Desktop conversation mention is unresolved when priorConversation is null and read_thread is unavailable

Summary

A ChatGPT conversation selected through the desktop app's built-in task/conversation mention UI is rendered as a chatgpt-conversation://… reference, but Codex cannot read it because the referenced-conversation context arrives with priorConversation: null while the read_thread tool is not available in the task.

This makes a correctly inserted conversation mention behave like a broken link from the agent's perspective.

Steps to reproduce

1. In the ChatGPT/Codex desktop app, start a new Codex task.
2. Use the built-in picker/mention UI to reference an existing ChatGPT conversation.
3. Ask Codex a question that requires reading that referenced conversation.
4. Observe the context supplied to the agent.

Actual behavior

The agent receives a reference shaped like:

{
  "conversationId": "<redacted>",
  "title": "<redacted>",
  "priorConversation": null
}

The accompanying instruction says to call read_thread when the preview is null, but no read_thread capability is exposed in the session. The agent therefore cannot inspect the referenced conversation and may incorrectly tell the user that this type of link is unsupported.

Expected behavior

At least one of the following should always be true for a conversation inserted through the official picker:

• priorConversation contains a usable bounded preview; or
• read_thread is exposed and authorized to resolve the supplied conversationId.

The UI should not create a reference that the receiving Codex task has no capability to resolve.

Environment

• ChatGPT desktop app with Codex
• macOS
• Observed: 2026-09-04
• The reference was inserted using the official UI, not pasted as a raw URI.

Privacy

Conversation and task identifiers and the conversation title are intentionally omitted from this public issue. They can be supplied through a private feedback channel if needed.

No files, tests, or entry points are named. Start by tracing desktop conversation-mention handling and the task capabilities exposed when priorConversation is null; reproduce with the official picker and verify that every inserted reference receives either a bounded preview or an authorized read_thread capability.