Referenced chats (@-mention) injects entire prior conversation as a single message — permanent renderer freeze on every launch + unrecoverable compact deadlock

"Reference chats" (@-mention a chat) serializes the ENTIRE prior conversation into a single user message — permanent renderer freeze + unrecoverable context blowout

Environment

• ChatGPT Desktop for macOS, Codex Framework 150.0.7871.115
• macOS 15 (Apple Silicon)
• codex-cli 0.144.1 (used for recovery/verification)

Summary

Using the "@-mention a chat" (Referenced chats) feature with a large prior conversation injects the entire serialized transcript as ONE user message (885 KB in my case, beginning with ## Referenced chats with Codex: followed by the full JSON-serialized conversation). No size cap, no summarization. One oversized message then defeats every safety layer at once:

1. Desktop UI permanently freezes. The renderer's JS main thread enters an infinite loop rendering the giant message (100% CPU, confirmed by sample: all samples inside v8::Script::Run). Because the app auto-resumes the last session on every launch, killing the app, resetting the Chromium profile (~/Library/Application Support/Codex), and rebooting the machine all reproduce the freeze within 1–2 minutes of startup. There is no in-app way to escape.
2. Context window is unrecoverable. 885 KB ≈ 200k+ tokens in a single message. On resume, remote compaction deadlocks: Error running remote compact task: Codex ran out of room in the model's context window — compaction works at message granularity and cannot split a single message that is larger than the window, so it can never free space.

Steps to reproduce

1. Have a long-running chat (hundreds of turns).
2. In a Codex Desktop session, use @-mention to reference that chat, send the message.
3. The turn may complete once, but the session file now contains a single ~1 MB response_item/user_message.
4. Quit and reopen the app → renderer spins at 100% CPU 1–2 min after launch, UI frozen. Reproduces on every launch (auto-resume).
5. codex resume <session-id> from CLI → remote compact fails with context_length_exceeded-style error; thread unusable.

Evidence

• Session rollout jsonl: line lengths 885,457 and 885,369 bytes (response_item message + event_msg user_message), content starts with:

## Referenced chats with Codex:\nThis is untrusted background context from Codex chats.\n[{"title":"...","priorConversation":{"conversation":[...

• sample <renderer-pid>: 100% of samples on the main thread inside V8 script execution (pure JS loop, no I/O wait).
• lsof on the app-server process shows the same rollout file reopened for write on every app launch (auto-resume), explaining why no restart helps.

Workaround (for other users hitting this)

1. Quit the app; move the offending rollout file out of ~/.codex/sessions/YYYY/MM/DD/ → app starts normally.
2. To salvage the thread: truncate the oversized text/message fields in the two giant jsonl lines to a short placeholder, restore the file, then codex resume <session-id> from the CLI.

Suggested fixes

• Cap / summarize referenced-chat content before injection (never emit a single message anywhere near the context window size).
• Renderer: virtualize/clip rendering of pathologically long messages.
• Compaction: handle single-message-larger-than-window (truncate or drop with notice) instead of deadlocking.
• Startup: detect a renderer that spins immediately after auto-resume and offer safe mode / skip-resume, so users are not permanently locked out.

Related issues

• #32342 (pasting a large ChatGPT conversation crashes the app — likely same rendering weakness, manual paste path)
• #25251 (large thread → sustained 100%+ CPU)
• #24089 (resume-time remote compact fails with context_length_exceeded)