diff --git a/src/services/add-a-windowed-parallel-catch.service.ts b/src/services/add-a-windowed-parallel-catch.service.ts
new file mode 100644
index 0000000..2f720db
--- /dev/null
+++ b/src/services/add-a-windowed-parallel-catch.service.ts
@@ -0,0 +1,40 @@
+export interface AddAWindowedParallelCatchReportInput {
+  marketKey: string;
+  primaryUnits: number;
+  secondaryUnits: number;
+  maxThreshold?: number;
+}
+
+export interface AddAWindowedParallelCatchReport {
+  marketKey: string;
+  primaryUnits: number;
+  secondaryUnits: number;
+  utilizationBps: number;
+  verdict: 'healthy' | 'degraded' | 'halted';
+}
+
+export function evaluateAddAWindowedParallelCatch(input: AddAWindowedParallelCatchReportInput): AddAWindowedParallelCatchReport {
+  const marketKey = input.marketKey.trim();
+  if (!marketKey) {
+    throw new Error('evaluateAddAWindowedParallelCatch: marketKey is required');
+  }
+  const primaryUnits = Math.max(0, Math.trunc(input.primaryUnits));
+  const secondaryUnits = Math.max(0, Math.trunc(input.secondaryUnits));
+  const limit = Math.max(1, Math.trunc(input.maxThreshold ?? 100));
+
+  const utilizationBps = Math.min(10000, Math.round(((primaryUnits + secondaryUnits) * 10000) / limit));
+  let verdict: 'healthy' | 'degraded' | 'halted' = 'healthy';
+  if (utilizationBps >= 8500) {
+    verdict = 'halted';
+  } else if (utilizationBps >= 5000) {
+    verdict = 'degraded';
+  }
+
+  return {
+    marketKey,
+    primaryUnits,
+    secondaryUnits,
+    utilizationBps,
+    verdict,
+  };
+}
diff --git a/tests/add-a-windowed-parallel-catch.test.ts b/tests/add-a-windowed-parallel-catch.test.ts
new file mode 100644
index 0000000..3d78997
--- /dev/null
+++ b/tests/add-a-windowed-parallel-catch.test.ts
@@ -0,0 +1,10 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { evaluateAddAWindowedParallelCatch } from '../src/services/add-a-windowed-parallel-catch.service';
+
+test('evaluateAddAWindowedParallelCatch: evaluates healthy, degraded, and halted verdicts', () => {
+  assert.equal(evaluateAddAWindowedParallelCatch({ marketKey: 'mkt-1', primaryUnits: 15, secondaryUnits: 10, maxThreshold: 100 }).verdict, 'healthy');
+  assert.equal(evaluateAddAWindowedParallelCatch({ marketKey: 'mkt-2', primaryUnits: 35, secondaryUnits: 25 }).verdict, 'degraded');
+  assert.equal(evaluateAddAWindowedParallelCatch({ marketKey: 'mkt-3', primaryUnits: 70, secondaryUnits: 20, maxThreshold: 100 }).verdict, 'halted');
+  assert.throws(() => evaluateAddAWindowedParallelCatch({ marketKey: '   ', primaryUnits: 1, secondaryUnits: 1 }), /marketKey is required/);
+});