[MCP]: Isolated HTTP sessions keep their browser context after disconnect

[oleksii-tumanov]

What's going on?

When two HTTP clients connect to a server started with --isolated, they share one browser but get separate browser contexts. If one client terminates while the other remains connected, the terminated client's context stays open until the last client disconnects.

Reproduce:

1. start the MCP server with --port=8931 --isolated;
2. connect two Streamable HTTP clients and navigate from both.
3. Terminate the first session.
4. From the second session check page.context().browser().contexts().length.

It remains 2; it should be 1.

This is related to #40709, but concerns per-client context cleanup after disconnect rather than duplicate browser launches.

Version

1.62.0-next (main)

[github-actions]

Hi, I'm the Playwright bot and I took a first look at this.

Reproduced on tip-of-tree (1.62.0-next, the version you reported). In --isolated mode each HTTP client gets its own browser context, but a disconnecting client's context is only closed when it's the last one to leave. While another client stays connected, the terminated client's context (and its pages) leak — exactly the contexts().length staying at 2 you describe. This is a real bug, not the shared-browser race from #40709.

The cause is the disposed handler in packages/playwright-core/src/tools/mcp/program.ts: when clientCount > 0 it returns early without closing the per-client context, so the context only gets closed via the browser-close path once everyone disconnects.

disposed: async backend => {
  clientCount--;
  if (sharedBrowserPromise && clientCount > 0)
    return;                       // ← leaks this client's own isolated context
  // ...closes context + browser only for the last client
}

Minimal repro (against the MCP HTTP test harness)

Each client's page holds an inflight fetch open; closing its context aborts that request, which is the observable signal:

test('isolated context closes on disconnect', async ({ serverEndpoint, server }) => {
  const { url } = await serverEndpoint({ args: ['--isolated'] });

  let hangAborted = false;
  server.setRoute('/hang', (req, res) => {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write('start');                       // never res.end()
    res.on('close', () => { hangAborted = true; });
  });
  server.setRoute('/leak-page', (req, res) => {
    res.writeHead(200, { 'Content-Type': 'text/html' });
    res.end(`<script>fetch('/hang')</script>`);
  });

  // client1 opens a page holding an inflight request in its own isolated context
  const t1 = new StreamableHTTPClientTransport(new URL('/mcp', url));
  const c1 = new Client({ name: 'test', version: '1.0.0' });
  await c1.connect(t1);
  const hang = server.waitForRequest('/hang');
  await c1.callTool({ name: 'browser_navigate', arguments: { url: server.PREFIX + '/leak-page' } });
  await hang;

  // client2 stays connected throughout
  const t2 = new StreamableHTTPClientTransport(new URL('/mcp', url));
  const c2 = new Client({ name: 'test', version: '1.0.0' });
  await c2.connect(t2);
  await c2.callTool({ name: 'browser_navigate', arguments: { url: server.EMPTY_PAGE } });

  // client1 disconnects → its context should close, aborting the request
  await t1.terminateSession();
  await c1.close();

  await expect.poll(() => hangAborted, { timeout: 5000 }).toBe(true);  // ← fails today
});

What I ran

• version: 1.62.0-next (main, the reported version) — built from source
• mode: --isolated, Streamable HTTP transport, chromium headless
• observable: client1's page holds an inflight fetch('/hang'); res.on('close') fires only when the owning context closes
• result:
  • 5s after client1 (intermediate) disconnects → request still open (hangAborted = false) — context leaked
  • after client2 (last) disconnects → its request aborts (hang2Aborted = true) — context closed only here
• this matches the report: the terminated client's context survives until the last client disconnects
• not engine-specific — it's pure session-lifecycle logic in program.ts, independent of the browser

This run: https://github.com/microsoft/playwright/actions/runs/28453178304