diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8b1edccf5..5835d846f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,9 +41,12 @@ jobs:
           PY
       - name: Compare production source
         env:
-          PR_BASE: ${{ github.event.pull_request.base.sha || inputs.base }}
+          # pull_request.base.sha is not refreshed when the base branch moves, so it can name a
+          # revision many commits behind and charge the PR for debt main introduced. The PR's own
+          # change starts at the merge-base with the base branch's current tip.
+          BASE_TIP: ${{ github.event.pull_request.base.ref && format('origin/{0}', github.event.pull_request.base.ref) || inputs.base }}
           PR_HEAD: ${{ github.event.pull_request.head.sha || inputs.head }}
-        run: PYTHONPATH="$PWD/.core-ratchet/src" PATH="$PWD/.venv/bin:$PATH" .venv/bin/python .core-ratchet/tools/debt_ratchet.py --root src/toad --base "$PR_BASE" --head "$PR_HEAD"
+        run: PYTHONPATH="$PWD/.core-ratchet/src" PATH="$PWD/.venv/bin:$PATH" .venv/bin/python .core-ratchet/tools/debt_ratchet.py --root src/toad --base "$(git merge-base "$BASE_TIP" "$PR_HEAD")" --head "$PR_HEAD"
       - name: Run surface guards
         run: .venv/bin/pytest tests/guards
   suite: