diff --git a/prek.toml b/prek.toml
index 33cdded..ea06e70 100644
--- a/prek.toml
+++ b/prek.toml
@@ -1,7 +1,7 @@
 #:schema https://www.schemastore.org/prek.json
 
 default_install_hook_types = ["pre-commit", "commit-msg"]
-# Local hooks default to the pre-commit stage; the versioner opts into
+# Local hooks default to the pre-commit stage; the commit lint opts into
 # commit-msg explicitly.
 default_stages = ["pre-commit"]
 
@@ -33,10 +33,12 @@ entry = "tofu fmt -check -diff"
 language = "system"
 files = '\.tf$'
 
+# Read-only: version bumping happens at release time (npm run bump), not per
+# commit, so parallel worktrees stop conflicting on the version files.
 [[repos.hooks]]
-id = "versioner"
-name = "conventional commits versioner"
-entry = "node scripts/version-bump.mjs"
+id = "commit-lint"
+name = "conventional commit header lint"
+entry = "node scripts/lint-commit-msg.mjs"
 language = "system"
 stages = ["commit-msg"]
 
diff --git a/scripts/conventional.mjs b/scripts/conventional.mjs
new file mode 100644
index 0000000..2157114
--- /dev/null
+++ b/scripts/conventional.mjs
@@ -0,0 +1,17 @@
+// Conventional Commits parsing shared by the commit-msg lint
+// (lint-commit-msg.mjs) and the release-time versioner (version-bump.mjs).
+
+export const TYPES = ['build', 'chore', 'ci', 'docs', 'feat', 'fix', 'perf', 'refactor', 'revert', 'style', 'test']
+export const HEADER = new RegExp(`^(${TYPES.join('|')})(\\([^()]+\\))?(!)?: .+`)
+
+// Messages git writes for us keep whatever shape git gave them.
+export const GIT_SHAPES = /^(Merge |Revert |fixup! |squash! |amend! )/
+
+// Semver bump for one already-matched header: breaking -> major, feat ->
+// minor, fix/perf -> patch, anything else -> null. `lines` is the whole
+// message, for BREAKING CHANGE footers.
+export function bumpFor(header, lines) {
+  const [, type, , bang] = HEADER.exec(header)
+  const breaking = bang === '!' || lines.some((line) => /^BREAKING[- ]CHANGE: /.test(line))
+  return breaking ? 'major' : type === 'feat' ? 'minor' : type === 'fix' || type === 'perf' ? 'patch' : null
+}
diff --git a/scripts/lint-commit-msg.mjs b/scripts/lint-commit-msg.mjs
new file mode 100644
index 0000000..52657af
--- /dev/null
+++ b/scripts/lint-commit-msg.mjs
@@ -0,0 +1,31 @@
+// Conventional Commits lint (prek commit-msg stage): reject a commit whose
+// header is not a Conventional Commit. Read-only — version bumping happens at
+// release time (scripts/version-bump.mjs), not per commit, so parallel
+// worktrees stop contending on package.json and the manifest.
+import { readFileSync } from 'node:fs'
+import { GIT_SHAPES, HEADER, TYPES } from './conventional.mjs'
+
+const SCISSORS = '# ------------------------ >8 ------------------------'
+
+const msgFile = process.argv[2]
+if (!msgFile) {
+  console.error('usage: lint-commit-msg.mjs <commit-msg-file>')
+  process.exit(2)
+}
+
+// Keep the message above the scissors line (git commit -v appends the diff
+// below one) and drop comment lines before looking for the header.
+const raw = readFileSync(msgFile, 'utf8')
+const cut = raw.indexOf(SCISSORS)
+const lines = (cut === -1 ? raw : raw.slice(0, cut))
+  .split('\n')
+  .filter((line) => !line.startsWith('#'))
+const header = lines.find((line) => line.trim() !== '') ?? ''
+
+if (GIT_SHAPES.test(header)) process.exit(0)
+
+if (!HEADER.test(header)) {
+  console.error(`commit-lint: not a Conventional Commit header: "${header}"`)
+  console.error(`commit-lint: expected "<type>(<scope>)!?: <description>" with a type in: ${TYPES.join(', ')}`)
+  process.exit(1)
+}
diff --git a/scripts/lint-commit-msg.test.mjs b/scripts/lint-commit-msg.test.mjs
new file mode 100644
index 0000000..5118427
--- /dev/null
+++ b/scripts/lint-commit-msg.test.mjs
@@ -0,0 +1,38 @@
+import { execFileSync } from 'node:child_process'
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterAll, describe, expect, it } from 'vitest'
+
+const script = join(import.meta.dirname, 'lint-commit-msg.mjs')
+const dir = mkdtempSync(join(tmpdir(), 'commit-lint-'))
+afterAll(() => rmSync(dir, { recursive: true, force: true }))
+
+function run(message) {
+  const msgFile = join(dir, 'msg')
+  writeFileSync(msgFile, message)
+  execFileSync(process.execPath, [script, msgFile])
+}
+
+describe('lint-commit-msg.mjs', () => {
+  it('accepts Conventional Commit headers', () => {
+    run('feat: add a thing\n')
+    run('fix(popup)!: change the close behavior\n')
+  })
+
+  it('accepts messages git writes for us', () => {
+    run("Merge branch 'claude/thing'\n")
+    run('fixup! feat: add a thing\n')
+  })
+
+  it('rejects a non-conventional header', () => {
+    expect(() => run('added some stuff\n')).toThrow()
+  })
+
+  it('ignores comments and everything below the scissors line', () => {
+    run(
+      '# please enter the commit message\nfix: close the leak\n\n' +
+        '# ------------------------ >8 ------------------------\nnot a header, just the diff\n',
+    )
+  })
+})