diff --git a/.github/workflows/quarto-publish.yml b/.github/workflows/quarto-publish.yml
index 59e2cd5a3..859fed68e 100644
--- a/.github/workflows/quarto-publish.yml
+++ b/.github/workflows/quarto-publish.yml
@@ -79,7 +79,10 @@ jobs:
       - name: Checkout repo
         uses: actions/checkout@v7
         with:
-          fetch-depth: 0
+          # Shallow checkout (issue #306, B5): the change-detection step
+          # below needs only the base commit, which it fetches on demand.
+          # Full history costs ~1 min of checkout against a ~5 GB .git.
+          fetch-depth: 1
 
       # ── Detect changed files ─────────────────────────────────
       # Determines which QMDs changed so we can:
@@ -106,6 +109,18 @@ jobs:
             exit 0
           fi
 
+          # Shallow checkout (issue #306, B5): fetch just the base commit so
+          # the diff below has both endpoints. Fails soft – on error (e.g.
+          # the base commit was force-pushed away) fall back to a full build.
+          if ! git fetch --no-tags --depth=1 origin "$BASE_SHA"; then
+            echo "⚠️ Could not fetch base $BASE_SHA – treating as full build"
+            echo "changed_qmds=" >> "$GITHUB_OUTPUT"
+            echo "has_changes=false" >> "$GITHUB_OUTPUT"
+            echo "need_julia=true" >> "$GITHUB_OUTPUT"
+            echo "need_python=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
           CHANGED_QMDS=$(git diff --name-only --diff-filter=ACMR "$BASE_SHA" HEAD -- '*.qmd' 2>/dev/null | tr '\n' ' ' || echo "")
           echo "changed_qmds=$CHANGED_QMDS" >> "$GITHUB_OUTPUT"