fix: cancel a local MCP tool call when its client disconnects

The /mcp route built its Request without a signal, so a tool kept working
after the client left. The request's signal now aborts when the connection
closes before the answer is sent.

server/mcp/plugin.js

@@ -64,6 +64,12 @@ export function localMcpPlugin({ createServer = createLocalMcpServer } = {}) {
         );
         return;
       }
+      // A client that disconnects before its answer cancels the tool call.
+      const disconnect = new AbortController();
+      const onClose = () => {
+        if (!res.writableFinished) disconnect.abort();
+      };
+      res.on('close', onClose);
       try {
         const body = req.method === 'POST' ? await readBody(req) : undefined;
         const request = new Request(`http://${host}/mcp`, {
@@ -72,11 +78,13 @@ export function localMcpPlugin({ createServer = createLocalMcpServer } = {}) {
             value === undefined ? [] : [[name, String(value)]],
           ),
           body,
+          signal: disconnect.signal,
         });
         const response = await handlerFor(`http://${host}`)(request);
         res.writeHead(response.status, Object.fromEntries(response.headers));
         res.end(Buffer.from(await response.arrayBuffer()));
       } catch (error) {
+        if (disconnect.signal.aborted) return;
         if (res.headersSent) return res.destroy();
         res.writeHead(error?.status === 413 ? 413 : 400, {
           'Content-Type': 'application/json',
@@ -87,6 +95,8 @@ export function localMcpPlugin({ createServer = createLocalMcpServer } = {}) {
             error: error?.status === 413 ? 'Request too large' : 'Bad request',
           }),
         );
+      } finally {
+        res.off('close', onClose);
       }
     });
   };

src/tools/localRoute.test.mjs

@@ -121,3 +121,46 @@ test('the /mcp route answers local MCP requests and refuses others', async (t) =
     405,
   );
 });
+
+test('a client that disconnects cancels its tool call', async (t) => {
+  let middleware;
+  let seen;
+  const started = Promise.withResolvers();
+  const aborted = Promise.withResolvers();
+  const plugin = localMcpPlugin({
+    createServer: () => ({
+      handle: (message, { signal }) => {
+        seen = signal;
+        signal.addEventListener('abort', () => aborted.resolve(), {
+          once: true,
+        });
+        started.resolve();
+        return new Promise(() => {});
+      },
+    }),
+  });
+  plugin.configureServer({
+    middlewares: { use: (path, handler) => (middleware = handler) },
+  });
+  const http = createServer((req, res) => middleware(req, res));
+  await new Promise((resolve) => http.listen(0, '127.0.0.1', resolve));
+  t.after(() => http.close());
+  const { port } = http.address();
+  const request = httpRequest({
+    host: '127.0.0.1',
+    port,
+    path: '/mcp',
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+      Host: `localhost:${port}`,
+    },
+  });
+  request.on('error', () => {});
+  request.end(JSON.stringify({ jsonrpc: '2.0', id: 1, method: 'tools/call' }));
+  await started.promise;
+  assert.equal(seen.aborted, false);
+  request.destroy();
+  await aborted.promise;
+  assert.equal(seen.aborted, true);
+});