diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8fc5686..ab6fa7b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,6 +7,32 @@ on:
     branches: [main]
 
 jobs:
+  commit-messages:
+    name: Commit Messages
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Validate commit messages for pull requests
+        if: github.event_name == 'pull_request'
+        run: scripts/commit-msg.sh --rev-range "${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}"
+
+      - name: Validate commit messages for pushes
+        if: github.event_name == 'push'
+        env:
+          BEFORE_SHA: ${{ github.event.before }}
+          CURRENT_SHA: ${{ github.sha }}
+        run: |
+          if [ "$BEFORE_SHA" = "0000000000000000000000000000000000000000" ]; then
+            rev_spec="$CURRENT_SHA"
+          else
+            rev_spec="$BEFORE_SHA..$CURRENT_SHA"
+          fi
+          scripts/commit-msg.sh --rev-range "$rev_spec"
+
   test:
     name: Test
     runs-on: ubuntu-latest
diff --git a/Makefile b/Makefile
index 088be01..f25b7dd 100644
--- a/Makefile
+++ b/Makefile
@@ -75,10 +75,12 @@ 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 hooks
 install-hooks:
 	@ln -sf ../../scripts/pre-commit.sh .git/hooks/pre-commit
+	@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..24a7b76 100644
--- a/README.md
+++ b/README.md
@@ -258,20 +258,60 @@ Each task has a default cooldown interval to prevent the same task from running
 
 ## Development
 
-### Pre-commit hooks
+### Git hooks
 
-Install the git pre-commit hook to catch formatting and vet issues before pushing:
+Install local git hooks to catch build issues and invalid commit subjects before pushing:
 
 ```bash
 make install-hooks
 ```
 
-This symlinks `scripts/pre-commit.sh` into `.git/hooks/pre-commit`. The hook runs:
-- **gofmt** — flags any staged `.go` files that need formatting
-- **go vet** — catches common correctness issues
-- **go build** — ensures the project compiles
+This symlinks:
+- `scripts/pre-commit.sh` into `.git/hooks/pre-commit`
+- `scripts/commit-msg.sh` into `.git/hooks/commit-msg`
 
-To bypass in a pinch: `git commit --no-verify`
+The hooks run:
+- **pre-commit** — `gofmt`, `go vet`, and `go build`
+- **commit-msg** — validates the first line of each new commit message
+
+### Commit message format
+
+Nightshift standardizes future commit subjects only. Existing history stays as-is.
+
+Required format:
+
+```text
+type(scope?): summary
+```
+
+Allowed types:
+- `build`
+- `chore`
+- `ci`
+- `docs`
+- `feat`
+- `fix`
+- `perf`
+- `refactor`
+- `revert`
+- `style`
+- `test`
+
+Valid examples:
+- `feat(cli): add commit message validator`
+- `fix: handle pull request commit ranges in CI`
+- `docs(readme): document local hook installation`
+
+These exception subjects remain valid:
+- `Merge pull request #17 from owner/branch`
+- `Revert "feat(cli): add commit message validator"`
+- `Bump version to v0.3.4`
+- `Release v0.3.4: changelog and binaries`
+
+If the `commit-msg` hook fails:
+- Amend the subject with `git commit --amend -m "fix(cli): normalize commit message validation"`
+- Re-run the validator with `scripts/commit-msg.sh .git/COMMIT_EDITMSG`
+- Bypass the hooks in a pinch with `git commit --no-verify`
 
