diff --git a/.claude-plugin/marketplace.json b/.claude-plugin/marketplace.json
index 91db5b7..ae437f2 100644
--- a/.claude-plugin/marketplace.json
+++ b/.claude-plugin/marketplace.json
@@ -9,7 +9,7 @@
     {
       "name": "autodev",
       "description": "Autonomous development workflow skills for coding agents",
-      "version": "6.3.0",
+      "version": "6.3.1",
       "source": "./",
       "author": {
         "name": "Jon Langevin",
diff --git a/.claude-plugin/plugin.json b/.claude-plugin/plugin.json
index 940fae4..d22c713 100644
--- a/.claude-plugin/plugin.json
+++ b/.claude-plugin/plugin.json
@@ -1,7 +1,7 @@
 {
   "name": "autodev",
   "description": "Autonomous development workflow skills for coding agents: design, review, planning, execution, monitoring, and retrospectives",
-  "version": "6.3.0",
+  "version": "6.3.1",
   "author": {
     "name": "Jon Langevin",
     "email": "jon@gocodealone.com"
diff --git a/.cursor-plugin/plugin.json b/.cursor-plugin/plugin.json
index 47f5913..3ec3935 100644
--- a/.cursor-plugin/plugin.json
+++ b/.cursor-plugin/plugin.json
@@ -2,7 +2,7 @@
   "name": "autodev",
   "displayName": "Autonomous Dev Kit",
   "description": "Autonomous development workflow skills for coding agents",
-  "version": "6.3.0",
+  "version": "6.3.1",
   "author": {
     "name": "Jon Langevin",
     "email": "jon@gocodealone.com"
diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 2c3ee1a..4530d1e 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -1,5 +1,23 @@
 # Autonomous Dev Kit Release Notes
 
+## v6.3.1 — 2026-06-01
+
+Bug fix for **#66** — PreCompact hook returned invalid JSON on **Codex**.
+
+- Root cause: `hooks/pre-compact-snapshot` emitted **empty stdout** on its no-locked-plans
+  path (the common case at compaction) and other guard paths. Claude Code tolerates empty
+  PreCompact output; Codex rejects it as "invalid PreCompact hook JSON output". The v6.3.0
+  wrapper recovered JSON behind diagnostics but still emitted nothing for empty output.
+- Fix (defense-in-depth, both invocation paths): every exit path now emits a valid JSON
+  object — `hooks/pre-compact-snapshot` emits a `{}` no-op instead of empty (covers Codex
+  invoking the hook directly), and `hooks/run-hook.cmd` emits `{}` for any empty hook output
+  (covers the wrapper path for every hook). `{}` is a universal no-op on Claude Code and
+  valid JSON for Codex.
+- New regression `tests/hook-contracts.sh::test_pre_compact_snapshot_emits_json_when_no_locked_plans`
+  runs the installed hook the way Codex invokes it (directly, no wrapper) and asserts valid
+  JSON on the no-locked-plans + disabled paths; `tests/hook-stdout-discipline.sh` case (e)
+  asserts the wrapper emits `{}` for empty output. Both CI-gated by `hooks-check.yml`.
+
 ## v6.3.0 — 2026-06-01
 
