SessionStart hook stdout is silently discarded — hook output is never displayed in the TUI

Summary

SessionStart hook commands execute, but their stdout is silently discarded — the hook output is never rendered in the TUI or appended to the conversation context. Users configuring a SessionStart hook (e.g. to print a startup message or inject session-start instructions) see nothing, despite the hook actually running.

Expected behavior

Per the hooks documentation (Return Values section):

0 — Normal exit, allow: Continue execution; stdout content (if any) may be appended to context

So for any hook (including observation-only events like SessionStart), stdout should be surfaced to the user — at minimum rendered in the TUI transcript, similar to how UserPromptSubmit hook results appear as UserPromptSubmit hook entries.

Actual behavior

The SessionStart hook runs (its side effects, e.g. writing a file, are visible), but its stdout goes nowhere:

• No hook.result event is emitted, so the TUI never renders a transcript entry.
• No context append happens, so the model never sees the output either.
• The returned HookResult[] from the trigger is simply dropped.

Steps to reproduce

# ~/.kimi-code/config.toml
[[hooks]]
event = "SessionStart"
command = "echo 'hello from SessionStart hook' && echo HOOK_RAN >> /tmp/hook-marker.log"

1. Start a new kimi session (or resume one).
2. Observe that /tmp/hook-marker.log gets written — the hook did run.
3. The string hello from SessionStart hook appears nowhere: not in the TUI transcript, not in the model context (checkable via kimi -p + inspecting wire.jsonl — no context.append_message entry for it).

Tested on kimi-code 0.35.0 (macOS).

Root cause

The session-scope external hooks service fires SessionStart fire-and-forget and discards the result:

• packages/agent-core-v2/src/session/externalHooks/externalHooksService.ts:137-149 — triggerSessionStart() calls await this.runner.trigger('SessionStart', …) and ignores the returned HookResult[] (no hook.result event, no context append, nothing).

Meanwhile, hook.result events — the only thing the TUI renders as a hook transcript entry (apps/kimi-code/src/tui/controllers/session-event-handler.ts → handleHookResult) — are only published by the agent-scope UserPromptSubmit handler:

• packages/agent-core-v2/src/agent/externalHooks/externalHooksService.ts:353-397 — runPromptSubmitHook() is the only path that appends hook stdout to the context and publishes type: 'hook.result'.

So observation-only events (SessionStart, SessionEnd, SubagentStart, PreCompact, Notification, …) currently have no output-surfacing path at all.

Suggested fix

Publish hook.result (and optionally append stdout to context) for observation-only events too, mirroring the UserPromptSubmit path in runPromptSubmitHook:

• Emit hook.result with the hook stdout for events such as SessionStart/SessionEnd so the TUI renders them as transcript entries.
• Or append stdout to the context for observation events (possibly gated behind an opt-in config field to avoid context pollution for noisy hooks).

Workaround

Until this is fixed, the same effect can be achieved with a UserPromptSubmit hook guarded by a per-session_id state file so it only emits on the session's first prompt — UserPromptSubmit is currently the only event whose stdout is actually surfaced.

Environment

• kimi-code CLI version: 0.35.0 (kimi --version)
• OS: macOS

Reproduce the SessionStart configuration, then read packages/agent-core-v2/src/session/externalHooks/externalHooksService.ts:137-149 and compare it with runPromptSubmitHook in packages/agent-core-v2/src/agent/externalHooks/externalHooksService.ts:353-397. Check how apps/kimi-code/src/tui/controllers/session-event-handler.ts handles hook.result. Done means SessionStart stdout is surfaced through the intended event or context path and appears in the TUI or model context.