Intermittent: post-action hook messages missing from the next-turn request after session resume (pre-action always delivered) #4174

Summary

A conformance rig that drives kimi headless (CLI, print/headless mode with session resume) observes that post-action hook messages are intermittently not delivered to the next turn, while pre-action hook messages are always delivered. The delivery mechanism itself works when the messages do arrive (user-role hook_result).

Scenario

• An external harness launches kimi headless against a scratch project with hooks configured.
• Hooks run checks around tool calls:
  • a pre-action check (before Write|Edit) that must stay silent in the turn it fires and appear in the next turn's request body as a user-role hook_result message ("before Write");
  • a post-action check (after Write|Edit) with the same held-delivery contract ("after Edit");
  • a post-guard on Bash with the same contract ("after Bash").
• The harness launches a session, then resumes it (--resume <session_id>, headless) and inspects the outgoing request bodies.

Observed

Seven assertions per run; six are consistently green. The intermittent one is the post-action Write|Edit warning:

Timeline (all runs on the identical binary, same sha256):

• 2026-10-08 06:13 UTC — post-action Edit check failed
• 2026-10-08 06:16 UTC — same tree, same binary: all 7 passed
• 2026-10-11 02:04 / 02:06 / 02:07 UTC — three consecutive runs on two different source trees (one of them a clean control tree): same single check failed each time

The two source trees differ substantially; the failure signature is identical and binary-independent.

Environment

• @moonshot-ai/kimi-code v2.1.1, Linux x64
• binary: /usr/bin/kimi → dist/main.mjs
• sha256 2563a338c94868a34e1bfd2e5579067d5a472acf4cb0bf350a3bfe1092bbe59d (identical across every pass and fail above; installed 2026-10-08 00:33 +0200, so the 10-08 pass and fail ran on this same build)

Expected vs actual

• Expected: held post-action warnings always appear in the next turn's request after resume, as user-role hook_result.
• Actual: the post-action ("after Edit") warning is sometimes absent from the next-turn body; the pre-action one is never absent.

Notes

• Since the binary is unchanged between pass and fail, the nondeterminism likely sits in the message-assembly/resume path — possibly influenced by model/server-side behavior — rather than in local code.
• Possibly related: UserPromptSubmit hook output hides user messages from chat history (desktop, Windows) #4132 (UserPromptSubmit hook output and chat-history visibility) — different surface (headless resume vs desktop TUI), same neighborhood of hook-output handling.
• I can provide redacted request/response captures from failing and passing runs on request; the harness is private, but the wire-level evidence is exportable.

Thanks for taking a look — this one intermittently blocks a release gate that requires an all-green run.