 Pipeline-hardening release closing seven recurring gate-miss / context-waste issues
diff --git a/docs/plans/2026-06-01-precompact-codex-empty-json-design.md b/docs/plans/2026-06-01-precompact-codex-empty-json-design.md
new file mode 100644
index 0000000..4741e32
--- /dev/null
+++ b/docs/plans/2026-06-01-precompact-codex-empty-json-design.md
@@ -0,0 +1,124 @@
+# Fix: PreCompact invalid-JSON on Codex (#66) — Design
+
+**Status:** Approved (autonomous bug-fix; user-directed; v6.3.1 release after merge)
+**Date:** 2026-06-01
+**Issue:** #66 (regression/uncovered-path of #41)
+**Type:** Bug fix (systematic-debugging root-caused)
+**Adversarial review:** design cycle 1 = FAIL (0C/2I/3m) → resolved this revision (I1 test-inventory + I2 jq-present scoping + m1 schema-vs-syntax + m2 TTY + m3 test-(b) note).
+
+## Root cause
+
+`hooks/pre-compact-snapshot` exits with **empty stdout** on five paths — the common one
+being **no locked plan in `docs/plans/`** (`[ -z "$state_section" ] && exit 0`, line 132),
+plus disabled / TTY-stdin / no-jq / empty-input. Claude Code tolerates empty PreCompact
+output (no-op); **Codex rejects it** ("hook returned invalid PreCompact hook JSON output").
+The v6.3.0 wrapper (#41) routes *non-JSON* to stderr and recovers JSON behind warnings, but
+its empty-output branch emits **nothing** (`: # nothing to emit`) — so the empty path is
+still empty on the wrapper route too. Most sessions at compaction have no autodev locked
+plan (incl. the reporter's workflow-compute session) → the empty path is the *common* case.
+
+Evidence constraint: cannot run Codex here. Conclusion derived from the hook's exit paths +
+the exact error wording (empty ≠ valid JSON) + the common-case match. This is the
+highest-confidence, verifiable-on-source-layout root cause.
+
+## Invariant (the fix)
+
+**Every hook emits a valid JSON object on stdout on every exit path; empty becomes `{}`.**
+`{}` is the minimal valid JSON object and a universal **no-op** across all Claude Code hook
+events (SessionStart=no context, PreToolUse/PostToolUse=no decision→proceed,
+UserPromptSubmit=no context/no block, Stop/SubagentStop=no block, PreCompact=no-op) — so it
+changes no Claude Code behavior — while satisfying Codex's "valid JSON" requirement.
+
+Fix at **two layers** (defense in depth, covers both invocation paths — #66 asks "not only
+the Claude wrapper path", "the installed hook the way Codex invokes it"):
+
+1. **`hooks/pre-compact-snapshot`** — replace each empty `exit 0` (lines 18/20/21/24/132)
+   with `printf '{}\n'; exit 0` (via a `noop_json` helper; `printf` needs no jq, so the
+   no-jq path is covered). The content path (line ~163, after `emit_additional_context`)
+   is unchanged. Covers **direct invocation** (Codex bypassing the wrapper). (Line 20 is the
+   TTY guard — an interactive-only path no host triggers with TTY stdin; adding `noop_json`
+   there is harmless, m2.)
+2. **`hooks/run-hook.cmd`** — the empty-hook-output branch emits `{}` instead of nothing.
+   Covers all hooks on the wrapper path **when jq is present** (the jq-absent branch is a
+   verbatim `exec bash` passthrough — that path is covered for pre-compact-snapshot only,
+   by its own `printf`-based fix; I2). Safe no-op on Claude Code for every event type.
+
+## Tests requiring update (I1 — these existing tests assert empty output)
+
+Three existing `hook-contracts.sh` tests run pre-compact-snapshot on a *no-snapshot*
+scenario and assert `[ -n "$output" ]` is empty; after the fix `output="{}"`, so they must
+change to "no *populated* snapshot" rather than "empty string":
+- `test_scope_lock_complete*` (~line 654): completed (not locked) plan → no snapshot.
+- `pre_compact_ignores_prose_mention_of_locked_status` (~line 1182): draft plan → no snapshot.
+- `pre_compact_ignores_single_workspace_lock_when_session_has_no_lock` (~line 1279).
+
+New assertion (robust to both old-empty and new-`{}`): **fail only if the output contains a
+populated snapshot** —
+```bash
+if printf '%s' "$output" | jq -e '.hookSpecificOutput.additionalContext' >/dev/null 2>&1; then
