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diff --git a/ci/github-script/lint-commits.js b/ci/github-script/lint-commits.js
new file mode 100644
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+++ b/ci/github-script/lint-commits.js
@@ -0,0 +1,177 @@
+// @ts-check
+const { classify } = require('../supportedBranches.js')
+const { getCommitDetailsForPR } = require('./get-pr-commit-details.js')
+
+// Supported Conventional Commit types for Project Tick
+const CONVENTIONAL_TYPES = [
+  'build',
+  'chore',
+  'ci',
+  'docs',
+  'feat',
+  'fix',
+  'perf',
+  'refactor',
+  'revert',
+  'style',
+  'test',
+]
+
+// Known project scopes in the monorepo
+const KNOWN_SCOPES = [
+  'archived',
+  'cgit',
+  'ci',
+  'cmark',
+  'corebinutils',
+  'forgewrapper',
+  'genqrcode',
+  'hooks',
+  'images4docker',
+  'json4cpp',
+  'libnbtplusplus',
+  'meshmc',
+  'meta',
+  'mnv',
+  'neozip',
+  'tomlplusplus',
+  'repo',
+  'deps',
+]
+
+/**
+ * Validates commit messages against Project Tick Conventional Commits conventions.
+ *
+ * Format: type(scope): subject
+ *   type  — one of: feat, fix, docs, style, refactor, perf, test, build, ci, chore, revert
+ *   scope — optional, should match a project directory or be a known scope
+ *   subject — imperative, no trailing period, no uppercase first letter
+ *
+ * @param {{
+ *  github: InstanceType<import('@actions/github/lib/utils').GitHub>,
+ *  context: import('@actions/github/lib/context').Context,
+ *  core: import('@actions/core'),
+ *  repoPath?: string,
+ * }} CheckCommitMessagesProps
+ */
+async function checkCommitMessages({ github, context, core, repoPath }) {
+  const pull_number = context.payload.pull_request?.number
+  if (!pull_number) {
+    core.info('This is not a pull request. Skipping checks.')
+    return
+  }
+
+  const pr = (
+    await github.rest.pulls.get({
+      ...context.repo,
+      pull_number,
+    })
+  ).data
+
+  const baseBranchType = classify(
+    pr.base.ref.replace(/^refs\/heads\//, ''),
+  ).type
+  const headBranchType = classify(
+    pr.head.ref.replace(/^refs\/heads\//, ''),
+  ).type
+
+  if (
+    baseBranchType.includes('development') &&
+    headBranchType.includes('development') &&
+    pr.base.repo.id === pr.head.repo?.id
+  ) {
+    core.info(
+      'This PR is from one development branch to another. Skipping checks.',
+    )
+    return
+  }
+
+  const commits = await getCommitDetailsForPR({ core, pr, repoPath })
+
+  const failures = new Set()
+  const warnings = new Set()
+
+  const conventionalRegex = new RegExp(
+    `^(${CONVENTIONAL_TYPES.join('|')})(\\(([^)]+)\\))?(!)?: .+$`,
+  )
+
+  for (const commit of commits) {
+    const msg = commit.subject
+    const logPrefix = `Commit ${commit.sha.slice(0, 12)}`
+
+    // Check: fixup/squash commits
+    const fixups = ['amend!', 'fixup!', 'squash!']
+    if (fixups.some((s) => msg.startsWith(s))) {
+      core.error(
+        `${logPrefix}: starts with "${fixups.find((s) => msg.startsWith(s))}". ` +
+          'Did you forget to run `git rebase -i --autosquash`?',
+      )
+      failures.add(commit.sha)
+      continue
+    }
+
+    // Check: Conventional Commit format
+    if (!conventionalRegex.test(msg)) {
+      core.error(
+        `${logPrefix}: "${msg}" does not follow Conventional Commits format. ` +
+          'Expected: type(scope): subject  (e.g. "feat(mnv): add keybinding")',
+      )
+      failures.add(commit.sha)
+      continue
+    }
+
+    // Extract parts
+    const match = msg.match(conventionalRegex)
+    const type = match[1]
+    const scope = match[3] || null
+
+    // Check: trailing period
+    if (msg.endsWith('.')) {
+      core.error(
+        `${logPrefix}: subject should not end with a period.`,
+      )
+      failures.add(commit.sha)
+    }
+
+    // Warning: unknown scope
+    if (scope && !KNOWN_SCOPES.includes(scope)) {
+      core.warning(
+        `${logPrefix}: scope "${scope}" is not a known project. ` +
+          `Known scopes: ${KNOWN_SCOPES.join(', ')}`,
+      )
+      warnings.add(commit.sha)
+    }
+
+    // Check: subject should not start with uppercase (after type(scope): )
+    const subjectStart = msg.indexOf(': ') + 2
+    if (subjectStart < msg.length) {
+      const firstChar = msg[subjectStart]
+      if (firstChar === firstChar.toUpperCase() && firstChar !== firstChar.toLowerCase()) {
+        core.warning(
+          `${logPrefix}: subject should start with lowercase letter.`,
+        )
+        warnings.add(commit.sha)
+      }
+    }
+
+    if (!failures.has(commit.sha)) {
+      core.info(`${logPrefix}: "${msg}" — passed.`)
+    }
+  }
+
+  if (failures.size !== 0) {
+    core.error(
+      'Please review the Conventional Commits guidelines at ' +
+        '<https://www.conventionalcommits.org/> and the project CONTRIBUTING.md.',
+    )
+    core.setFailed(
+      `${failures.size} commit(s) do not follow commit conventions.`,
+    )
+  } else if (warnings.size !== 0) {
+    core.warning(
+      `${warnings.size} commit(s) have minor issues (see warnings above).`,
+    )
+  }
+}
+
+module.exports = checkCommitMessages