 ## Uninstalling
 
diff --git a/scripts/commit-msg.sh b/scripts/commit-msg.sh
new file mode 100755
index 0000000..c66526f
--- /dev/null
+++ b/scripts/commit-msg.sh
@@ -0,0 +1,141 @@
+#!/usr/bin/env bash
+# commit-msg hook and CI validator for Nightshift
+# Install: make install-hooks  (or: ln -sf ../../scripts/commit-msg.sh .git/hooks/commit-msg)
+set -euo pipefail
+
+readonly ALLOWED_TYPES='build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test'
+readonly CONVENTIONAL_PATTERN="^(${ALLOWED_TYPES})(\\([a-z0-9][a-z0-9._/-]*\\))?(!)?: [^ ].*$"
+readonly MERGE_PATTERN='^Merge '
+readonly REVERT_PATTERN='^Revert ".+"$'
+readonly VERSION_BUMP_PATTERN='^Bump version to v[0-9]+(\.[0-9]+)+([.-][0-9A-Za-z.-]+)?$'
+readonly RELEASE_PATTERN='^Release v[0-9]+(\.[0-9]+)+([.-][0-9A-Za-z.-]+)?(: .+)?$'
+
+usage() {
+  cat <<'EOF'
+Usage:
+  scripts/commit-msg.sh <commit-message-file>
+  scripts/commit-msg.sh --rev-range <git-revision-or-range>
+
+Expected subject format:
+  type(scope?): summary
+
+Allowed types:
+  build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test
+EOF
+}
+
+print_error() {
+  local source="$1"
+  local subject="$2"
+
+  {
+    echo "Invalid commit subject in ${source}:"
+    echo "  ${subject}"
+    echo ""
+    echo "Expected first line to match:"
+    echo "  type(scope?): summary"
+    echo ""
+    echo "Allowed types:"
+    echo "  build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test"
+    echo ""
+    echo "Examples:"
+    echo "  feat(cli): add commit message validator"
+    echo "  fix: handle pull request commit ranges in CI"
+    echo "  docs(readme): document local hook installation"
+    echo ""
+    echo "Allowed exceptions:"
+    echo "  Merge pull request #123 from owner/branch"
+    echo "  Revert \"feat(cli): add commit message validator\""
+    echo "  Bump version to v0.3.4"
+    echo "  Release v0.3.4: changelog and binaries"
+  } >&2
+}
+
+is_valid_subject() {
+  local subject="$1"
+
+  [[ "$subject" =~ $CONVENTIONAL_PATTERN ]] && return 0
+  [[ "$subject" =~ $MERGE_PATTERN ]] && return 0
+  [[ "$subject" =~ $REVERT_PATTERN ]] && return 0
+  [[ "$subject" =~ $VERSION_BUMP_PATTERN ]] && return 0
+  [[ "$subject" =~ $RELEASE_PATTERN ]] && return 0
+  return 1
+}
+
+validate_subject() {
+  local source="$1"
+  local subject="$2"
+
+  if ! is_valid_subject "$subject"; then
+    print_error "$source" "$subject"
+    return 1
+  fi
+}
+
+validate_message_file() {
+  local message_file="$1"
+  local subject
+
+  if [[ ! -f "$message_file" ]]; then
+    echo "Commit message file not found: $message_file" >&2
+    return 1
+  fi
+
+  subject=$(sed -n '1{s/[[:space:]]*$//;p;}' "$message_file")
+  validate_subject "$message_file" "$subject"
+}
+
+validate_rev_range() {
+  local rev_spec="$1"
+  local commits=()
+  local rev
+  local subject
+
+  if [[ "$rev_spec" == *..* ]]; then
+    while IFS= read -r rev; do
+      commits+=("$rev")
+    done < <(git rev-list --reverse "$rev_spec")
+  else
+    commits=("$rev_spec")
+  fi
+
+  if [[ ${#commits[@]} -eq 0 ]]; then
+    echo "No commits found for revision range: $rev_spec" >&2
+    return 1
+  fi
+
+  for rev in "${commits[@]}"; do
+    subject=$(git log -1 --format=%s "$rev")
+    validate_subject "$rev" "$subject"
+    printf '✓ %s %s\n' "${rev:0:7}" "$subject"
+  done
+}
+
+main() {
+  case "${1-}" in
+    "")
+      usage >&2
+      exit 1
+      ;;
+    -h|--help)
+      usage
+      exit 0
+      ;;
+    --rev-range)
+      if [[ $# -ne 2 ]]; then
+        usage >&2
+        exit 1
+      fi
+      validate_rev_range "$2"
+      ;;
+    *)
+      if [[ $# -ne 1 ]]; then
+        usage >&2
+        exit 1
+      fi
+      validate_message_file "$1"
+      ;;
+  esac
+}
+
+main "$@"