diff --git a/web/scripts/vercel-ignore.sh b/web/scripts/vercel-ignore.sh
new file mode 100755
index 00000000..3c0861fd
--- /dev/null
+++ b/web/scripts/vercel-ignore.sh
@@ -0,0 +1,15 @@
+#!/bin/bash
+# Vercel `ignoreCommand`: exit 0 to skip the build, non-zero to build.
+# Skips the build when neither web/ nor common/ changed since the previous deployed commit.
+# Kept in a script (rather than inline in vercel.json) because `ignoreCommand` is capped at 256 chars.
+
+git fetch origin --unshallow || git fetch origin --deepen=200 || true
+
+echo "prev=$VERCEL_GIT_PREVIOUS_SHA cur=$VERCEL_GIT_COMMIT_SHA"
+
+if [ -z "$VERCEL_GIT_PREVIOUS_SHA" ] || ! git cat-file -e "$VERCEL_GIT_PREVIOUS_SHA^{commit}" 2>/dev/null; then
+  echo 'no usable previous sha - building'
+  exit 1
+fi
+
+git diff --quiet "$VERCEL_GIT_PREVIOUS_SHA" "$VERCEL_GIT_COMMIT_SHA" -- ./ ../common/
diff --git a/web/vercel.json b/web/vercel.json
index 34f03ab2..9f7f2753 100644
--- a/web/vercel.json
+++ b/web/vercel.json
@@ -9,5 +9,5 @@
       "headers": [{"key": "Cache-Control", "value": "no-cache"}]
     }
   ],
-  "ignoreCommand": "git fetch origin --unshallow || git fetch origin --deepen=200 || true; echo \"prev=$VERCEL_GIT_PREVIOUS_SHA cur=$VERCEL_GIT_COMMIT_SHA\"; if [ -z \"$VERCEL_GIT_PREVIOUS_SHA\" ] || ! git cat-file -e \"$VERCEL_GIT_PREVIOUS_SHA^{commit}\" 2>/dev/null; then echo 'no usable previous sha - building'; exit 1; fi; git diff --quiet \"$VERCEL_GIT_PREVIOUS_SHA\" \"$VERCEL_GIT_COMMIT_SHA\" -- ./ ../common/"
+  "ignoreCommand": "bash scripts/vercel-ignore.sh"
 }