continuity via X-Hermes-Session-Id header)
 - POST /v1/responses               — OpenAI Responses API format (stateful via previous_response_id)
 - GET  /v1/responses/{response_id} — Retrieve a stored response
 - DELETE /v1/responses/{response_id} — Delete a stored response
@@ -300,6 +300,7 @@ class APIServerAdapter(BasePlatformAdapter):
         self._runner: Optional["web.AppRunner"] = None
         self._site: Optional["web.TCPSite"] = None
         self._response_store = ResponseStore()
+        self._session_db: Optional[Any] = None  # Lazy-init SessionDB for session continuity
 
     @staticmethod
     def _parse_cors_origins(value: Any) -> tuple[str, ...]:
@@ -496,7 +497,23 @@ class APIServerAdapter(BasePlatformAdapter):
                 status=400,
             )
 
+        # Allow caller to continue an existing session by passing X-Hermes-Session-Id.
+        # When provided, history is loaded from state.db instead of from the request body.
+        provided_session_id = request.headers.get("X-Hermes-Session-Id", "").strip()
+        if provided_session_id:
+            session_id = provided_session_id
+            try:
+                if self._session_db is None:
+                    from hermes_state import SessionDB
+                    self._session_db = SessionDB()
+                history = self._session_db.get_messages_as_conversation(session_id)
+            except Exception as e:
+                logger.warning("Failed to load session history for %s: %s", session_id, e)
+                history = []
+        else:
             session_id = str(uuid.uuid4())
+            # history already set from request body above
+
         completion_id = f"chatcmpl-{uuid.uuid4().hex[:29]}"
         model_name = body.get("model", "hermes-agent")
         created = int(time.time())
@@ -540,7 +557,7 @@ class APIServerAdapter(BasePlatformAdapter):
 
             return await self._write_sse_chat_completion(
                 request, completion_id, model_name, created, _stream_q,
-                agent_task, agent_ref,
+                agent_task, agent_ref, session_id=session_id,
             )
 
         # Non-streaming: run the agent (with optional Idempotency-Key)
@@ -599,11 +616,11 @@ class APIServerAdapter(BasePlatformAdapter):
             },
         }
 
-        return web.json_response(response_data)
+        return web.json_response(response_data, headers={"X-Hermes-Session-Id": session_id})
 
     async def _write_sse_chat_completion(
         self, request: "web.Request", completion_id: str, model: str,
-        created: int, stream_q, agent_task, agent_ref=None,
+        created: int, stream_q, agent_task, agent_ref=None, session_id: str = None,
     ) -> "web.StreamResponse":
         """Write real streaming SSE from agent's stream_delta_callback queue.
 
@@ -620,6 +637,8 @@ class APIServerAdapter(BasePlatformAdapter):
         cors = self._cors_headers_for_origin(origin) if origin else None
         if cors:
             sse_headers.update(cors)
+        if session_id:
+            sse_headers["X-Hermes-Session-Id"] = session_id
         response = web.StreamResponse(status=200, headers=sse_headers)
         await response.prepare(request)
 

tests/gateway/test_api_server.py

@@ -1576,3 +1576,110 @@ class TestConversationParameter:
                 assert resp.status == 200
                 # Conversation mapping should NOT be set since store=false
                 assert adapter._response_store.get_conversation("ephemeral-chat") is None
+
+
+# ---------------------------------------------------------------------------
+# X-Hermes-Session-Id header (session continuity)
+# ---------------------------------------------------------------------------
+
+
+class TestSessionIdHeader:
+    @pytest.mark.asyncio
+    async def test_new_session_response_includes_session_id_header(self, adapter):
+        """Without X-Hermes-Session-Id, a new session is created and returned in the header."""
+        mock_result = {"final_response": "Hello!", "messages": [], "api_calls": 1}
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+                resp = await cli.post(
+                    "/v1/chat/completions",
+                    json={"model": "hermes-agent", "messages": [{"role": "user", "content": "Hi"}]},
+                )
+            assert resp.status == 200
+            assert resp.headers.get("X-Hermes-Session-Id") is not None
+
+    @pytest.mark.asyncio
+    async def test_provided_session_id_is_used_and_echoed(self, adapter):
+        """When X-Hermes-Session-Id is provided, it's passed to the agent and echoed in the response."""
+        mock_result = {"final_response": "Continuing!", "messages": [], "api_calls": 1}
+        mock_db = MagicMock()
+        mock_db.get_messages_as_conversation.return_value = [
+            {"role": "user", "content": "previous message"},
+            {"role": "assistant", "content": "previous reply"},
+        ]
+        adapter._session_db = mock_db
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+
+                resp = await cli.post(
+                    "/v1/chat/completions",
+                    headers={"X-Hermes-Session-Id": "my-session-123"},
+                    json={"model": "hermes-agent", "messages": [{"role": "user", "content": "Continue"}]},
+                )
+
+            assert resp.status == 200
+            assert resp.headers.get("X-Hermes-Session-Id") == "my-session-123"
+            call_kwargs = mock_run.call_args.kwargs
+            assert call_kwargs["session_id"] == "my-session-123"
+
+    @pytest.mark.asyncio
+    async def test_provided_session_id_loads_history_from_db(self, adapter):
+        """When X-Hermes-Session-Id is provided, history comes from SessionDB not request body."""
+        mock_result = {"final_response": "OK", "messages": [], "api_calls": 1}
+        db_history = [
+            {"role": "user", "content": "stored message 1"},
+            {"role": "assistant", "content": "stored reply 1"},
+        ]
+        mock_db = MagicMock()
+        mock_db.get_messages_as_conversation.return_value = db_history
+        adapter._session_db = mock_db
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+
+                resp = await cli.post(
+                    "/v1/chat/completions",
+                    headers={"X-Hermes-Session-Id": "existing-session"},
+                    # Request body has different history — should be ignored
+                    json={
+                        "model": "hermes-agent",
+                        "messages": [
+                            {"role": "user", "content": "old msg from client"},
+                            {"role": "assistant", "content": "old reply from client"},
+                            {"role": "user", "content": "new question"},
+                        ],
+                    },
+                )
+
+            assert resp.status == 200
+            call_kwargs = mock_run.call_args.kwargs
+            # History must come from DB, not from the request body
+            assert call_kwargs["conversation_history"] == db_history
+            assert call_kwargs["user_message"] == "new question"
+
+    @pytest.mark.asyncio
+    async def test_db_failure_falls_back_to_empty_history(self, adapter):
+        """If SessionDB raises, history falls back to empty and request still succeeds."""
+        mock_result = {"final_response": "OK", "messages": [], "api_calls": 1}
+        # Simulate DB failure: _session_db is None and SessionDB() constructor raises
+        adapter._session_db = None
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run, \
+                 patch("hermes_state.SessionDB", side_effect=Exception("DB unavailable")):
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+
+                resp = await cli.post(
+                    "/v1/chat/completions",
+                    headers={"X-Hermes-Session-Id": "some-session"},
+                    json={"model": "hermes-agent", "messages": [{"role": "user", "content": "Hi"}]},
+                )
+
+            assert resp.status == 200
+            call_kwargs = mock_run.call_args.kwargs
+            assert call_kwargs["conversation_history"] == []
+            assert call_kwargs["session_id"] == "some-session"