feat(codex): re-inject the bootstrap after context compaction via SessionStart hook

Codex 0.145 re-fires SessionStart with source:"compact" after every
context compaction, and injects hook stdout into the live model context
(both verified with sentinel probes on codex-cli 0.145.0). Compaction
replaces the transcript with a summary that sheds the using-superpowers
bootstrap and the active skill's instructions; in instrumented SDD runs
the first post-compaction dispatch that lacked a freshly printed
reminder reverted to harness defaults (fork_turns=all, inherited
frontier model) and the drift then self-perpetuated. Claude Code never
exhibits this because its SessionStart matcher (startup|clear|compact)
re-injects the bootstrap at the same moment — this hook restores that
parity on Codex.

On source:"startup" the hook emits nothing: the native plugin path owns
session-start injection, and duplicating it would recreate the
redundancy that led to the original session-start-codex removal. Output
is plain text (Codex consumes raw stdout, unlike the JSON envelopes
hooks/session-start emits for other harnesses), and every failure path
is fail-open: bad stdin, missing skill file, or any error yields empty
output and exit 0 so a hook problem can never break a session.

Ships with hooks-codex.json.example for the user-level ~/.codex/hooks.json
merge and tests covering source filtering, whitespace-tolerant matching,
decoy fields, and fail-open behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/hooks/test-session-start-codex.sh

@@ -0,0 +1,110 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+HOOK_UNDER_TEST="$REPO_ROOT/hooks/session-start-codex"
+EXAMPLE_UNDER_TEST="$REPO_ROOT/hooks/hooks-codex.json.example"
+
+FAILURES=0
+
+pass() {
+    echo "  [PASS] $1"
+}
+
+fail() {
+    echo "  [FAIL] $1"
+    FAILURES=$((FAILURES + 1))
+}
+
+# run_hook <stdin-payload> — echoes hook stdout; fails the calling test on
+# non-zero exit. env -i mirrors the codex hook executor's clean environment.
+run_hook() {
+    printf '%s' "$1" | env -i PATH="${PATH:-}" bash "$HOOK_UNDER_TEST"
+}
+
+echo "Codex SessionStart hook tests"
+
+startup_payload='{"session_id":"s","hook_event_name":"SessionStart","model":"gpt-5.6-terra","source":"startup"}'
+if output="$(run_hook "$startup_payload")" && [ -z "$output" ]; then
+    pass "source=startup emits nothing and exits 0"
+else
+    fail "source=startup emits nothing and exits 0"
+    printf '%s\n' "$output" | head -3 | sed 's/^/    /'
+fi
+
+compact_payload='{"session_id":"s","hook_event_name":"SessionStart","model":"gpt-5.6-terra","source":"compact"}'
+if output="$(run_hook "$compact_payload")"; then
+    ok=1
+    for needle in \
+        "<EXTREMELY_IMPORTANT>" \
+        "You have superpowers." \
+        "name: using-superpowers" \
+        "<CONTEXT_RESTORED>" \
+        "subagent-driven-development/SKILL.md" \
+        "references/codex-tools.md"; do
+        if [[ "$output" != *"$needle"* ]]; then
+            ok=0
+            echo "    missing: $needle"
+        fi
+    done
+    if [ "$ok" -eq 1 ]; then
+        pass "source=compact emits bootstrap plus re-read addendum"
+    else
+        fail "source=compact emits bootstrap plus re-read addendum"
+    fi
+else
+    fail "source=compact emits bootstrap plus re-read addendum (hook exited non-zero)"
+fi
+
+# Whitespace-tolerant source matching (serializers vary).
+spaced_payload='{"hook_event_name":"SessionStart", "source" : "compact"}'
+if output="$(run_hook "$spaced_payload")" && [[ "$output" == *"<CONTEXT_RESTORED>"* ]]; then
+    pass "whitespace around the source key still triggers injection"
+else
+    fail "whitespace around the source key still triggers injection"
+fi
+
+if output="$(printf '' | env -i PATH="${PATH:-}" bash "$HOOK_UNDER_TEST")" && [ -z "$output" ]; then
+    pass "empty stdin fails open to no output, exit 0"
+else
+    fail "empty stdin fails open to no output, exit 0"
+fi
+
+if output="$(run_hook 'not json at all {{{')" && [ -z "$output" ]; then
+    pass "garbage stdin fails open to no output, exit 0"
+else
+    fail "garbage stdin fails open to no output, exit 0"
+fi
+
+# A compact mention inside some other field must not trigger injection.
+decoy_payload='{"hook_event_name":"SessionStart","source":"startup","cwd":"/tmp/compact"}'
+if output="$(run_hook "$decoy_payload")" && [ -z "$output" ]; then
+    pass "compact appearing outside the source field does not trigger"
+else
+    fail "compact appearing outside the source field does not trigger"
+fi
+
+if node -e '
+const example = JSON.parse(require("fs").readFileSync(process.argv[1], "utf8"));
+const entry = example.hooks.SessionStart[0].hooks[0];
+if (entry.type !== "command") {
+  console.error(`example hook type is ${JSON.stringify(entry.type)}, expected "command"`);
+  process.exit(1);
+}
+if (!/session-start-codex"$/.test(entry.command)) {
+  console.error(`unexpected example command shape: ${entry.command}`);
+  process.exit(1);
+}
+' "$EXAMPLE_UNDER_TEST"; then
+    pass "hooks-codex.json.example parses and invokes session-start-codex"
+else
+    fail "hooks-codex.json.example parses and invokes session-start-codex"
+fi
+
+if [[ "$FAILURES" -gt 0 ]]; then
+    echo "STATUS: FAILED ($FAILURES failure(s))"
+    exit 1
+fi
+
+echo "STATUS: PASSED"