Extension canvas.open emits malformed opened event and does not reliably focus

Problem

A user-scoped Copilot CLI extension can register and manually render a canvas, but calling the documented low-level session.rpc.canvas.open route on resume does not reliably make the canvas visible and emits a malformed event warning in the extension process.

Copilot CLI version: 1.0.71-0 on macOS arm64.

Reproduction

1. Register a canvas through joinSession({ canvases: [...], requestCanvasRenderer: true }).
2. After detecting a resumed session, resolve the canvas through session.rpc.canvas.list().
3. Call session.rpc.canvas.open with the exact returned provider ID:

await session.rpc.canvas.open({
  extensionId: declared.extensionId,
  canvasId: declared.canvasId,
  instanceId: "stable-instance",
  input: { key: "example" },
});

A complete implementation and focused tests are available in the Session Briefing adapter and its test.

Actual behavior

• The canvas.open promise resolves; the extension's bounded failure diagnostic does not run.
• The extension process logs failed to deserialize session.canvas.opened payload.
• The resumed CLI does not reliably show/focus the canvas.
• The SDK rejects the event because the payload does not satisfy OpenCanvasInstance (provider identity, instance/canvas IDs, reopen, and availability).

Expected behavior

The returned/opened snapshot and session.canvas.opened event satisfy the bundled SDK schema, and reopening a stable instance visibly focuses the canvas.

Additional evidence

• The same canvas renders successfully when opened through the host's open_canvas tool with the explicit provider ID.
• Omitting extensionId and passing the exact ID returned by canvas.list() both produce the malformed-event warning.
• The canvas ID is unique in the session.

A small diagnostic option to log or expose the rejected event shape would also make this class of failure much easier to debug without instrumenting the SDK.

Empieza por el adaptador Session Briefing y su adapter.test.mjs; después, sigue la ruta session.rpc.canvas.open y el manejo del evento session.canvas.opened. Compara el snapshot y el evento devueltos con el esquema OpenCanvasInstance incluido. Se considera terminado cuando el evento se deserializa correctamente y volver a abrir una instancia estable enfoca visiblemente el canvas.