diff --git a/.github/workflows/no-ai-lines.yml b/.github/workflows/no-ai-lines.yml
new file mode 100644
index 0000000..fef9cbb
--- /dev/null
+++ b/.github/workflows/no-ai-lines.yml
@@ -0,0 +1,62 @@
+# Fails when a pull request or a push to main carries an AI session link,
+# an AI co-author line or a "generated with" line, in the PR title, the PR
+# body or any commit message. A pull request with more than 250 commits
+# fails, because the API lists at most 250 and the rest cannot be checked.
+name: no-ai-lines
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+  pull-requests: read
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for AI lines
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          EVENT_NAME: ${{ github.event_name }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          set -euo pipefail
+          banned='claude-session:|claude\.ai/code|co-authored-by:.*(claude|anthropic|copilot|cursor|grok|gemini|openai|codex|chatgpt|gpt|aider|devin|windsurf|deepseek|qwen|kimi)|generated (with|by) .*(claude|ai\b)|🤖'
+          fail=0
+          check() {
+            if grep -qiE "$banned" <<< "$2"; then
+              echo "::error::AI line found in $1"
+              echo "Matched in $1:"
+              grep -iE "$banned" <<< "$2" | sed 's/^/  /'
+              fail=1
+            fi
+          }
+          if [ "$EVENT_NAME" = pull_request ]; then
+            total=$(jq -r '.pull_request.commits // 0' "$GITHUB_EVENT_PATH")
+            if [ "$total" -gt 250 ]; then
+              echo "::error::This pull request has $total commits. The API lists at most 250, so the commits could not all be checked."
+              exit 1
+            fi
+            check "PR title" "$(jq -r '.pull_request.title // ""' "$GITHUB_EVENT_PATH")"
+            check "PR body" "$(jq -r '.pull_request.body // ""' "$GITHUB_EVENT_PATH")"
+            gh api --paginate "repos/$REPO/pulls/$PR_NUMBER/commits" \
+              --jq '.[] | [.sha, (.commit.message | @base64)] | @tsv' > commits.tsv
+            while IFS=$'\t' read -r sha msg; do
+              check "commit $sha" "$(printf '%s' "$msg" | base64 -d)"
+            done < commits.tsv
+          else
+            jq -r '.commits[] | [.id, (.message | @base64)] | @tsv' "$GITHUB_EVENT_PATH" > commits.tsv
+            while IFS=$'\t' read -r sha msg; do
+              check "commit $sha" "$(printf '%s' "$msg" | base64 -d)"
+            done < commits.tsv
+          fi
+          if [ "$fail" -ne 0 ]; then
+            echo "Fix: remove the AI lines. Edit the PR title or body, or rewrite the commit message (git commit --amend, or git rebase -r with reword) and force-push the branch."
+            exit 1
+          fi
+          echo "No AI lines found."