diff --git a/src/__tests__/billing-index.test.ts b/src/__tests__/billing-index.test.ts
index feeac247..478a57f3 100644
--- a/src/__tests__/billing-index.test.ts
+++ b/src/__tests__/billing-index.test.ts
@@ -8,11 +8,13 @@
  * Confirms the hot /api/billing filter on `developer_id` creates and uses
  * `idx_billing_requests_lookup_hot`, and that the rollback migration drops it.
  */
+import fc from 'fast-check';
 import { execFileSync } from 'node:child_process';
 import { mkdtempSync, readFileSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import path from 'node:path';
 import { newDb } from 'pg-mem';
+import { billingInternals } from '../services/billing.js';
 
 const migrationsDir = path.join(process.cwd(), 'migrations');
 
@@ -31,6 +33,13 @@ INSERT INTO billing_requests (id, request_id, developer_id, api_id, endpoint_id,
 VALUES ('req_1', 'req_id_1', 'dev_a', 'api_1', 'ep_1', 'key_1', '5.00');
 `;
 
+// --- USDC unit conversion properties (issue: property-test round trips) ---
+// Use the real exported helpers from src/services/billing.ts so these
+// round-trip invariants exercise production code instead of a local copy.
+const { parseUsdcToContractUnits, formatContractUnitsToUsdc } = billingInternals;
+const USDC_DECIMALS = 7;
+const USDC_SCALE = 10n ** BigInt(USDC_DECIMALS);
+
 const HOT_PATH_QUERY = `
 SELECT id, request_id, developer_id, api_id, endpoint_id, api_key_id, amount_usdc, created_at
 FROM billing_requests
@@ -48,6 +57,101 @@ function isSqliteAvailable(): boolean {
   }
 }
 
+describe('USDC unit conversion — fast-check properties', () => {
+  it('round-trips format(parse(x)) for 10k generated amounts', () => {
+    const amountArb = fc
+      .tuple(
+        fc.bigInt({ min: 0n, max: 10n ** 18n }),
+        fc.integer({ min: 0, max: USDC_DECIMALS }),
+        fc.array(fc.integer({ min: 0, max: 9 }), {
+          minLength: USDC_DECIMALS,
+          maxLength: USDC_DECIMALS,
+        }),
+      )
+      .map(([whole, fracLen, digits]) => {
+        const frac = digits.slice(0, fracLen).join('');
+        const raw = frac.length > 0 ? `${whole}.${frac}` : `${whole}`;
+        return raw;
+      })
+      .filter((raw) => {
+        try {
+          parseUsdcToContractUnits(raw);
+          return true;
+        } catch {
+          return false;
+        }
+      });
+
+    fc.assert(
+      fc.property(amountArb, (raw) => {
+        const units = parseUsdcToContractUnits(raw);
+        const formatted = formatContractUnitsToUsdc(units);
+        const reparsed = parseUsdcToContractUnits(formatted);
+        expect(reparsed).toBe(units);
+      }),
+      { numRuns: 10_000 },
+    );
+  });
+
+  it('rejects inputs with more than 7 fractional digits', () => {
+    fc.assert(
+      fc.property(
+        fc.bigInt({ min: 0n, max: 10n ** 12n }),
+        fc.array(fc.integer({ min: 0, max: 9 }), { minLength: 8, maxLength: 12 }),
+        (whole, digits) => {
+          const raw = `${whole}.${digits.join('')}`;
+          expect(() => parseUsdcToContractUnits(raw)).toThrow();
+        },
+      ),
+      { numRuns: 1_000 },
+    );
+  });
+
+  it('rejects negative amounts', () => {
+    fc.assert(
+      fc.property(fc.bigInt({ min: 1n, max: 10n ** 12n }), (whole) => {
+        expect(() => parseUsdcToContractUnits(`-${whole}`)).toThrow();
+      }),
+      { numRuns: 1_000 },
+    );
+  });
+
+  it("throws 'must be greater than zero' for zero", () => {
+    expect(() => parseUsdcToContractUnits('0')).toThrow('must be greater than zero');
+    expect(() => parseUsdcToContractUnits('0.0')).toThrow('must be greater than zero');
+    expect(() => parseUsdcToContractUnits('0.0000000')).toThrow('must be greater than zero');
+  });
+
+  it('never emits trailing zeros in formatted output', () => {
+    fc.assert(
+      fc.property(
+        fc.bigInt({ min: 1n, max: 10n ** 24n }),
+        (units) => {
+          const formatted = formatContractUnitsToUsdc(units);
+          expect(formatted).not.toMatch(/\.\d*0$/);
+          expect(formatted).not.toMatch(/\.$/);
+        },
+      ),
+      { numRuns: 10_000 },
+    );
+  });
+
+  it('preserves precision for very large whole numbers', () => {
+    fc.assert(
+      fc.property(
+        fc.bigInt({ min: 10n ** 18n, max: 10n ** 30n }),
+        (whole) => {
+          const raw = `${whole}`;
+          const units = parseUsdcToContractUnits(raw);
+          expect(units).toBe(whole * USDC_SCALE);
+          expect(formatContractUnitsToUsdc(units)).toBe(raw);
+        },
+      ),
+      { numRuns: 1_000 },
+    );
+  });
+});
+
 describe('migrations/billing_index.sql — EXPLAIN-verified hot path [b#057]', () => {
   let db: ReturnType<typeof newDb>;
 
@@ -61,6 +165,7 @@ describe('migrations/billing_index.sql — EXPLAIN-verified hot path [b#057]', (
     expect(() => db.public.none(upSql)).not.toThrow();
   });
 
+
   it('uses idx_billing_requests_lookup_hot for the hot developer_id filter', () => {
     const upSql = readFileSync(path.join(migrationsDir, 'billing_index.sql'), 'utf8');
     db.public.none(upSql);