fix(ci): promote performance baseline only on successful runs

.github/workflows/performance.yml

@@ -117,12 +117,17 @@ jobs:
         # results for just that one benchmark, silently disabling
         # regression checks for the other benchmarks until the next
         # full run happens to succeed.
-        if: always() && (github.event.inputs.benchmark_target == 'all' || github.event.inputs.benchmark_target == '')
+        # success(), not always(): a run that fails the regression check must
+        # not become the next baseline, or each regression lowers the bar for
+        # the run after it. To accept a slowdown deliberately, delete the
+        # perf-baseline-macOS-* caches (`gh cache list --limit 100 --json key --jq '.[] | select(.key | startswith("perf-baseline-macOS-")) | .key' | while read -r key; do gh cache delete "$key"; done`); the next run has no baseline, passes,
+        # and becomes the new one.
+        if: success() && (github.event.inputs.benchmark_target == 'all' || github.event.inputs.benchmark_target == '')
         run: cp performance_results.json performance_baseline.json
 
       - name: Cache updated baseline for next run
         uses: actions/cache/save@v6
-        if: always() && (github.event.inputs.benchmark_target == 'all' || github.event.inputs.benchmark_target == '')
+        if: success() && (github.event.inputs.benchmark_target == 'all' || github.event.inputs.benchmark_target == '')
         with:
           path: performance_baseline.json
           key: perf-baseline-${{ runner.os }}-${{ github.run_id }}

CHANGELOG.md

@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- **Weekly performance benchmark promoted failing runs to baseline**: the `Save new baseline` and
+  `Cache updated baseline for next run` steps in `.github/workflows/performance.yml` ran under `always()`, so a
+  run that failed the >20% regression check still cached its slower numbers as the next run's baseline — each
+  regression lowered the bar for the following week (e.g. the 2026-09-13 run compared `apps` against 2.732s,
+  the exact figure from the failed 2026-09-06 run). Both steps are now gated on `success()`; to accept a
+  slowdown deliberately, delete the `perf-baseline-macOS-*` caches and the next run re-baselines. Guarded by
+  `test_performance_baseline_promoted_only_on_success`.
+
 ## [1.2.0] - 2026-08-15
 
 ### Security

tests/test_project_consistency.py

@@ -181,6 +181,26 @@ def test_ci_python_versions(self):
         # Validate all supported versions are tested
         self._validate_ci_versions(supported_versions, ci_versions)
 
+    def test_performance_baseline_promoted_only_on_success(self):
+        """Test that a run failing the regression check never becomes the next baseline.
+
+        With ``always()``, each failing run's slower numbers were cached as the
+        next baseline, so every regression lowered the bar for the following run.
+        """
+        perf_path = get_project_root() / ".github" / "workflows" / "performance.yml"
+        with open(perf_path, encoding="utf-8") as f:
+            perf_config = yaml.safe_load(f)
+
+        step_names = [step.get("name") for step in perf_config["jobs"]["performance-test"]["steps"]]
+        steps = dict(zip(step_names, perf_config["jobs"]["performance-test"]["steps"], strict=True))
+        compare_index = step_names.index("Compare with baseline (fail on >20% regression)")
+
+        for name in ("Save new baseline", "Cache updated baseline for next run"):
+            condition = steps[name]["if"]
+            assert step_names.index(name) > compare_index, f"'{name}' must run after the regression check"
+            assert "always()" not in condition, f"'{name}' must not run when the regression check fails"
+            assert condition.startswith("success()"), f"'{name}' should be gated on success()"
+
     def _assert_version_in_ci(self, version: str, ci_versions: list[str]) -> None:
         """Assert that a specific version is tested in CI."""
         error_msg = f"Python {version} is supported but not tested in CI"