diff --git a/services/trinity-core/index.js b/services/trinity-core/index.js
index da13767..3c9e3ca 100644
--- a/services/trinity-core/index.js
+++ b/services/trinity-core/index.js
@@ -233,6 +233,14 @@ app.all('/mcp', auth, async (req, res) => {
       };
       const mcpServer = createMcpServer();
       await mcpServer.connect(transport);
+    } else if (sessionId && !activeSessions.has(sessionId)) {
+      // Stale session (e.g. after service restart) — tell client to re-initialize
+      log(`StreamableHTTP stale session ${sessionId} — returning 404`);
+      return res.status(404).json({
+        jsonrpc: '2.0',
+        error: { code: -32001, message: 'Session not found. Please re-initialize.' },
+        id: null
+      });
     } else if (!sessionId && req.method === 'GET') {
       // Legacy SSE client connecting via GET /mcp
       return legacySSE(req, res);