Expose a permissioned in-app browser API for web pages to send follow-up messages to the current Codex thread

What variant of Codex are you using?

App

What feature would you like to see?

Feature request

Codex MCP Apps widgets can send follow-up messages to the current Codex thread through the host bridge, e.g. app.sendMessage(...). This enables workflows like Cowart, where a canvas widget can export an annotated screenshot and ask Codex to continue from that visual context.

I would like a similar, permissioned API for pages opened in the Codex in-app browser.

Use case

Apps like Flare have an existing web canvas. A user may annotate an image or select canvas objects, then click "Send to Codex". The page should be able to ask Codex to append a follow-up message to the current thread, with explicit user confirmation.

Today the alternatives are awkward:

• Copy prompt to clipboard and ask the user to paste it into Codex.
• Rebuild the app as an MCP widget just to access app.sendMessage.
• Run a local app-server bridge, which adds setup and security complexity.

Proposed API shape

A host-mediated API, for example:

await window.openai?.sendFollowUpMessage({
  role: "user",
  content: [
    { type: "text", text: prompt },
    { type: "image", data: base64Png, mimeType: "image/png" }
  ],
  metadata: {
    source: "flare",
    projectId,
    annotationId
  }
});

### Additional information

_No response_