Lesson 4 · 15 min
A tool is not forever: the page registers it when the function exists and removes it when it leaves the scene. In the 10/16/2026 spec there is no unregisterTool; what removes it is the signal of an AbortController. And the caller can cancel an execution midway. In this lesson you see the whole cycle and the events the browser fires.
Start the lesson
Goal: register, list, remove with AbortSignal, and cancel an execution, knowing which events the browser fires at each step.
Click each step.
Measured on 10/19/2026: Chromium 156 fires all three events; polyfill 6.0.3 fires only toolchange.
Excerpt from kit/scripts/ciclo.mjs (runs inside the page). Two controllers: one for registration, one for the call.
const registro = new AbortController();
await mc.registerTool({
name: 'tarefa_lenta', description: 'Leva 2 segundos. Existe só para ensinar cancelamento.',
async execute(_, { signal }) {
return new Promise((ok, falha) => {
const t = setTimeout(() => ok({ terminou: true }), 2000);
signal.addEventListener('abort', () => { clearTimeout(t); falha(new DOMException('cancelada', 'AbortError')); });
});
}
}, { signal: registro.signal });
const t = (await mc.getTools()).find(x => x.name === 'tarefa_lenta');
const chamada = new AbortController();
setTimeout(() => chamada.abort(), 100);
try { await mc.executeTool(t, {}, { signal: chamada.signal }); }
catch (e) { /* AbortError */ }
registro.abort(); // removes the tool: there is no unregisterToolcd ~/projetos/formacao-webmcp-site-v72/kit node scripts/ciclo.mjs nativo; echo "saída: $?" node scripts/ciclo.mjs polyfill; echo "saída: $?"
== Chromium 156.0.8078.4 (Playwright) · modo: nativo antes: 4 ferramentas registrou tarefa_lenta: 5 ferramentas nome repetido: recusado (InvalidStateError) execução cancelada pelo chamador: AbortError depois do abort(): 4 ferramentas eventos: toolchange toolactivated:tarefa_lenta toolcancel:tarefa_lenta toolchange saída: 0 == Chromium 156.0.8078.4 (Playwright) · modo: polyfill antes: 4 ferramentas registrou tarefa_lenta: 5 ferramentas nome repetido: recusado (InvalidStateError) execução cancelada pelo chamador: AbortError depois do abort(): 4 ferramentas eventos: toolchange toolchange saída: 0
Same sequence in both. The difference is in the "eventos" line: only native fires toolactivated and toolcancel.
If execute doesn't listen to signal, canceling rejects the call, but the work keeps going: the setTimeout fires, the fetch completes, the booking happens. For a tool with side effects, check signal.aborted before writing.
Same rule in any framework: register at the start, abort at the end.
Register after the rules are ready and abort on pagehide.
This is what iniciarWebMCP does in the kit.
addEventListener('pagehide', () => pagina.abort(), { once: true })
A tool for a single screen exists only while that screen is mounted.
When the route changes: abort() and register the new screen's tools.
In React, register in the effect and abort in the cleanup; strict mode mounts twice, and the repeated name would throw InvalidStateError.
The pattern holds for other frameworks; lesson 7 shows the model.
If the third of four fails, the first two are left hanging.
The kit's registrar() uses one controller per batch and aborts everything on failure.
Rehearsal in both modes with an API. Each exercise ends with a done when.
Run the cycle block and read it line by line.
Done when: node scripts/ciclo.mjs nativo → nome repetido: recusado (InvalidStateError), execução cancelada pelo chamador: AbortError, depois do abort(): 4 ferramentas and eventos: toolchange toolactivated:tarefa_lenta toolcancel:tarefa_lenta toolchange.
Run the same in polyfill and note in your status-webmcp.md (lesson 1) what the polyfill doesn't do.
Done when: node scripts/ciclo.mjs polyfill → eventos: toolchange toolchange and saída: 0; the line is in your file.
Run the kit's rule tests: one of them simulates the 2nd tool failing at registration.
Done when: node --test 2>&1 | grep -E "^ℹ (pass|fail)" → ℹ pass 4 and ℹ fail 0.
The explanation appears after your answer.
Question 1 of 4
How do you remove a tool in the 10/16/2026 spec?
Question 2 of 4
The caller aborts the execution. What happens to execute?
Question 3 of 4
Your test waits for toolactivated and runs only on polyfill 6.0.3. What happens?
Question 4 of 4
In a React component, where does abort() go?
Pick an answer. The explanation shows up after you try.