diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index 06b3b88..ba8700d 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -21,7 +21,8 @@ jobs:
     steps:
       - uses: actions/checkout@v7
       - uses: astral-sh/setup-uv@v7
-      - run: uv sync --no-dev --group docs
+      # --locked: fail when uv.lock is out of date with pyproject.toml.
+      - run: uv sync --locked --no-dev --group docs
       - run: cp CHANGELOG.md docs/changelog.md
       # --no-dev on `uv run` too: without it the implicit sync reinstalls the
       # whole dev group just to build docs.