diff --git a/lib/memory-commit.sh b/lib/memory-commit.sh
index 4e109e4..52b7065 100755
--- a/lib/memory-commit.sh
+++ b/lib/memory-commit.sh
@@ -10,6 +10,10 @@
 #   memory-commit.sh pending             # exit 0 if memory/tasks have changes, 1 if clean
 #   memory-commit.sh commit "<message>"  # surgical commit; exit 0 ok/no-op, 3 unsafe state
 #
+# Output contract for `commit`: diagnostics go to stderr; on a real commit the
+# short hash of the MEMORY commit is the ONLY thing on stdout (empty on no-op or
+# unsafe), so callers can capture it: `mem_hash=$(memory-commit.sh commit "msg")`.
+#
 # Sourceable: `memory_pending` and `commit_memory` for the v2 hook.
 
 set -uo pipefail
@@ -52,20 +56,21 @@ memory_pending() {
 }
 
 # Surgical commit of the scoped paths only. Returns 0 (ok or no-op), 3 (unsafe).
+# On a real commit, prints the memory-commit short hash to stdout (stderr = diag).
 commit_memory() {
   local msg="${1:?commit message required}"
   _in_git_repo || {
-    echo "memory-commit: not a git repo — skip"
+    echo "memory-commit: not a git repo — skip" >&2
     return 3
   }
   if _unsafe_state; then
-    echo "memory-commit: detached HEAD or merge/rebase in progress — skip (no commit)"
+    echo "memory-commit: detached HEAD or merge/rebase in progress — skip (no commit)" >&2
     return 3
   fi
   local changed
   mapfile -t changed < <(_changed_paths)
   if [ "${#changed[@]}" -eq 0 ]; then
-    echo "memory-commit: nothing pending — no-op"
+    echo "memory-commit: nothing pending — no-op" >&2
     return 0
   fi
   # Re-stage working-tree content of the scoped paths over any stale index entry,
@@ -73,10 +78,13 @@ commit_memory() {
   # commit: other staged files (dangling code) are not recorded.
   git add -- "${changed[@]}"
   if git diff --cached --quiet -- "${changed[@]}"; then
-    echo "memory-commit: only ignored/no-op changes — no-op"
+    echo "memory-commit: only ignored/no-op changes — no-op" >&2
     return 0
   fi
-  git commit -m "$msg" -- "${changed[@]}"
+  # Contract: diagnostics go to stderr; on success ONLY the memory-commit short
+  # hash goes to stdout, so a caller can do `mem_hash=$(... commit "msg")`.
+  git commit -q -m "$msg" -- "${changed[@]}"
+  git rev-parse --short HEAD
 }
 
 main() {
diff --git a/lib/tests/run-deterministic.sh b/lib/tests/run-deterministic.sh
index 25c4c3b..1a5a78a 100755
--- a/lib/tests/run-deterministic.sh
+++ b/lib/tests/run-deterministic.sh
@@ -110,6 +110,37 @@ printf '    committed: [%s]\n' "$committed"
 if printf '%s\n' "$committed" | grep -q '^.claude/tasks/TODO.md$'; then ok "TODO.md embarked"; else ko "TODO.md not embarked"; fi
 rm -rf "$R"
 
+echo "T6 — stdout contract: commit→hash, no-op→empty, unsafe→empty"
+R="$(new_repo)"
+printf 'CHG\n' >>"$R/.claude/memory/decisions.md"
+out="$( (cd "$R" && "$HELPER" commit "chore(memory): T6") 2>/dev/null )"
+expected="$(git -C "$R" rev-parse --short HEAD)"
+printf '    commit  stdout=[%s] HEAD=[%s]\n' "$out" "$expected"
+if [ -n "$out" ] && [ "$out" = "$expected" ]; then ok "commit emits the memory-commit hash on stdout"; else ko "hash mismatch [$out] != [$expected]"; fi
+out="$( (cd "$R" && "$HELPER" commit "chore(memory): T6-noop") 2>/dev/null )"
+printf '    no-op   stdout=[%s]\n' "$out"
+if [ -z "$out" ]; then ok "no-op emits nothing on stdout"; else ko "no-op leaked stdout [$out]"; fi
+printf 'CHG2\n' >>"$R/.claude/memory/decisions.md"
+: >"$R/.git/MERGE_HEAD"
+out="$( (cd "$R" && "$HELPER" commit "chore(memory): T6-unsafe") 2>/dev/null )"
+rc=$?
+printf '    unsafe  stdout=[%s] exit=%s\n' "$out" "$rc"
+if [ -z "$out" ] && [ "$rc" -eq 3 ]; then ok "unsafe emits nothing on stdout, exit 3"; else ko "unsafe leaked stdout [$out] or rc $rc"; fi
+rm -rf "$R"
+
+echo "T7 — double run: at most one commit (real run, not by construction)"
+R="$(new_repo)"
+printf 'ONCE\n' >>"$R/.claude/memory/decisions.md"
+base="$(git -C "$R" rev-list --count HEAD)"
+h1="$( (cd "$R" && "$HELPER" commit "chore(memory): T7-run1") 2>/dev/null )"
+after1="$(git -C "$R" rev-list --count HEAD)"
+h2="$( (cd "$R" && "$HELPER" commit "chore(memory): T7-run2") 2>/dev/null )"
+after2="$(git -C "$R" rev-list --count HEAD)"
+printf '    counts base=%s after1=%s after2=%s ; h1=[%s] h2=[%s]\n' "$base" "$after1" "$after2" "$h1" "$h2"
+if [ "$after1" -eq "$((base + 1))" ] && [ -n "$h1" ]; then ok "run1 created exactly one commit (hash emitted)"; else ko "run1 commit count wrong"; fi
+if [ "$after2" -eq "$after1" ] && [ -z "$h2" ]; then ok "run2 is a no-op (no 2nd commit, empty stdout)"; else ko "run2 was not a no-op"; fi
+rm -rf "$R"
+
 echo
 printf 'RESULT: %d passed, %d failed\n' "$PASS" "$FAIL"
 [ "$FAIL" -eq 0 ]