src/mcplocal/src/http/project-mcp-endpoint.ts

@@ -30,6 +30,8 @@ import { AuditCollector } from '../audit/collector.js';
 import { WireNameCodec, routeWithWireNames } from '../util/wire-names.js';
 import { newTraceCode } from '../util/trace-code.js';
 import { runInRequestScope } from '../request-context.js';
+import { adoptSession } from './session-adopt.js';
+import { degradationNotice } from '../util/degrade.js';
 
 interface ProjectCacheEntry {
   router: McpRouter;
@@ -48,6 +50,12 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
   let resolvedUserName: string | null | undefined; // undefined = not yet resolved
   const projectCache = new Map<string, ProjectCacheEntry>();
   const sessions = new Map<string, SessionEntry>();
+  /**
+   * Sessions recreated after a restart, awaiting their one-shot ⚠ notice.
+   * Delivered on the next tools/call result — the only response a model
+   * reliably reads as prose.
+   */
+  const pendingRecreationNotice = new Set<string>();
   // Wire-name codecs are keyed per project and OUTLIVE the router cache TTL:
   // a client may call a tool it listed minutes ago through a refreshed router,
   // and the mapping must still resolve.
@@ -217,6 +225,27 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
   }
 
   // POST /projects/:projectName/mcp — JSON-RPC requests
+  /**
+   * Prepend the recreation notice to the first tool result after a session was
+   * rebuilt. Silent recreation would hand an agent an ungated tool list with no
+   * signal that its gate state had vanished.
+   */
+  function maybeAnnotateRecreation(response: unknown, sid: string, method?: string): unknown {
+    if (method !== 'tools/call' || !pendingRecreationNotice.has(sid)) return response;
+    pendingRecreationNotice.delete(sid);
+
+    const r = response as { result?: { content?: Array<{ type?: string; text?: string }> } };
+    const first = r.result?.content?.[0];
+    if (!first || first.type !== 'text' || typeof first.text !== 'string') return response;
+
+    first.text = degradationNotice(
+      'Session state',
+      `mcplocal restarted; session ${sid.slice(0, 8)} was recreated`,
+      'Gate state and cached results were lost — if an expected tool is missing, call begin_session again.',
+    ) + first.text;
+    return response;
+  }
+
   app.post<{ Params: { projectName: string } }>('/projects/:projectName/mcp', async (request, reply) => {
     const { projectName } = request.params;
     const sessionId = request.headers['mcp-session-id'] as string | undefined;
@@ -229,10 +258,15 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
       return;
     }
 
-    if (sessionId && !sessions.has(sessionId)) {
+    // An unknown session id means mcplocal restarted (or the session was
-      reply.code(404).send({ error: 'Session not found' });
+    // evicted) while the client still holds a perfectly good id. Replying 404
-      return;
+    // is a dead end: it lands OUTSIDE the JSON-RPC envelope with no id, so the
-    }
+    // client cannot correlate it and stalls until its own timeout — 3.5ms
+    // server-side became 1800s client-side. Recreate instead, keep the id the
+    // client already has, and tell the model what it lost.
+    const recreatingSessionId = sessionId !== undefined && !sessions.has(sessionId)
+      ? sessionId
+      : undefined;
 
     // New session — get/create project router
     let router: McpRouter;
@@ -243,9 +277,11 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
       return;
     }
 
-    const transport = new StreamableHTTPServerTransport({
+    // Called by onsessioninitialized on the normal path, and explicitly on the
-      sessionIdGenerator: () => randomUUID(),
+    // recreation path — where onsessioninitialized never fires. Missing any of
-      onsessioninitialized: (id) => {
+    // this on the adopt path would give recreated sessions null userName on
+    // every audit event.
+    const registerSession = (id: string): void => {
         sessions.set(id, { transport, projectName });
         trafficCapture?.emit({
           timestamp: new Date().toISOString(),
@@ -283,7 +319,11 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
           verified: true,
           payload: { projectName },
         });
-      },
+    };
+
+    const transport = new StreamableHTTPServerTransport({
+      sessionIdGenerator: () => randomUUID(),
+      onsessioninitialized: (id) => { registerSession(id); },
     });
 
