Codex Desktop create_thread rejects valid project worktree requests as invalid arguments

What variant of Codex are you using?

Codex Desktop on macOS. The exact About-dialog version was not exposed to the reporting task.

Summary

The Desktop-owned codex_app__create_thread tool is exposed and its project/model combinations are discoverable, but valid project task requests are rejected before thread creation with only:

create_thread received invalid arguments

No field-level validation detail, task ID, client task ID, or worktree is returned.

Reproduction

1. In Codex Desktop, list saved projects through codex_app__list_projects.
2. Select a returned project with isGitRepository: true.
3. Call codex_app__create_thread with a supported model and reasoning effort:

{
  "projectId": "<VALID_PROJECT_ID_FROM_LIST_PROJECTS>",
  "model": "gpt-5.6-luna",
  "thinking": "high",
  "target": {
    "type": "project",
    "environment": {
      "type": "worktree",
      "startingState": {
        "type": "working-tree"
      }
    }
  },
  "prompt": "<NONEMPTY_TASK_PROMPT>"
}

1. Observe create_thread received invalid arguments.

The same failure reproduced with:

• a saved project whose folder is itself a linked Git worktree;
• the saved main repository project;
• environment.type = local where supported by the exposed schema;
• environment.type = worktree without startingState;
• startingState.type = branch naming an existing committed branch;
• startingState.type = working-tree;
• gpt-5.6-sol with medium reasoning;
• gpt-5.6-luna with high reasoning.

The project registry was refreshed immediately before the final attempt and returned the same valid project IDs and advertised Git/worktree capability.

Expected behavior

A valid request should create or queue exactly one task and return threadId/hostId or clientThreadId for worktree setup.

If a request is invalid, the tool should return a typed validation error naming the rejected field, expected schema, and received value.

Actual behavior

Every request fails before task creation with the same opaque invalid arguments message. No task or worktree appears.

Impact

This blocks orchestrated workflows that require independently configured worktree tasks. In this reproduction, a governed build workflow required gpt-5.6-luna High implementation tasks and had to stop rather than silently substitute another execution mode.

Additional information

• The parent task retained working list_projects, list_threads, and other thread-management tools.
• Git state was clean and the requested starting branch was committed.
• Local project names, paths, repository names, client data, and thread IDs are intentionally omitted.
• Existing issues were searched for the exact error and adjacent create_thread failures; no matching opaque-validation report was found.