+  fail "... expected no lock snapshot, got a populated one: $output"
+fi
+```
+(`jq -e .hookSpecificOutput.additionalContext` is false for both `{}` and empty, true only
+for a real snapshot.) The existing with-locked-plan test (asserts the populated
+`hookSpecificOutput`) is unchanged.
+
+`tests/hook-stdout-discipline.sh` test (b) (noise-only fixture) is **unaffected** (noise
+routes via the wrapper's `else` branch, not the empty branch, so stdout stays empty there);
+the new wrapper test is a **distinct case (e): a fixture that emits nothing at all → wrapper
+emits `{}`**.
+
+## Tests (the regression #66 explicitly requests)
+
+- `tests/hook-contracts.sh`: `test_pre_compact_snapshot_emits_json_when_no_locked_plans` —
+  run the hook **directly** (`run_hook`, NOT the wrapper — Codex-style) against a tmp cwd
+  with no `docs/plans/`, assert stdout parses as JSON (`jq -e .`) and is non-empty. Plus the
+  disabled-env path (`SUPERPOWERS_HOOKS_DISABLE=1`) also emits `{}`. The existing
+  with-locked-plan test continues to assert the `hookSpecificOutput` shape.
+- `tests/hook-stdout-discipline.sh`: a fixture that emits empty → the wrapper emits `{}`
+  (valid JSON), not empty.
+- CI: `hooks-check.yml` (added in v6.3.0) gates both on Linux.
+
+## Global Design Guidance
+
+`Guidance: README §Cross-LLM. The contract is host-neutral: stdout must be valid JSON for
+the strictest host (Codex). `{}` is the lowest-common-denominator no-op.`
+
+## Security Review
+
+`{}` carries no directive — a PreToolUse/Stop hook emitting `{}` = no block = proceed,
+identical to today's empty/exit-0 behavior. No block decision is weakened (the v6.3.0
+discipline still recovers an explicit `{"decision":"block"}` behind a warning). No secrets,
+no network.
+
+## Infrastructure Impact
+
+None. Hook + wrapper + tests + version bump (v6.3.0 → v6.3.1, patch). Release via
+`release-tag.yml`.
+
+## Multi-Component Validation
+
+The `test_pre_compact_snapshot_emits_json_when_no_locked_plans` runs the **real** hook the
+way Codex invokes it (direct, no wrapper) — the actual boundary #66 names. The wrapper test
+runs the **real** `run-hook.cmd` against an empty-output fixture.
+
+## Assumptions
+
+| id | assumption | challenge | fallback |
+|---|---|---|---|
+| A1 | Codex rejects empty PreCompact stdout but accepts a valid JSON object | Could reject `{}` specifically if it enforces a strict typed schema | `{}` is the minimal valid object; if Codex needs specific fields, that requires Codex-side repro (documented limitation — I cannot run Codex). Emitting valid JSON is strictly better than empty. **`{}` solves only the "not valid JSON" rejection class — NOT a "wrong schema shape" rejection.** If Codex enforces a typed PreCompact schema that rejects unknown objects, a *different* error will surface; the issue-close note must capture this distinction so the next investigator isn't misled (m1). |
+| A2 | `{}` is a no-op for every Claude Code hook event | Some event might warn on `{}` | All events treat `{}` as "no directives"; the existing with-content tests confirm no regression to the populated path. |
+| A3 | Codex invokes the installed hook (direct and/or via wrapper) | Unknown exact path | Fixed at BOTH layers so either path emits valid JSON. |
+
+## Rollback
+
+Revert the PR; the hook returns to empty-exit, the wrapper to empty-branch. No migration.
+Do not tag v6.3.1 if reverted pre-merge.
+
+## Honesty note (verification-before-completion)
+
+I cannot reproduce on Codex from here. This fix is verified on the **source layout** by the
+new direct-invocation regression (the hook now emits valid JSON on every path, including the
+empty/no-locked-plans case that previously emitted nothing). That removes the
+highest-confidence root cause (empty ≠ valid JSON). Final Codex confirmation belongs to a
+Codex session; the issue close note will state this.
diff --git a/hooks/pre-compact-snapshot b/hooks/pre-compact-snapshot
index 5bc19ef..24178b8 100755
--- a/hooks/pre-compact-snapshot
+++ b/hooks/pre-compact-snapshot
@@ -15,13 +15,19 @@
 
