fix(hooks): address Codex review — tighten codec to the reference schemas, preserve stdout

Codex's PR-E review found three protocol-fidelity blockers + two doc gaps, all
verified against ~/repos/refs:

- (A) Top-level `decision` accepted allow/deny/ask, but both reference schemas
  reserve those for hookSpecificOutput.permissionDecision — the legacy top-level
  decision is approve/block ONLY. Split topLevelDecisionOf (approve/block) from
  permissionDecisionOf (allow/deny/ask), so an out-of-band {"decision":"deny"} is
  now invalid and ignored instead of becoming a real blocking decision.
- (A) hookSpecificOutput was parsed without its hookEventName discriminator.
  HookOutput now surfaces hookEventName so a bridge can discard a block whose
  claimed event doesn't match the firing one (the schemas key the block by event).
- (A) runHook discarded raw stdout. HookOutput now carries `stdout` (trimmed,
  verbatim) so a bridge can reproduce CC's plain-stdout rendering / Codex's
  plain-stdout-as-additionalContext behavior.
- (B) hook/* SessionEventMap variants were only named in prose; added a payload/role
  table to core-data-structures/session.md (a maintained catalog surface).
- (B) Removed PR-stack-position references (PR-F / "future bridge packages") from a
  test comment and the RFC, per the current-state-wording rule.

New codec tests: top-level allow/deny/ask invalid+ignored, hookEventName capture,
raw stdout preserved on plain + JSON + empty stdout. 51 tests, per-file 100%.

packages/hooks/hook-protocol/src/codec.ts

@@ -42,14 +42,19 @@ function obj(value: unknown): Record<string, unknown> | undefined {
     : undefined
 }
 
-/** Normalize a raw `decision`/`permissionDecision` string to the neutral enum. */
-function decisionOf(value: string | undefined): HookOutput['decision'] {
-  switch (value) {
-    case 'approve': case 'allow': case 'block': case 'deny': case 'ask':
-      return value
-    default:
-      return undefined
-  }
+/**
+ * The legacy TOP-LEVEL `decision` is only `approve`/`block` in both reference
+ * schemas — `allow`/`deny`/`ask` are reserved for `hookSpecificOutput.
+ * permissionDecision`. So an out-of-band `{"decision":"deny"}` is invalid and
+ * ignored here (it must not become a real blocking decision).
+ */
+function topLevelDecisionOf(value: string | undefined): HookOutput['decision'] {
+  return value === 'approve' || value === 'block' ? value : undefined
+}
+
+/** A `hookSpecificOutput.permissionDecision` is `allow`/`deny`/`ask` only. */
+function permissionDecisionOf(value: string | undefined): HookOutput['decision'] {
+  return value === 'allow' || value === 'deny' || value === 'ask' ? value : undefined
 }
 
 /**
@@ -62,7 +67,11 @@ function decisionOf(value: string | undefined): HookOutput['decision'] {
  */
 export function parseHookOutput(exitCode: number | undefined, stdout: string, stderr: string): HookOutput {
   const trimmedErr = stderr.trim()
-  const output: HookOutput = { exitCode, stderr: trimmedErr }
+  const trimmedOut = stdout.trim()
+  // Keep the raw stdout verbatim: a clean-exit hook may emit PLAIN text the
+  // protocol renders/uses (CC output; Codex SessionStart/UserPromptSubmit
+  // additionalContext), so the bridge needs it even when there's no JSON.
+  const output: HookOutput = { exitCode, stderr: trimmedErr, stdout: trimmedOut }
 
   // Exit 2 is a blocking error in both dialects: stderr is the reason. Surface
   // it as a `block` decision so the bridge maps it uniformly with a structured
@@ -76,7 +85,6 @@ export function parseHookOutput(exitCode: number | undefined, stdout: string, st
   // the stderr channel is authoritative. A non-zero/undefined exit other than 2
   // carries no decision (the bridge records it as a non-blocking error).
   if (exitCode === 0) {
-    const trimmedOut = stdout.trim()
     // Only attempt JSON when stdout looks like a JSON object — matches the
     // reference engines, which treat other stdout as plain text, not an error.
     if (trimmedOut.startsWith('{')) {
@@ -106,18 +114,23 @@ function applyStructured(output: HookOutput, parsed: Record<string, unknown>): v
   const sysMsg = str(parsed, 'systemMessage')
   if (sysMsg !== undefined) output.systemMessage = sysMsg
 
