Codex desktop live voice: delegated browser agent cannot access the visible embedded tab

What version of the Codex App are you using (From “About Codex” dialog)?

Unknown (not captured during reproduction)

What subscription do you have?

Unknown (not captured during reproduction)

What platform is your computer?

macOS; exact OS version and architecture not captured

What issue are you seeing?

During live voice, a delegated browser agent could operate a separate hidden in-app browser game, but could not access the embedded page I was watching. Observed September 14–15, 2026.

• A hidden in-app tab supported actual WebMCP start_game, look, and move calls, reaching five moves.
• Creating a visible in-app tab from the subagent returned: "IAB visibility is not supported in a subagent thread".
• Parent open_in_codex requests using the URL, tool tab ID, and provider tab ID returned "queued", but did not reliably show the controlled game. I saw empty panes, then a separate introduction after manual navigation.
• Later browser inventories exposed external Chrome only; the visible embedded browser was absent. An ordinary UI-control attempt therefore could not access that embedded page either.

What steps can reproduce the bug?

1. Start live voice in Codex desktop on macOS.
2. Ask the agent to open and play https://googlechromelabs.github.io/webmcp-tools/demos/webmcp-maze/ in the visible embedded browser while continuing the voice conversation.
3. Route browser operation through a delegated agent (required by this task’s configuration).
4. Attempt to show the controlled tab in the parent task.
5. Ask for ordinary browser UI interaction with the visible embedded tab as an alternative.

This describes the observed session; reproducibility in a fresh default configuration has not been established.

What is the expected behavior?

The agent acts on the embedded page the user is watching, or clearly reports that this session/delegation combination is unsupported before creating separate hidden browser state.

Additional information

No root cause is established. Task routing or delegated-session support may contribute; this report does not establish that WebMCP is universally broken.

As a secondary check, external Chrome loaded the maze but the agent connection advertised pageAssets only, without a WebMCP capability. This persisted after I reported enabling the Chrome WebMCP testing flag and restarting/reconnecting. Native Chrome support and the agent bridge’s support have not been independently isolated.

No private session identifiers or raw logs are included.

Start by reproducing the live voice flow on macOS with the WebMCP maze URL, using a delegated browser agent and the visible embedded tab. Trace the open_in_codex requests and browser inventories to determine whether the visible tab is reachable or the session should report unsupported behavior. Done means the agent controls the page the user is watching or clearly reports the unsupported delegation combination.