 set -euo pipefail
 
-[ "${SUPERPOWERS_HOOKS_DISABLE:-}" = "1" ] && exit 0
+# #66: every exit path must emit a valid JSON object on stdout. Codex rejects empty
+# PreCompact output ("invalid PreCompact hook JSON output"); Claude Code treats `{}` as a
+# no-op. `printf` needs no jq, so this also covers the jq-absent path. Works whether the
+# hook is invoked directly (Codex) or via run-hook.cmd.
+noop_json() { printf '{}\n'; exit 0; }
 
-[ -t 0 ] && exit 0
-command -v jq >/dev/null 2>&1 || exit 0
+[ "${SUPERPOWERS_HOOKS_DISABLE:-}" = "1" ] && noop_json
+
+[ -t 0 ] && noop_json
+command -v jq >/dev/null 2>&1 || noop_json
 
 hook_input=$(cat || true)
-[ -z "$hook_input" ] && exit 0
+[ -z "$hook_input" ] && noop_json
 
 cwd_dir=$(printf '%s' "$hook_input" | jq -r '.cwd // empty' 2>/dev/null || true)
 [ -z "$cwd_dir" ] && cwd_dir="${PWD}"
@@ -128,8 +134,9 @@ if [ -d "$plans_dir" ]; then
 fi
 
 if [ -z "$state_section" ]; then
-    # No active locked plans found — nothing to snapshot. Exit silently.
-    exit 0
+    # No active locked plans — nothing to snapshot. Emit a `{}` no-op (not empty) so
+    # Codex accepts the PreCompact output (#66). This is the common case.
+    noop_json
 fi
 
 # ── Append to autodev-state file ─────────────────────────────────────────
diff --git a/hooks/run-hook.cmd b/hooks/run-hook.cmd
index 73f5872..5edcfaf 100755
--- a/hooks/run-hook.cmd
+++ b/hooks/run-hook.cmd
@@ -63,7 +63,7 @@ if command -v jq >/dev/null 2>&1; then
   hook_out="$(bash "${SCRIPT_DIR}/${SCRIPT_NAME}" "$@")"
   hook_rc=$?
   if [ -z "$hook_out" ]; then
-    : # nothing to emit
+    printf '{}\n'   # #66: emit a `{}` no-op (not empty) so strict hosts (Codex) accept it
   elif printf '%s' "$hook_out" | jq -e . >/dev/null 2>&1; then
     printf '%s\n' "$hook_out"                       # valid JSON as a whole
   else
diff --git a/tests/hook-contracts.sh b/tests/hook-contracts.sh
index 6defdf3..38c3e89 100755
--- a/tests/hook-contracts.sh
+++ b/tests/hook-contracts.sh
@@ -419,6 +419,24 @@ test_pretool_pr_review_json() {
   assert_hook_context_json "pretool-pr-review-reminder" "PreToolUse" "$output"
 }
 
+test_pre_compact_snapshot_emits_json_when_no_locked_plans() {
+  # #66: Codex rejects empty PreCompact stdout. The no-locked-plans path (common case)
+  # must emit a valid JSON object, not empty. Run the hook DIRECTLY (Codex-style — no wrapper).
+  local tmp; tmp="$(mktemp -d)"
+  trap 'rm -rf "$tmp"' RETURN
+  local out; out="$(run_hook pre-compact-snapshot '{"cwd":"'"$tmp"'"}')"
+  if printf '%s' "$out" | jq -e . >/dev/null 2>&1; then
+    pass "pre-compact-snapshot: emits valid JSON (not empty) with no locked plans (#66)"
+  else
+    fail "pre-compact-snapshot: no-locked-plans must emit valid JSON, got: [$out]"
+  fi
+  # The disabled path must also emit valid JSON (the host still invokes the hook).
+  local outd; outd="$(export SUPERPOWERS_HOOKS_DISABLE=1; run_hook pre-compact-snapshot '{"cwd":"'"$tmp"'"}')"
+  printf '%s' "$outd" | jq -e . >/dev/null 2>&1 \
+    && pass "pre-compact-snapshot: emits valid JSON when disabled (#66)" \
+    || fail "pre-compact-snapshot: disabled path must emit valid JSON, got: [$outd]"
+}
+
 test_pr_reminder_dedup() {
   local tmp; tmp="$(mktemp -d)"
   trap 'rm -rf "$tmp"' RETURN
@@ -650,7 +668,7 @@ PLAN
     return
   fi
   compact_output="$(run_hook pre-compact-snapshot '{"cwd":"'"$tmp"'","transcript_path":"'"$transcript"'"}')"
-  if [ -n "$compact_output" ]; then
+  if printf '%s' "$compact_output" | jq -e '.hookSpecificOutput.additionalContext' >/dev/null 2>&1; then
     fail "scope-lock-complete: expected completed plan to produce no pre-compact lock snapshot, got: ${compact_output}"
     return
   fi
@@ -1176,7 +1194,7 @@ test_pre_compact_ignores_prose_mention_of_locked_status() {
   mkdir -p "$tmp/docs/plans"
   emit_draft_fixture "$tmp/docs/plans/draft.md" "draft"
   output="$(run_hook pre-compact-snapshot '{"cwd":"'"$tmp"'","transcript_path":"'"$transcript"'"}')"
-  if [ -n "$output" ]; then
+  if printf '%s' "$output" | jq -e '.hookSpecificOutput.additionalContext' >/dev/null 2>&1; then
     fail "pre-compact-snapshot: prose mention of Locked status triggered snapshot, output: ${output}"
     return
   fi
@@ -1273,7 +1291,7 @@ test_pre_compact_ignores_single_workspace_lock_when_session_has_no_lock() {
   mkdir -p "$tmp/docs/plans"
   emit_locked_fixture "$tmp/docs/plans/active.md" "active"
   output="$(run_hook pre-compact-snapshot '{"cwd":"'"$tmp"'","transcript_path":"'"$transcript"'"}')"
-  if [ -n "$output" ]; then
+  if printf '%s' "$output" | jq -e '.hookSpecificOutput.additionalContext' >/dev/null 2>&1; then
     fail "pre-compact-snapshot: single workspace lock falsely triggered fallback, output: ${output}"
     return
   fi
@@ -1811,6 +1829,7 @@ test_prompt_strict_uses_session_locked_plan_only
 test_prompt_strict_ignores_prose_mention_of_locked_status
 test_pretool_pr_review_json
 test_pr_reminder_dedup
+test_pre_compact_snapshot_emits_json_when_no_locked_plans
 test_posttool_pr_created_json
 test_pre_compact_snapshot_json
 test_wrapper_suppresses_pre_compact_locale_noise
diff --git a/tests/hook-stdout-discipline.sh b/tests/hook-stdout-discipline.sh
index 196859b..0fcb075 100755
--- a/tests/hook-stdout-discipline.sh
+++ b/tests/hook-stdout-discipline.sh
@@ -10,7 +10,7 @@ command -v jq >/dev/null 2>&1 || { echo "SKIP: jq required for a/b/c"; exit 0; }
 
