diff --git a/.gitea/workflows/harness-replays.yml b/.gitea/workflows/harness-replays.yml
index 9186f673..694d167e 100644
--- a/.gitea/workflows/harness-replays.yml
+++ b/.gitea/workflows/harness-replays.yml
@@ -68,6 +68,14 @@ jobs:
       run: ${{ steps.decide.outputs.run }}
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # fetch-depth: 0 is required so the base.sha (PR merge-base) is
+          # present for the git diff in the next step. Without it, the default
+          # fetch-depth: 1 only fetches the PR head commit; the base ref is
+          # absent and git diff silently returns empty, causing the detect-changes
+          # step to report no relevant changes even on files that ARE relevant.
+          # Verified 2026-05-11 against harness-replays run failures.
+          fetch-depth: 0
       - id: decide
         run: |
           # workflow_dispatch: always run (manual trigger)