-  // Top-level legacy `decision` + `reason` (CC approve/block; Codex block).
-  const topDecision = decisionOf(str(parsed, 'decision'))
+  // Top-level legacy `decision` (approve/block ONLY — allow/deny/ask there are
+  // invalid per both schemas) + its `reason`.
+  const topDecision = topLevelDecisionOf(str(parsed, 'decision'))
   if (topDecision !== undefined) output.decision = topDecision
   const topReason = str(parsed, 'reason')
   if (topReason !== undefined) output.reason = topReason
 
-  // hookSpecificOutput: the per-event channel. permissionDecision (allow/deny/
-  // ask) OVERRIDES the legacy top-level decision when present; additionalContext
-  // and updatedInput live here too.
+  // hookSpecificOutput: the per-event channel, keyed by `hookEventName`. We
+  // surface that discriminator so the bridge can DISCARD a block whose event
+  // doesn't match the firing one (the schemas make it the discriminator). The
+  // permissionDecision (allow/deny/ask) OVERRIDES the legacy top-level decision;
+  // additionalContext and updatedInput live here too.
   const hso = obj(parsed.hookSpecificOutput)
   if (hso) {
-    const permission = decisionOf(str(hso, 'permissionDecision'))
+    const eventName = str(hso, 'hookEventName')
+    if (eventName !== undefined) output.hookEventName = eventName
+    const permission = permissionDecisionOf(str(hso, 'permissionDecision'))
     if (permission !== undefined) output.decision = permission
     const permissionReason = str(hso, 'permissionDecisionReason')
     if (permissionReason !== undefined) output.reason = permissionReason

packages/hooks/hook-protocol/src/types.ts

@@ -100,6 +100,14 @@ export interface HookOutput {
   exitCode: number | undefined
   /** Trimmed stderr — the block-reason source on a blocking (exit 2) hook. */
   stderr: string
+  /**
+   * Trimmed stdout, verbatim. On a clean exit a hook may emit PLAIN (non-JSON)
+   * stdout that the protocol renders as output (CC) or treats as
+   * `additionalContext` (Codex SessionStart/UserPromptSubmit) — so the bridge
+   * needs the raw text, not just the parsed structured fields. Empty string when
+   * the hook produced no stdout.
+   */
+  stdout: string
   /**
    * `false` ⇒ the hook asked to halt (CC/Codex `continue:false`); pairs with
    * {@link stopReason}. `true`/absent ⇒ proceed.
@@ -110,14 +118,26 @@ export interface HookOutput {
   /** Hide the hook's stdout from the transcript (CC `suppressOutput`). */
   suppressOutput?: boolean
   /**
-   * The blocking decision a hook expressed via structured stdout (CC's
-   * `decision` / `hookSpecificOutput.permissionDecision`): `'block'`/`'deny'`
-   * forbid the action, `'approve'`/`'allow'` permit it, `'ask'` requests
-   * confirmation. Absent ⇒ no explicit decision (exit code governs).
+   * The neutral blocking decision a hook expressed, folded from the two channels
+   * the reference protocols keep DISTINCT: the legacy top-level `decision`
+   * (`approve`/`block` only) and `hookSpecificOutput.permissionDecision`
+   * (`allow`/`deny`/`ask`). We normalize them to one enum — `'block'`/`'deny'`
+   * forbid, `'approve'`/`'allow'` permit, `'ask'` requests confirmation — but
+   * `'allow'`/`'deny'`/`'ask'` arise ONLY from a `permissionDecision`, never from
+   * a top-level `decision` (an out-of-band `{"decision":"deny"}` is invalid and
+   * ignored, matching the schemas). Absent ⇒ no explicit decision (exit code governs).
    */
   decision?: 'approve' | 'allow' | 'block' | 'deny' | 'ask'
   /** The reason/explanation accompanying {@link decision}. */
   reason?: string
+  /**
+   * The `hookSpecificOutput.hookEventName` discriminator, when the hook emitted
+   * a `hookSpecificOutput` block. The reference schemas key that block by event;
+   * a bridge compares this to the firing event and DISCARDS a mismatched block
+   * (a hook claiming `PreToolUse` output on a `Stop` event is malformed). Absent
+   * when the hook emitted no `hookSpecificOutput`.
+   */
+  hookEventName?: string
   /** Extra context to inject for the next model request (CC `additionalContext`). */
   additionalContext?: string
   /** A warning surfaced to the user (CC `systemMessage`). */