diff --git a/.githooks/commit-msg b/.githooks/commit-msg
new file mode 100755
index 0000000..d3205e1
--- /dev/null
+++ b/.githooks/commit-msg
@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Enforce Conventional Commits on the first line of the commit message.
+# https://www.conventionalcommits.org/en/v1.0.0/
+#
+# Activate per clone with:
+#   git config core.hooksPath .githooks
+
+set -euo pipefail
+
+msg_file="${1:?commit-msg: missing message file path}"
+first_line="$(awk 'NR==1{print; exit}' "$msg_file")"
+
+case "$first_line" in
+  "Merge "*|"Revert "*|"fixup! "*|"squash! "*|"amend! "*) exit 0 ;;
+esac
+
+pattern='^(feat|fix|docs|style|refactor|perf|test|build|ci|chore|revert)(\([a-z0-9._-]+\))?!?: .+'
+
+if ! printf '%s' "$first_line" | grep -qE "$pattern"; then
+  printf 'commit-msg: aborting — message does not follow Conventional Commits.\n' >&2
+  printf '  expected: <type>[(<scope>)][!]: <description>\n' >&2
+  printf '  types:    feat, fix, docs, style, refactor, perf, test, build, ci, chore, revert\n' >&2
+  printf '  got:      %s\n' "$first_line" >&2
+  exit 1
+fi
diff --git a/CLAUDE.md b/CLAUDE.md
index 5c65203..8baa7ab 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -51,6 +51,11 @@ Code skills used while working in this repo) — don't conflate them.
 - Research notes live in `docs/research/`.
 - Low dependencies by default. The project is bash-first; ask before adding new
   tools, runtimes, or package managers.
+- Commit messages follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/):
+  `<type>[(scope)][!]: <description>`, where `<type>` is one of `feat`, `fix`,
+  `docs`, `style`, `refactor`, `perf`, `test`, `build`, `ci`, `chore`, `revert`.
+  A `commit-msg` hook in `.githooks/` enforces this. Activate it once per clone
+  with `git config core.hooksPath .githooks`.
 
 ## Intended design