Codex Desktop subagent completions render as raw notifications and create unknown-conversation errors

This issue was drafted and submitted by Codex on behalf of the user.

Summary

multi_agent_v2 can turn a subagent completion/rendering bug into a durable Codex Desktop session corruption bug: after using subagents, the affected conversation can no longer be opened in the app. Opening that conversation hits the LocalConversationPage / markdown renderer error boundary.

After disabling multi_agent_v2 and keeping legacy multi_agent enabled, the same two-subagent smoke test no longer made the conversation unopenable, but it still emitted raw <subagent_notification> blocks into the main chat and logged unknown conversation / No turns for conversation errors. So v2 appears to amplify an underlying subagent notification/state-race problem into a renderer-breaking persisted session record.

Relationship to possible duplicates

• Related to #23174, which also describes Codex Desktop sessions becoming impossible to open because malformed/raw subagent notification records are persisted in local session history. This issue adds a minimal two-subagent repro, the multi_agent_v2 severity difference, and concrete renderer/log excerpts.
• Not a duplicate of #23588. That issue is about agent_path UUID being shown instead of a nickname. This issue is about raw subagent payloads being rendered/persisted in a way that can break the desktop conversation renderer and make a conversation unopenable.

Environment

• Codex Desktop: release=26.513.31313 in desktop logs
• Codex CLI in subagent session metadata: 0.131.0-alpha.9
• Platform: macOS / darwin
• Latest repro config after mitigation:
• features.multi_agent = true
• features.multi_agent_v2.enabled = false
• Earlier failing state: multi_agent_v2 was enabled before it was disabled as a mitigation.

Reproduction / timeline

1. Use Codex Desktop with subagent support enabled.
2. With multi_agent_v2 enabled, run a minimal two-subagent smoke test. Each subagent only reports pwd and local date.
3. A raw subagent completion JSON fragment is rendered into the main thread, beginning with "completed":"Task /root/agent_smoke_two is alive....
4. The desktop app hits a renderer error boundary: invalid syntax at line 1 col 2.
5. User impact: the affected conversation becomes unopenable in Codex Desktop.
6. Disable multi_agent_v2, restart Codex Desktop, and rerun the same two-subagent smoke test.
7. With v2 disabled, both subagents complete and the conversation remains usable, but raw <subagent_notification> blocks and conversation-state errors still appear.

Latest subagent IDs from the v2-disabled repro:

• 019e48ac-2bc9-7bb3-94b8-1254b9256a72
• 019e48ac-398b-7ed1-b2ed-a8cc1fda7ef9

Expected

• Subagent completions should be consumed as internal structured notifications, not persisted/rendered as raw XML or raw JSON in the main chat.
• A malformed subagent notification should never make an existing conversation impossible to open.
• The desktop renderer should fail soft on malformed historical records instead of crashing LocalConversationPage.
• No unknown conversation, No turns for conversation, empty rollout, or renderer markdown parse crash should occur.

Actual

With multi_agent_v2.enabled=true:

• Raw subagent completion JSON was rendered into the main thread.
• The renderer hit an error boundary with invalid syntax at line 1 col 2.
• The affected conversation could no longer be opened in the Codex Desktop app.

With multi_agent_v2.enabled=false after restart:

• Both subagents completed successfully.
• Raw <subagent_notification> blocks still appeared in the main conversation.
• Desktop logs showed repeated unknown conversation and No turns for conversation errors for subagent conversation IDs.
• The severe “conversation cannot be opened” symptom did not recur in the latest v2-disabled smoke test.

Inline log excerpts

These are copied directly from local Codex Desktop logs because local log file paths are not accessible to maintainers.

v2-enabled run that produced the renderer crash:

2026-05-21T03:42:11.952Z error [electron-message-handler] error boundary name=LocalConversationPage errorMessage="invalid syntax at line 1 col 2:

1  \"completed\":\"Task `/root/agent_smoke_two` is alive. `pwd` returned `~/Documents/Codex/2026-05-18/codex`, and local time is `2026-05-21 11:41:16 CST +0800`. No files changed.\"}
    ^"

v2-disabled run after restart:

2026-05-21T03:55:07.045Z error [electron-message-handler] Received turn/started for unknown conversation conversationId=019e48ac-2bc9-7bb3-94b8-1254b9256a72
2026-05-21T03:55:09.286Z error [electron-message-handler] Received turn/started for unknown conversation conversationId=019e48ac-398b-7ed1-b2ed-a8cc1fda7ef9
2026-05-21T03:55:10.665Z error [electron-message-handler] No turns for conversation conversationId=019e48ac-2bc9-7bb3-94b8-1254b9256a72
2026-05-21T03:55:11.217Z error [electron-message-handler] No turns for conversation conversationId=019e48ac-398b-7ed1-b2ed-a8cc1fda7ef9
2026-05-21T03:55:21.505Z error [electron-message-handler] Received turn/completed for unknown conversation conversationId=019e48ac-42f6-70f1-b971-688027a69d54
2026-05-21T03:55:35.204Z error [electron-message-handler] Received turn/completed for unknown conversation conversationId=019e48ac-39cd-75c2-9abc-65668e3f880d

Example raw notification still visible with v2 disabled

<subagent_notification>
{"agent_path":"019e48ac-398b-7ed1-b2ed-a8cc1fda7ef9","status":{"completed":"agent_smoke_two is alive. `pwd`: `~/Documents/Codex/2026-05-21/019e3a4c-f6af-7a31-9b6d-78b411626059-agent`; local time: `Thu May 21 11:55:15 CST 2026`."}}
</subagent_notification>