diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8fc5686..150b09c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,6 +7,17 @@ on:
     branches: [main]
 
 jobs:
+  commit-format:
+    name: Commit Format
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Validate pull request title
+        run: bash ./scripts/commit-msg.sh --title "${{ github.event.pull_request.title }}"
+
   test:
     name: Test
     runs-on: ubuntu-latest
diff --git a/Makefile b/Makefile
index 088be01..77529ca 100644
--- a/Makefile
+++ b/Makefile
@@ -75,10 +75,13 @@ help:
 	@echo "  check         - Run tests and lint"
 	@echo "  install       - Build and install to Go bin directory"
 	@echo "  calibrate-providers - Compare local Claude/Codex session usage for calibration"
-	@echo "  install-hooks  - Install git pre-commit hook"
+	@echo "  install-hooks - Install git pre-commit and commit-msg hooks"
 	@echo "  help          - Show this help"
 
-# Install git pre-commit hook
+# Install git pre-commit and commit-msg hooks
 install-hooks:
+	@mkdir -p .git/hooks
 	@ln -sf ../../scripts/pre-commit.sh .git/hooks/pre-commit
-	@echo "✓ pre-commit hook installed (.git/hooks/pre-commit → scripts/pre-commit.sh)"
+	@ln -sf ../../scripts/commit-msg.sh .git/hooks/commit-msg
+	@echo "✓ pre-commit hook installed (.git/hooks/pre-commit -> scripts/pre-commit.sh)"
+	@echo "✓ commit-msg hook installed (.git/hooks/commit-msg -> scripts/commit-msg.sh)"
diff --git a/README.md b/README.md
index 84f92cd..6eb1e2d 100644
--- a/README.md
+++ b/README.md
@@ -258,20 +258,47 @@ Each task has a default cooldown interval to prevent the same task from running
 
 ## Development
 
-### Pre-commit hooks
+### Git hooks and commit messages
 
-Install the git pre-commit hook to catch formatting and vet issues before pushing:
+Install the local git hooks before pushing:
 
 ```bash
 make install-hooks
 ```
 
-This symlinks `scripts/pre-commit.sh` into `.git/hooks/pre-commit`. The hook runs:
+This symlinks `scripts/pre-commit.sh` into `.git/hooks/pre-commit` and `scripts/commit-msg.sh` into `.git/hooks/commit-msg`.
+
+The `pre-commit` hook runs:
 - **gofmt** — flags any staged `.go` files that need formatting
 - **go vet** — catches common correctness issues
 - **go build** — ensures the project compiles
 
-To bypass in a pinch: `git commit --no-verify`
+The `commit-msg` hook validates the first non-comment line of each commit message. Use Conventional Commits:
+- `type: summary`
+- `type(scope): summary`
+
+Accepted types:
+- `build`
+- `chore`
+- `ci`
+- `docs`
+- `feat`
+- `fix`
+- `perf`
+- `refactor`
+- `style`
+- `test`
+
+Examples:
+- `feat(run): add pause command`
+- `fix(config): preserve provider YAML keys`
+- `docs(readme): explain hook installation`
+
+Git-generated `Merge ...` and `Revert ...` subjects are allowed automatically.
+
+Pull request titles are validated in CI with the same script so squash-merge commits on `main` stay consistent even when local hooks are skipped.
+
+To bypass local hooks in a pinch: `git commit --no-verify`
 
 ## Uninstalling
 
diff --git a/scripts/commit-msg.sh b/scripts/commit-msg.sh
new file mode 100755
index 0000000..6963709
--- /dev/null
+++ b/scripts/commit-msg.sh
@@ -0,0 +1,89 @@
+#!/usr/bin/env bash
+# commit-msg hook for nightshift
+# Install: make install-hooks
+set -euo pipefail
+
+readonly CONVENTIONAL_TYPES='build|chore|ci|docs|feat|fix|perf|refactor|style|test'
+readonly CONVENTIONAL_PATTERN="^(${CONVENTIONAL_TYPES})(\\([[:alnum:]./_-]+\\))?: [^[:space:]].*"
+
+usage() {
+  echo "Usage: scripts/commit-msg.sh <commit-msg-file> | scripts/commit-msg.sh --title \"<subject>\"" >&2
+}
+
+read_subject() {
+  if [[ $# -eq 2 && "$1" == "--title" ]]; then
+    awk '
+      {
+        line = $0
+        sub(/\r$/, "", line)
+        sub(/^[[:space:]]+/, "", line)
+        sub(/[[:space:]]+$/, "", line)
+        if (line != "" && line !~ /^#/) {
+          print line
+          exit
+        }
+      }
+    ' <<<"$2"
+    return
+  fi
+
+  if [[ $# -eq 1 ]]; then
+    awk '
+      {
+        line = $0
+        sub(/\r$/, "", line)
+        sub(/^[[:space:]]+/, "", line)
+        sub(/[[:space:]]+$/, "", line)
+        if (line != "" && line !~ /^#/) {
+          print line
+          exit
+        }
+      }
+    ' "$1"
+    return
+  fi
+
+  usage
+  exit 2
+}
+
+print_failure() {
+  cat >&2 <<'EOF'
+Commit subject must use Conventional Commits:
+  type: summary
+  type(scope): summary
+
+Accepted types: build, chore, ci, docs, feat, fix, perf, refactor, style, test
+Allowed exceptions: Merge ..., Revert ...
+
+Examples:
+  feat(run): add pause command
+  fix(config): preserve provider YAML keys
+  docs(readme): explain hook installation
+EOF
+}
+
+if [[ $# -eq 1 && ( "$1" == "-h" || "$1" == "--help" ) ]]; then
+  usage
+  exit 0
+fi
+
+subject="$(read_subject "$@")"
+
+if [[ -z "$subject" ]]; then
+  echo "Commit subject is empty." >&2
+  print_failure
+  exit 1
+fi
+
+if [[ "$subject" =~ ^(Merge|Revert)\  ]]; then
+  exit 0
+fi
+
+if [[ "$subject" =~ $CONVENTIONAL_PATTERN ]]; then
+  exit 0
+fi
+
+echo "Invalid commit subject: $subject" >&2
+print_failure
+exit 1