fix(agent): surface error for unsupported slash commands instead of hanging

[lost-particles]

Closes #2158

Summary

• When the user submits a Claude Code slash command that PostHog Code does not implement (e.g. /plugin install slack), the SDK consumes the input but emits no echo or local_command_output. The prompt loop in ClaudeAcpAgent.prompt then sits on the first session_state_changed → idle notification with promptReplayed still false, breaks out of the switch, and goes back to query.next() forever. The chat shows "Discombobulating…" indefinitely with no way to recover.
• This change detects the case specifically: when an idle arrives before any prompt replay AND the original user input was a slash command we did not handle locally, emit a clear agent message and return end_turn so the renderer can clear its spinner.
• Regular prompts keep the existing skip behavior, so background idles (the original purpose of the skip) do not get misclassified as failures.

Behavior

Input	Old behavior	New behavior
/context, /heapdump, /extra-usage (local-only commands)	Works (isLocalOnlyCommand, promptReplayed=true)	Unchanged
/plugin install slack and other SDK commands that silently consume	Loop hangs forever, "Discombobulating…"	Emits agent_message_chunk with "Unsupported slash command: \/plugin`. PostHog Code does not implement this command."and returnsend_turn`
Regular prompt where an unrelated background idle arrives before replay	Skipped, waits for prompt replay	Unchanged

Test plan

• pnpm --filter @posthog/agent test src/adapters/claude/claude-agent.slash-command.test.ts — 2 new tests:
  • Slash command + early idle → emits the unsupported-command chunk and returns end_turn.
  • Regular prompt + unrelated early idle → does NOT emit the chunk; falls through to the existing Session did not end in result path.
• pnpm --filter @posthog/agent test — all 446 tests pass, including the existing claude-agent.refresh.test.ts.
• pnpm --filter @posthog/agent typecheck and biome check clean on changed files.
• Manual: open a new task in PostHog Code, type /plugin install slack, submit. The chat should show the "Unsupported slash command" message and stay responsive instead of spinning.

Notes

• The detection reuses the existing commandMatch regex (/^(\/\S+)/) that the function already computes at line 350, so the diff is small and the matching rules stay consistent with the LOCAL_ONLY_COMMANDS check.
• The error is emitted as a regular agent_message_chunk (the same channel local-only commands use to forward their results), so it renders inline in the conversation rather than as a toast or error banner — matching the reporter's "surface a clear error in the chat" expectation.
• This does not enumerate which slash commands PostHog Code supports vs not (that depends on the SDK's supportedCommands() and on what each command needs at runtime). It just catches the "SDK swallowed it and produced nothing" failure mode, which is the actual user-visible bug.

[lost-particles]

Hi maintainers, this PR targets #2158. Whenever convenient, could a maintainer take a look and let me know if the failure-mode detection (idle-before-replay + original input is a slash command we did not handle locally) is the right place to draw the line? I considered also validating slash commands in the renderer against the availableCommands list before sending, but /plugin is in the SDK reported command set so that approach would not have caught the reported case. Happy to iterate on wording, scope, or placement based on your feedback. Thanks!

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
packages/agent/src/adapters/claude/claude-agent.ts:478
The `!isLocalOnlyCommand` guard is always `true` inside this branch. When `isLocalOnlyCommand` is `true`, `promptReplayed` is also set to `true` at line 353, so the enclosing `if (!promptReplayed)` block is never entered with a local-only command. The check is superfluous (simplicity rule: no superfluous parts), and a reader has to trace back to line 353 to convince themselves it's harmless.

```suggestion
                if (commandMatch) {
```

### Issue 2 of 2
packages/agent/src/adapters/claude/claude-agent.slash-command.test.ts:79-162
**Non-parameterized tests** — the two cases share identical setup (make agent, install session, send idle, complete) and differ only in the prompt text and expected outcome. The project convention is to always prefer parameterised tests. Both scenarios could be driven by a single `it.each` table that supplies `{ prompt, expectedStopReason, expectsErrorChunk }` (or similar), which would also make it trivial to add future slash-command variants without duplicating the fixture plumbing.

_{Reviews (1): Last reviewed commit: "fix(agent): surface error for slash comm..." | Re-trigger Greptile}