diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b0ea5e0..9ca3388 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,6 +9,30 @@ permissions:
   contents: read
 
 jobs:
+  # A pull request must include the latest release of its base branch. One made from an old main undoes what
+  # was fixed since, and conflicts. Lagging a few commits behind is fine: it only gets a notice.
+  base:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          # the branch as pushed, not the merge GitHub prepares (that one always contains the base branch)
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+      - name: The branch includes the latest release
+        env:
+          BASE: ${{ github.event.pull_request.base.ref }}
+        run: |
+          git fetch --quiet origin "+refs/heads/$BASE:refs/remotes/origin/$BASE" '+refs/tags/*:refs/tags/*'
+          behind=$(git rev-list --count HEAD..origin/$BASE)
+          tag=$(git tag --merged origin/$BASE --list 'v[0-9]*.[0-9]*.[0-9]*' --sort=-v:refname | head -n 1)
+          if [ -n "$tag" ] && ! git merge-base --is-ancestor "$tag" HEAD; then
+            echo "::error::This branch starts before $tag: it is $behind commits behind $BASE. Update it from $BASE (git fetch upstream && git rebase upstream/$BASE, or git merge upstream/$BASE), then push again."
+            exit 1
+          fi
+          if [ "$behind" -gt 0 ]; then echo "::notice::This branch includes $tag but is $behind commits behind $BASE."; else echo "Up to date with $BASE."; fi
+
   test:
     runs-on: ubuntu-latest
     strategy:
diff --git a/README.md b/README.md
index 193967d..cba8c7f 100644
--- a/README.md
+++ b/README.md
@@ -592,6 +592,8 @@ The tests start a real server on a temporary data folder, create one account per
 
 The `CI` workflow runs them on Node 20 and 22 on every push to `main` and every pull request. A change that touches the API must come with a test.
 
+A pull request must start from the latest release or later: the `base` job fails when the branch does not include the latest `v…` tag of `main`, and says how to update it (rebase or merge `main`). A few commits behind `main` only gets a notice.
+
 `npm run test:e2e` walks through the interface in Chromium with Playwright (`tests/e2e.mjs`): an admin loads the demo and sees the 3D model and the 2D diagram, a contributor proposes a change, an approver sees it highlighted and approves it. Any JavaScript error in a page fails it. The first time, install the browser with `npx playwright install chromium`. The `CI` workflow runs it too (`e2e` job). A change to the interface must keep it green.
 
 ### Releasing