diff --git a/.github/actions/setup-diffbio/action.yml b/.github/actions/setup-diffbio/action.yml
index 6cf799f..58cb7bf 100644
--- a/.github/actions/setup-diffbio/action.yml
+++ b/.github/actions/setup-diffbio/action.yml
@@ -43,6 +43,14 @@ runs:
       shell: bash
       env:
         UV_CONCURRENT_DOWNLOADS: "4"
+        # uv sync is a project command and selects its own interpreter; it does not
+        # inherit the one `uv venv --python` just created. Without these it resolved
+        # against CPython 3.14 and refused the lock, because tensorflow 2.20.0 has no
+        # cp314 wheel. The same three variables are exported for later steps further
+        # down; they are needed here too, before the first uv sync runs.
+        UV_PYTHON: ${{ inputs.python-version }}
+        UV_PYTHON_DOWNLOADS: never
+        UV_PYTHON_PREFERENCE: only-system
       run: |
         set -euo pipefail
 
@@ -51,18 +59,31 @@ runs:
           editable_target="${{ inputs.macos-editable-target }}"
         fi
 
+        # Resolve from uv.lock rather than from the pyproject floors. Installing
+        # unpinned let every tool drift to its newest release between runs, so a
+        # run could fail on a dependency change no commit had made.
+        sync_args=(--locked)
+        if [[ "$editable_target" == *"["* ]]; then
+          extras="${editable_target#*[}"
+          extras="${extras%]}"
+          IFS=',' read -ra extra_names <<< "$extras"
+          for extra in "${extra_names[@]}"; do
+            sync_args+=(--extra "${extra// /}")
+          done
+        fi
+
         for attempt in 1 2 3 4 5; do
-          if uv pip install -e "$editable_target"; then
+          if uv sync "${sync_args[@]}"; then
             break
           fi
 
           if [[ "$attempt" -eq 5 ]]; then
-            echo "Failed to install $editable_target after 5 attempts" >&2
+            echo "Failed to sync $editable_target after 5 attempts" >&2
             exit 1
           fi
 
           delay=$((attempt * 10))
-          echo "Install attempt $attempt failed, retrying in ${delay} seconds..." >&2
+          echo "Sync attempt $attempt failed, retrying in ${delay} seconds..." >&2
           sleep "$delay"
         done