fix(audit): require integer drift minimum sample thresholds

Enforce non-negative integer minCandidateSampledEvents/minBaselineSampledEvents in drift gate evaluation and CLI parsing; add regression coverage. Architecture/protocol diagrams reviewed and no updates were needed for this validation-only change.

docs/plans/state.json

@@ -507,6 +507,19 @@
       ],
       "test_status": "pnpm test:run src/audit/phase0BaselineSummary.test.ts + pnpm typecheck passing"
     },
+    "phase0-live-baseline-drift-min-threshold-integer-validation": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Hardened phase-0 backend drift gating by requiring `minCandidateSampledEvents` and `minBaselineSampledEvents` to be non-negative integers in both gate evaluation and CLI parsing, preventing fractional count thresholds.",
+      "files_modified": [
+        "src/audit/phase0BaselineDrift.ts",
+        "src/audit/phase0BaselineDrift.test.ts",
+        "scripts/check-phase0-baseline-backend-drift.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/audit/phase0BaselineDrift.test.ts + pnpm typecheck passing"
+    },
     "phase0-instrumentation-ticket-checklist": {
       "status": "completed",
       "date": "2026-02-25",

scripts/check-phase0-baseline-backend-drift.ts

@@ -69,8 +69,8 @@ function usage(): string {
     '  --out <path>                               Write output to file instead of stdout',
     '',
     'Drift thresholds (optional):',
-    '  --min-candidate-sampled-events <number>',
-    '  --min-baseline-sampled-events <number>',
+    '  --min-candidate-sampled-events <integer>',
+    '  --min-baseline-sampled-events <integer>',
     '  --max-sampled-events-drop-pct <number>',
     '  --max-run-outcomes-drop-pct <number>',
     '  --max-completion-rate-drop-pp <number>',
@@ -106,6 +106,20 @@ function parseOptionalNumber(raw: string | undefined, flag: string): number | un
   return parsed;
 }
 
+function parseOptionalInteger(raw: string | undefined, flag: string): number | undefined {
+  const parsed = parseOptionalNumber(raw, flag);
+  if (parsed === undefined) {
+    return undefined;
+  }
+  if (!Number.isInteger(parsed)) {
+    throw new Error(`Invalid ${flag} value "${raw}". Expected an integer.`);
+  }
+  if (parsed < 0) {
+    throw new Error(`${flag} must be greater than or equal to 0.`);
+  }
+  return parsed;
+}
+
 function parseBackends(raw: string | undefined): Phase0BackendTarget[] {
   const values = parseCsv(raw) ?? ['pi_embedded', 'native'];
   const parsed: Phase0BackendTarget[] = [];
@@ -162,8 +176,8 @@ async function writeOutput(pathValue: string, output: string): Promise<void> {
 function buildThresholds(values: Record<string, string | boolean | undefined>): Phase0BaselineDriftGateThresholds {
   return {
     requireBaselineHistory: Boolean(values['require-baseline-history']),
-    minCandidateSampledEvents: parseOptionalNumber(values['min-candidate-sampled-events'] as string | undefined, '--min-candidate-sampled-events'),
-    minBaselineSampledEvents: parseOptionalNumber(values['min-baseline-sampled-events'] as string | undefined, '--min-baseline-sampled-events'),
+    minCandidateSampledEvents: parseOptionalInteger(values['min-candidate-sampled-events'] as string | undefined, '--min-candidate-sampled-events'),
+    minBaselineSampledEvents: parseOptionalInteger(values['min-baseline-sampled-events'] as string | undefined, '--min-baseline-sampled-events'),
     maxSampledEventsDropPct: parseOptionalNumber(values['max-sampled-events-drop-pct'] as string | undefined, '--max-sampled-events-drop-pct'),
     maxRunOutcomesDropPct: parseOptionalNumber(values['max-run-outcomes-drop-pct'] as string | undefined, '--max-run-outcomes-drop-pct'),
     maxCompletionRateDropPp: parseOptionalNumber(values['max-completion-rate-drop-pp'] as string | undefined, '--max-completion-rate-drop-pp'),

src/audit/phase0BaselineDrift.test.ts

@@ -382,4 +382,49 @@ describe('phase0BaselineDrift', () => {
       minCandidateSampledEvents: -5,
     })).toThrow('minCandidateSampledEvents');
   });
+
+  it('rejects non-integer sampled-event minimum thresholds', () => {
+    const comparison = comparePhase0BaselineDrift({
+      sampled_event_count: 10,
+      summary: {
+        event_counts: {
+          run_state: 0,
+          run_cancel: 0,
+          reaction_match: 0,
+          reaction_skip: 0,
+        },
+        run_outcomes: {
+          overall: {
+            total_outcomes: 5,
+            complete: 4,
+            cancelled: 1,
+            error: 0,
+            cancel_requested: 0,
+            start: 5,
+            completion_rate_pct: 80,
+            cancel_rate_pct: 20,
+            error_rate_pct: 0,
+          },
+          by_channel: [],
+          by_session: [],
+        },
+        cancel_latency_ms: null,
+        reactions: {
+          matched: 0,
+          skipped: 0,
+          total: 0,
+          match_rate_pct: null,
+          skip_rate_pct: null,
+          skip_reasons: [],
+        },
+      },
+    });
+
+    expect(() => evaluatePhase0BaselineDriftGate(comparison, {
+      minCandidateSampledEvents: 10.5,
+    })).toThrow('minCandidateSampledEvents');
+    expect(() => evaluatePhase0BaselineDriftGate(comparison, {
+      minBaselineSampledEvents: 8.2,
+    })).toThrow('minBaselineSampledEvents');
+  });
 });

src/audit/phase0BaselineDrift.ts

@@ -80,6 +80,17 @@ function readThreshold(value: unknown, thresholdName: string): number | undefine
   return parsed;
 }
 
+function readIntegerThreshold(value: unknown, thresholdName: string): number | undefined {
+  const parsed = readThreshold(value, thresholdName);
+  if (parsed === undefined) {
+    return undefined;
+  }
+  if (!Number.isInteger(parsed)) {
+    throw new Error(`${thresholdName} must be an integer greater than or equal to 0.`);
+  }
+  return parsed;
+}
+
 function toPctDelta(baseline: number, candidate: number): number | null {
   if (!Number.isFinite(baseline) || baseline <= 0 || !Number.isFinite(candidate)) {
     return null;
@@ -166,7 +177,7 @@ export function evaluatePhase0BaselineDriftGate(
     });
   }
 
-  const minCandidateSampledEvents = readThreshold(thresholds.minCandidateSampledEvents, 'minCandidateSampledEvents');
+  const minCandidateSampledEvents = readIntegerThreshold(thresholds.minCandidateSampledEvents, 'minCandidateSampledEvents');
   if (typeof minCandidateSampledEvents === 'number') {
     criteria.push({
       criterion: 'candidate_sampled_events',
@@ -176,7 +187,7 @@ export function evaluatePhase0BaselineDriftGate(
     });
   }
 
-  const minBaselineSampledEvents = readThreshold(thresholds.minBaselineSampledEvents, 'minBaselineSampledEvents');
+  const minBaselineSampledEvents = readIntegerThreshold(thresholds.minBaselineSampledEvents, 'minBaselineSampledEvents');
   if (typeof minBaselineSampledEvents === 'number') {
     if (!baseline) {
       criteria.push({