 tmp="$(mktemp -d)"
 # Cleanup trap set BEFORE any fixture is copied into hooks/ (rm -f on absent files is safe).
-trap 'rm -f "$REPO_ROOT"/hooks/fix-warn-then-json "$REPO_ROOT"/hooks/fix-noise "$REPO_ROOT"/hooks/fix-clean; rm -rf "$tmp"' EXIT
+trap 'rm -f "$REPO_ROOT"/hooks/fix-warn-then-json "$REPO_ROOT"/hooks/fix-noise "$REPO_ROOT"/hooks/fix-clean "$REPO_ROOT"/hooks/fix-empty; rm -rf "$tmp"' EXIT
 mkfix() { printf '%s\n' "$1" > "$tmp/$2"; chmod +x "$tmp/$2"; }
 
 # Fixture A: a warning leaks to stdout, then a block JSON on stdout.
@@ -23,8 +23,11 @@ echo "just a diagnostic line"' fix-noise
 # Fixture C: clean single-line JSON.
 mkfix '#!/usr/bin/env bash
 printf "%s\n" "{\"hookSpecificOutput\":{\"hookEventName\":\"X\"}}"' fix-clean
+# Fixture E: emits NOTHING at all (#66) — wrapper must emit a `{}` no-op, not empty.
+mkfix '#!/usr/bin/env bash
+exit 0' fix-empty
 
-for f in fix-warn-then-json fix-noise fix-clean; do cp "$tmp/$f" "$REPO_ROOT/hooks/$f"; done
+for f in fix-warn-then-json fix-noise fix-clean fix-empty; do cp "$tmp/$f" "$REPO_ROOT/hooks/$f"; done
 
 run() { OUT="$("$WRAPPER" "$1" 2>"$tmp/err")"; RC=$?; ERR="$(cat "$tmp/err")"; }
 
@@ -58,4 +61,10 @@ OUTD="$(PATH="$nojq" "$WRAPPER" fix-warn-then-json 2>/dev/null)"
   && pass "(d) jq-absent → verbatim passthrough (no discipline applied)" \
   || fail "(d) expected verbatim passthrough with jq absent, got: $OUTD"
 
+# (e) empty hook output (#66) → wrapper emits a `{}` no-op (valid JSON), not empty.
+run fix-empty
+printf '%s' "$OUT" | jq -e '. == {}' >/dev/null 2>&1 \
+  && pass "(e) empty hook output → wrapper emits {} no-op (valid JSON)" \
+  || fail "(e) expected {} for empty hook output, got: [$OUT]"
+
 echo ""; echo "Results: $failures failure(s)"; [ "$failures" -eq 0 ]