Unsupported slash commands hang the chat instead of erroring

[charlesvien]

Problem

Typing an unsupported Claude Code slash command (e.g. /plugin install slack from the Claude Code plugin install guide) hangs the chat indefinitely. The status bar shows "Discombobulating..." with a running timer (observed at 103m 34s, 0K/1.0M tokens) and never resolves or errors.

Thread: https://posthog.slack.com/archives/C09G8Q32R6F/p1778495163792959

Expected behavior

When the user submits a slash command that PostHog Code does not implement, the chat should immediately surface a clear error (e.g. "Unknown command: /plugin") and stay responsive, instead of silently entering an unrecoverable spinning state.

Steps to reproduce

1. Open a new task in PostHog Code
2. Type /plugin install slack and submit
3. Observe the chat enters "Discombobulating..." and never returns

Notes

• Related to We do not support all slash commands #670 (we do not support all slash commands), but this issue is specifically about the failure mode rather than the missing functionality. Even after We do not support all slash commands #670 ships, unknown commands should still error gracefully.
• Reporter noted this was discovered while following the Claude Code plugin install guide inside PostHog Code.

[lost-particles]

Opened #2345 with a fix for this. The PR catches the specific failure mode by detecting "first session_state_changed → idle arrives with promptReplayed=false AND the user input was a slash command we did not handle locally" and emits a clear agent_message_chunk ("Unsupported slash command: /plugin. PostHog Code does not implement this command.") before returning end_turn, so the renderer can clear its spinner. Regular prompts keep the existing skip behavior so background idles do not get misclassified. Would appreciate a review whenever convenient.