Authorized Windows dot cannot discover or read an existing local Codex thread; placement region 12 and CloudThreadNotFoundError #53124

Environment (2026-10-11): Windows desktop package 26.1002.7124.0; app-reported version 26.1002.52244 build13536 prod; bundled core 0.162.0-alpha.2. No successful updater availability result (busy).

1. An existing user-created local Codex thread is readable from a local Codex context using native read_thread(hostId=local).
2. The user relayed the dot's live computer state: connected, attached=true, is_authorized_for_tasks=true. A stale offline same-name computer is not authorized.
3. The dot reports cloud_threads_read/send_message/list_threads as its current tools for this operation, without native local read/send equivalents. The user subsequently relayed exact tool-directory searches for mcp__codex_app__list_threads, mcp__codex_app__read_thread, and mcp__codex_app__send_message_to_thread: all not found. This is user-relayed tool-directory evidence, not an independently captured server-side inventory.
4. At 2026-10-11 13:02:58 UTC, cloud_threads_read({threadId:LOCAL_THREAD_ID,limit:3}) returned:
   AppServerBackendRequestError: Client specified an invalid argument: post-cutover thread ID LOCAL_THREAD_ID has unsupported placement region 12
5. Around13:03 UTC, cloud_threads_send_message({threadId:LOCAL_THREAD_ID,model:gpt-6-luna,prompt:READ_ONLY_DIAGNOSTIC}) returned:
   CloudThreadNotFoundError: The cloud thread was not found.
   error_code=NOT_FOUND; error_data.type=cloud_thread_not_found.
   Neither failure returned a request ID or new turn ID.
6. The user relayed a complete account-scope unarchived list (children_only=false,limit=50; 50+18 entries, final nextCursor=null). The target was absent. Entries contained only threadId. Archived list was not queried. This establishes absence from that returned visible set, not absence from all backend registries.

Please clarify the supported route and capability requirements for dot to continue this authorized existing local thread, and investigate placement parsing/routing versus visibility/authorization filtering. The documentation explicitly supports existing local Codex continuation. Please provide a correlatable request ID and actionable error.

Related public reports: #52873, #49482, #49753. These are symptom comparisons, not proof of a common root cause.

A separate Windows sandbox failure in local execution matches the MAXIMUM_ALLOWED runtime-ACL issue addressed by #51822. We are not assuming it causes the cloud-thread lookup errors. No thread ID/database changes, Codex/REPL process termination, or repeated message submission were used for this round of dot diagnosis. An isolated sandbox backport is being built separately and has not been deployed into the desktop app.