fix(audit): validate non-negative drift thresholds

Reject negative phase0 drift gate thresholds with explicit parameter names and add regression tests. No architecture/protocol flow changes; diagram files reviewed and no updates were needed.

docs/plans/state.json

@@ -451,6 +451,18 @@
       ],
       "test_status": "pnpm test:run src/audit/phase0BaselineArtifactRetention.test.ts + pnpm typecheck passing"
     },
+    "phase0-live-baseline-drift-threshold-validation-hardening": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Hardened phase-0 drift gate threshold handling by rejecting negative threshold values with explicit errors, preventing nonsensical gate configurations from silently producing misleading pass/fail outcomes.",
+      "files_modified": [
+        "src/audit/phase0BaselineDrift.ts",
+        "src/audit/phase0BaselineDrift.test.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",

src/audit/phase0BaselineDrift.test.ts

@@ -337,4 +337,49 @@ describe('phase0BaselineDrift', () => {
       'sampled_events_drop_pct',
     ]);
   });
+
+  it('rejects negative threshold values', () => {
+    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, {
+      maxSampledEventsDropPct: -1,
+    })).toThrow('maxSampledEventsDropPct');
+    expect(() => evaluatePhase0BaselineDriftGate(comparison, {
+      minCandidateSampledEvents: -5,
+    })).toThrow('minCandidateSampledEvents');
+  });
 });

src/audit/phase0BaselineDrift.ts

@@ -69,11 +69,14 @@ function readFiniteNumberOrNull(value: unknown): number | null {
   return typeof parsed === 'number' ? parsed : null;
 }
 
-function readThreshold(value: unknown): number | undefined {
+function readThreshold(value: unknown, thresholdName: string): number | undefined {
   const parsed = readFiniteNumber(value);
   if (typeof parsed !== 'number') {
     return undefined;
   }
+  if (parsed < 0) {
+    throw new Error(`${thresholdName} must be greater than or equal to 0.`);
+  }
   return parsed;
 }
 
@@ -163,7 +166,7 @@ export function evaluatePhase0BaselineDriftGate(
     });
   }
 
-  const minCandidateSampledEvents = readThreshold(thresholds.minCandidateSampledEvents);
+  const minCandidateSampledEvents = readThreshold(thresholds.minCandidateSampledEvents, 'minCandidateSampledEvents');
   if (typeof minCandidateSampledEvents === 'number') {
     criteria.push({
       criterion: 'candidate_sampled_events',
@@ -173,7 +176,7 @@ export function evaluatePhase0BaselineDriftGate(
     });
   }
 
-  const minBaselineSampledEvents = readThreshold(thresholds.minBaselineSampledEvents);
+  const minBaselineSampledEvents = readThreshold(thresholds.minBaselineSampledEvents, 'minBaselineSampledEvents');
   if (typeof minBaselineSampledEvents === 'number') {
     if (!baseline) {
       criteria.push({
@@ -192,7 +195,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxSampledEventsDropPct = readThreshold(thresholds.maxSampledEventsDropPct);
+  const maxSampledEventsDropPct = readThreshold(thresholds.maxSampledEventsDropPct, 'maxSampledEventsDropPct');
   if (typeof maxSampledEventsDropPct === 'number') {
     const delta = comparison.deltas.sampled_event_count_pct;
     if (delta === null) {
@@ -213,7 +216,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxRunOutcomesDropPct = readThreshold(thresholds.maxRunOutcomesDropPct);
+  const maxRunOutcomesDropPct = readThreshold(thresholds.maxRunOutcomesDropPct, 'maxRunOutcomesDropPct');
   if (typeof maxRunOutcomesDropPct === 'number') {
     const delta = comparison.deltas.run_total_outcomes_pct;
     if (delta === null) {
@@ -234,7 +237,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxCompletionRateDropPp = readThreshold(thresholds.maxCompletionRateDropPp);
+  const maxCompletionRateDropPp = readThreshold(thresholds.maxCompletionRateDropPp, 'maxCompletionRateDropPp');
   if (typeof maxCompletionRateDropPp === 'number') {
     const delta = comparison.deltas.completion_rate_pp;
     if (delta === null) {
@@ -255,7 +258,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxCancelRateIncreasePp = readThreshold(thresholds.maxCancelRateIncreasePp);
+  const maxCancelRateIncreasePp = readThreshold(thresholds.maxCancelRateIncreasePp, 'maxCancelRateIncreasePp');
   if (typeof maxCancelRateIncreasePp === 'number') {
     const delta = comparison.deltas.cancel_rate_pp;
     if (delta === null) {
@@ -276,7 +279,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxErrorRateIncreasePp = readThreshold(thresholds.maxErrorRateIncreasePp);
+  const maxErrorRateIncreasePp = readThreshold(thresholds.maxErrorRateIncreasePp, 'maxErrorRateIncreasePp');
   if (typeof maxErrorRateIncreasePp === 'number') {
     const delta = comparison.deltas.error_rate_pp;
     if (delta === null) {
@@ -297,7 +300,7 @@ export function evaluatePhase0BaselineDriftGate(
     }
   }
 
-  const maxCancelLatencyP95IncreaseMs = readThreshold(thresholds.maxCancelLatencyP95IncreaseMs);
+  const maxCancelLatencyP95IncreaseMs = readThreshold(thresholds.maxCancelLatencyP95IncreaseMs, 'maxCancelLatencyP95IncreaseMs');
   if (typeof maxCancelLatencyP95IncreaseMs === 'number') {
     const delta = comparison.deltas.cancel_latency_p95_ms;
     if (delta === null) {