diff --git a/.github/workflows/build-gossamer-gui.yml b/.github/workflows/build-gossamer-gui.yml
index 7c0f377..8dfa384 100644
--- a/.github/workflows/build-gossamer-gui.yml
+++ b/.github/workflows/build-gossamer-gui.yml
@@ -152,7 +152,13 @@ jobs:
       - name: Install just
         uses: taiki-e/install-action@184183c2401be73c3bf42c2e61268aa5855379c1 # v2.78.1
         with:
-          tool: just
+          # Pin an explicit modern just: the Justfile uses `import?`
+          # (optional import, just >= 1.19.0). Unversioned `tool: just`
+          # resolves via this action's bundled manifest, which can ship
+          # an old just (e.g. 1.14.0) -> `error: Unknown start of token`
+          # at `import? "contractile.just"`, silently breaking the build.
+          # Estate Tooling Version Integrity policy (root cause: burble#39).
+          tool: just@1.34.0
 
       - name: Cache Ephapax build
         id: cache-ephapax