fix(hw-gate): a stale decision.json blocked #702 with #682's verdict

.github/workflows/hw-gate.yml

@@ -476,7 +476,13 @@ jobs:
           # previous PR's evidence files and home (run 33866758629 on #702
           # carried #700's fable-summary.md and #686's route outputs) and can
           # cite them as its own. Every session starts empty.
-          rm -rf fable-evidence fable-home
+          # decision.json is the same hazard, one level worse: upload-artifact
+          # runs `if: always()`, so a decide phase that dies before writing its
+          # own file publishes the PREVIOUS run's verdict as this run's. #702's
+          # decide job failed and the run published #682's decision -- "six
+          # refused loads ... master says 'no model loaded'" -- blocking a PR
+          # whose own lanes were 8/8 pass.
+          rm -rf fable-evidence fable-home decision.json
           mkdir -p fable-evidence fable-home
           exec 9>/home/kaden/actions-runner/_cache/hw-gate-gpu.lock
           flock --exclusive --timeout 3600 9
@@ -575,6 +581,21 @@ jobs:
           # floor-applied verdict under `.decision_final`; `.decision` is a JSON
           # object, so the old jq yielded "[object]" and every run — including
           # a successful merge-staging (#689, run 33889229683) — went red.
+          # A decision is only this PR's if it names this PR's head. The runner
+          # workspace is reused and upload-artifact runs `if: always()`, so a
+          # decide phase that dies can publish the previous run's verdict:
+          # #702 was blocked on #682's decision while its own lanes were 8/8
+          # pass. An artifact from another commit is a gate malfunction, not a
+          # verdict, so it fails as one rather than being obeyed.
+          want_head="${{ needs.select.outputs.head_sha }}"
+          got_head=$(jq -r '.head // ""' decision/decision.json)
+          if [ -n "$got_head" ] && [ "$got_head" != "$want_head" ]; then
+            echo "::error::decision artifact is for $got_head but this run is $want_head — stale decision.json from a reused workspace; re-run the gate"
+            exit 1
+          fi
+          if [ -z "$got_head" ]; then
+            echo "::warning::decision artifact predates head attribution; cannot verify it belongs to $want_head"
+          fi
           decision=$(jq -r '.decision_final // .decision.decision // "hold"' decision/decision.json)
           hard=$(jq -r '(.floor.hard // []) | join(", ")' decision/decision.json)
           echo "decision=$decision hard_floor=[$hard] human_reviewed=${{ needs.select.outputs.human_reviewed }}"

scripts/hw-gate/review.py

@@ -1690,6 +1690,17 @@ def _has_hold(r: str) -> bool:
         "version": 1,
         "seat": "fable",
         "model": model,
+        # The decision must be attributable to the commit it judged. The
+        # runner workspace is reused, so a decide phase that dies before
+        # writing its own decision.json leaves the PREVIOUS run's file in
+        # place, and `upload-artifact: if: always()` publishes it as this
+        # run's decision. That happened on 2026-09-04: #702's decide job
+        # failed and the run published #682's verdict -- "six refused loads
+        # ... master says 'no model loaded'" -- blocking a PR whose own lanes
+        # were 8/8 pass. The status job cross-checks these against the run's
+        # head, so a stale artifact is caught instead of obeyed.
+        "base": args.base,
+        "head": args.head,
         "decision": decision,
         "floor": {"hard": hard, "soft": soft},
         "decision_final": decision_final,

scripts/hw-gate/tests/test_review.py

@@ -874,3 +874,26 @@ class R: returncode, stdout, stderr = 0, "", ""
     assert "--thinking" in cmd and cmd[cmd.index("--thinking") + 1] == "xhigh"
     assert cmd[cmd.index("--max-time") + 1] == f"{minutes}m"
     assert "GH_TOKEN" not in captured["env"] and captured["env"]["HW_GATE_DEVICES"] == "0,1,2,3,4"
+
+
+def test_decision_records_the_commit_it_judged():
+    """A decision is only usable if it names the commit it judged.
+
+    The runner workspace is reused and upload-artifact runs `if: always()`, so a
+    decide phase that dies publishes the previous run's decision.json as this
+    run's. On 2026-09-04 #702 was blocked by #682's verdict -- announcement and
+    all -- while #702's own lanes were 8/8 pass. The status job cross-checks
+    `.head`, which only works if review.py records it.
+    """
+    tmp = Path(tempfile.mkdtemp())
+    result, out_path, gh_log, gh_comments, omp_log, base, head, checkout = _run_decide(
+        tmp, sol_final="greenlight",
+        fable_response={"phase": "decide", "decision": "merge-staging", "agrees_with_sol": True,
+                        "override": None, "regressions": [], "further_evidence_wanted": [],
+                        "rationale": "r", "announcement": "a"},
+    )
+    data = json.loads(out_path.read_text())
+    assert data["head"] == head, data.get("head")
+    assert data["base"] == base, data.get("base")
+
+