     // Per-request correlationId map for linking client ↔ upstream event pairs.
@@ -395,7 +435,9 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
         });
 
         requestCorrelations.delete(requestId);
-        await transport.send(response as unknown as JSONRPCMessage);
+        await transport.send(
+          maybeAnnotateRecreation(response, sid, method) as unknown as JSONRPCMessage,
+        );
       }
     };
 
@@ -414,6 +456,19 @@ export function registerProjectMcpEndpoint(app: FastifyInstance, mcpdClient: Mcp
       }
     };
 
+    if (recreatingSessionId !== undefined) {
+      // Adopt the client's existing id rather than minting a new one: a
+      // re-handshake would invalidate everything the client has cached about
+      // this session.
+      adoptSession(transport, recreatingSessionId);
+      registerSession(recreatingSessionId);
+      pendingRecreationNotice.add(recreatingSessionId);
+      console.error(
+        `[mcp] recreated session ${recreatingSessionId.slice(0, 8)} for project '${projectName}' `
+        + '(mcplocal restarted or the session was evicted); gate state was lost',
+      );
+    }
+
     await transport.handleRequest(request.raw, reply.raw, request.body);
     reply.hijack();
   });

src/mcplocal/src/http/server.ts

@@ -42,6 +42,24 @@ export async function createHttpServer(
     methods: ['GET', 'POST', 'PUT', 'DELETE', 'PATCH'],
   });
 
+  // MCP clients close a session with DELETE + `Content-Type: application/json`
+  // and an EMPTY body. Fastify's default JSON parser rejects that with
+  // FST_ERR_CTP_EMPTY_JSON_BODY before the route handler ever runs, so
+  // sessions.delete() never happened and every "closed" session leaked --
+  // which is a direct producer of the stale-session condition that used to
+  // 404. Treat an empty body as no body.
+  app.addContentTypeParser('application/json', { parseAs: 'string' }, (_req, body, done) => {
+    if (body === '' || body === undefined) {
+      done(null, undefined);
+      return;
+    }
+    try {
+      done(null, JSON.parse(body as string) as unknown);
+    } catch (err) {
+      done(err as Error, undefined);
+    }
+  });
+
   // Health endpoint
   app.get('/health', async (_request, reply) => {
     const upstreams = deps.router.getUpstreamNames();

src/mcplocal/src/http/session-adopt.ts

@@ -0,0 +1,47 @@
+/**
+ * Let a freshly-built transport adopt a session ID the client already holds.
+ *
+ * mcplocal keeps MCP sessions in memory only, so any restart — a deploy, an RPM
+ * install, `scripts/release.sh` — invalidates every live client's session id.
+ * The old behaviour was a bare `404 {"error":"Session not found"}` outside the
+ * JSON-RPC envelope: 3.5ms server-side, but the client cannot correlate it and
+ * stalls until its own timeout (1800s in Claude Code). Recreating the session
+ * is strictly better, and keeping the client's EXISTING id is the point — a
+ * re-handshake would change it and every assumption the client has cached.
+ *
+ * The SDK assigns `sessionId` and `_initialized` in exactly one place: while
+ * handling an `initialize` POST (webStandardStreamableHttp.js:419-420, SDK
+ * 1.26.0). A non-initialize message on a fresh transport is rejected by
+ * `validateSession` with "Bad Request: Server not initialized". Replaying those
+ * two assignments is far less fragile than synthesising an `initialize` request
+ * through hono's request listener.
+ *
+ * This reaches into SDK internals, so it fails LOUDLY rather than silently: if
+ * the shape moves in an SDK upgrade, `session-adopt.test.ts` goes red at
+ * `pnpm test` instead of in production.
+ */
+import type { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
+
+interface WebStandardInternals {
+  sessionId?: string | undefined;
+  _initialized?: boolean;
+}
+
+export function adoptSession(transport: StreamableHTTPServerTransport, sessionId: string): void {
+  const inner = (transport as unknown as { _webStandardTransport?: WebStandardInternals })
+    ._webStandardTransport;
+
+  // Guard on `_initialized` only: the SDK sets it in the constructor, whereas
+  // `sessionId` is not assigned until an initialize arrives — so on the fresh
+  // transport we are adopting onto, the property genuinely does not exist yet.
+  if (!inner || !('_initialized' in inner)) {
+    throw new Error(
+      'MCP SDK internals changed: cannot adopt an existing session id '
+      + '(_webStandardTransport._initialized / .sessionId not found). '
+      + 'See src/mcplocal/src/http/session-adopt.ts and its test.',
+    );
+  }
+
+  inner.sessionId = sessionId;
+  inner._initialized = true;
+}

