fix: Codex SessionStart hook rejects no-op fallback as invalid JSON

[thedotmack]

Summary

• hook-command.ts fell back to a bare {continue:true, suppressOutput:true} whenever a hook errored before its handler ran (missing session_id, invalid cwd, missing transcript path) — regardless of which hook event fired
• Codex's strict SessionStart validator rejects that bare shape as "hook returned invalid session start JSON output", since the context handler is the SessionStart producer on every platform
• buildNoOpResult(event) now attaches a minimal valid hookSpecificOutput: { hookEventName: 'SessionStart', additionalContext: '' } when the fallback fires for the context handler, keeping the no-op harmless everywhere else

This is the remaining half of the bug tracked in #2972 — the description-field parse rejection was already fixed separately, but the continue:true leak on the no-op fallback path was not. Supersedes #2953 (unmerged, only patched the shipped codex-hooks.json shell script rather than the root cause in worker-service.cjs).

Closes #2947
Closes #2972

Test plan

• bun test — 2225 pass, 0 fail (added coverage for buildNoOpResult in tests/hook-command.test.ts)
• tsc --noEmit clean
• npm run build-and-sync — rebuilt bundle, restarted worker, confirmed fix compiled into plugin/scripts/worker-service.cjs
• End-to-end: piped a Codex SessionStart payload with no session_id through the built worker-service.cjs hook codex context — before: {"continue":true} (invalid); after: {"continue":true,"hookSpecificOutput":{"hookEventName":"SessionStart"}} (valid)

🤖 Generated with Claude Code

https://claude.ai/code/session_011YMT5iCF1kfikq7fetR6NF

[greptile-apps]

Greptile Summary

This PR fixes Codex SessionStart no-op fallback output. The main changes are:

• Adds buildNoOpResult for early graceful fallback paths.
• Emits minimal SessionStart hookSpecificOutput for the context hook.
• Preserves empty-string additionalContext in Codex output formatting.
• Adds regression coverage for the fallback shape and adapter serialization.
• Regenerates the bundled worker script with the same behavior.

Confidence Score: 5/5

Safe to merge with minimal risk.

The change is narrowly scoped to no-op fallback serialization and is covered by targeted regression tests.

No files require special attention.

T-Rex Logs

What T-Rex did

• The base run of the contract-validation harness was executed to capture the initial state, which showed base stdout continuing true but missing the required SessionStart hookSpecificOutput and produced an INVALID validation.
• The after-run capture was executed to obtain the head state, which showed continue true and a hookSpecificOutput with hookEventName SessionStart and empty additionalContext, yielding a VALID validation.
• Checks demonstrated that neither the base nor the head artifacts leaked hookSpecificOutput into the non-context fallback.
• The additionalContext_empty_string_preserved status was observed to flip from false in the base state to true in the head state.
• The helper script artifact documenting the runtime harness used to generate the before/after captures was recorded.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
src/cli/hook-command.ts	Adds buildNoOpResult so early graceful fallback for the context hook emits minimal SessionStart-specific output.
src/cli/adapters/codex.ts	Preserves empty-string additionalContext in Codex output formatting so minimal SessionStart payloads remain schema-valid.
plugin/scripts/worker-service.cjs	Rebuilds the bundled worker with the same no-op fallback and empty-string serialization changes.
tests/hook-command.test.ts	Adds unit coverage for context no-op SessionStart fallback and ensures other hook events retain the bare no-op shape.
tests/hook-lifecycle.test.ts	Adds Codex adapter coverage confirming explicit empty additionalContext is serialized.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
  participant Codex
  participant HookCommand as hookCommand(context)
  participant Adapter as codexAdapter
  participant Handler as contextHandler

  Codex->>HookCommand: SessionStart payload
  HookCommand->>Adapter: normalizeInput(raw)
  alt input rejected before handler runs
    Adapter-->>HookCommand: AdapterRejectedInput
    HookCommand->>HookCommand: buildNoOpResult("context")
    HookCommand->>Adapter: formatOutput(no-op SessionStart)
    Adapter-->>Codex: valid SessionStart no-op JSON
  else input accepted
    Adapter-->>HookCommand: NormalizedHookInput
    HookCommand->>Handler: execute(input)
    Handler-->>HookCommand: HookResult
    HookCommand->>Adapter: formatOutput(result)
    Adapter-->>Codex: formatted hook JSON
  end

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sequenceDiagram
  participant Codex
  participant HookCommand as hookCommand(context)
  participant Adapter as codexAdapter
  participant Handler as contextHandler

  Codex->>HookCommand: SessionStart payload
  HookCommand->>Adapter: normalizeInput(raw)
  alt input rejected before handler runs
    Adapter-->>HookCommand: AdapterRejectedInput
    HookCommand->>HookCommand: buildNoOpResult("context")
    HookCommand->>Adapter: formatOutput(no-op SessionStart)
    Adapter-->>Codex: valid SessionStart no-op JSON
  else input accepted
    Adapter-->>HookCommand: NormalizedHookInput
    HookCommand->>Handler: execute(input)
    Handler-->>HookCommand: HookResult
    HookCommand->>Adapter: formatOutput(result)
    Adapter-->>Codex: formatted hook JSON
  end

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_{Reviews (2): Last reviewed commit: "fix: preserve empty-string additionalCon..." | Re-trigger Greptile}