diff --git a/.gitea/workflows/harness-replays.yml b/.gitea/workflows/harness-replays.yml
index 9186f673..3390b2cb 100644
--- a/.gitea/workflows/harness-replays.yml
+++ b/.gitea/workflows/harness-replays.yml
@@ -68,6 +68,17 @@ jobs:
       run: ${{ steps.decide.outputs.run }}
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # fetch-depth: 0 is required here. The step runs `git diff "$base_sha"
+          # "$head_sha"` — if only the head commit is fetched (default
+          # fetch-depth: 1), the base ref is not in the local history and
+          # git diff fails silently (2>/dev/null), leaving DIFF empty, the
+          # step exits non-zero, and detect-changes reports "failure" despite
+          # continue-on-error: true. With fetch-depth: 0 the full history is
+          # available so both refs exist locally. The PR head and the base
+          # branch tip are always on the remote; full history is needed only
+          # to make them accessible offline in the container.
+          fetch-depth: 0
       - id: decide
         run: |
           # workflow_dispatch: always run (manual trigger)