src/mcplocal/tests/project-mcp-endpoint.test.ts

@@ -113,18 +113,26 @@ describe('registerProjectMcpEndpoint', () => {
     expect(res.json().error).toContain('Failed to load project');
   });
 
-  it('returns 404 for unknown session ID', async () => {
+  it('recreates an unknown session instead of 404ing it', async () => {
+    // A 404 here lands OUTSIDE the JSON-RPC envelope with no id, so a client
+    // cannot correlate it and stalls until its own timeout — a 3.5ms failure
+    // that cost 1800s in production. mcplocal restarts routinely (deploys, RPM
+    // installs, release.sh), which is exactly when clients hold stale ids.
     const res = await app.inject({
       method: 'POST',
       url: '/projects/smart-home/mcp',
       payload: { jsonrpc: '2.0', id: 1, method: 'tools/list', params: {} },
       headers: {
         'content-type': 'application/json',
+        'accept': 'application/json, text/event-stream',
         'mcp-session-id': 'nonexistent-session',
       },
     });
 
-    expect(res.statusCode).toBe(404);
+    expect(res.statusCode).not.toBe(404);
+    // The client keeps the id it already holds — a re-handshake would
+    // invalidate everything it has cached about the session.
+    expect(res.headers['mcp-session-id'] ?? 'nonexistent-session').toBe('nonexistent-session');
   });
 
   it('returns 400 for GET without session', async () => {

src/mcplocal/tests/session-adopt.test.ts

@@ -0,0 +1,40 @@
+import { describe, it, expect } from 'vitest';
+import { randomUUID } from 'node:crypto';
+import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
+import { adoptSession } from '../src/http/session-adopt.js';
+
+/**
+ * Canary for SDK upgrades. adoptSession() reaches into
+ * _webStandardTransport.{sessionId,_initialized}, which the SDK sets only while
+ * handling an `initialize`. If a future SDK moves them, this fails here rather
+ * than turning every session recovery into a 400 in production.
+ */
+describe('adoptSession', () => {
+  it('marks a fresh transport as initialized with the given session id', () => {
+    const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: () => randomUUID() });
+    expect(transport.sessionId).toBeUndefined();
+
+    const id = randomUUID();
+    adoptSession(transport, id);
+
+    expect(transport.sessionId).toBe(id);
+  });
+
+  it('throws loudly when the SDK internals are not what we expect', () => {
+    expect(() => adoptSession({} as never, 'x')).toThrow(/MCP SDK internals changed/);
+    expect(() => adoptSession({ _webStandardTransport: undefined } as never, 'x'))
+      .toThrow(/MCP SDK internals changed/);
+  });
+
+  it('satisfies the SDK session check that rejects uninitialized transports', () => {
+    const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: () => randomUUID() });
+    const id = randomUUID();
+    adoptSession(transport, id);
+
+    // validateSession() rejects on !_initialized before it ever compares ids,
+    // so this flag is the half that actually unblocks a non-initialize message.
+    const inner = (transport as unknown as { _webStandardTransport: { _initialized: boolean } })
+      ._webStandardTransport;
+    expect(inner._initialized).toBe(true);
+  });
+});