diff --git a/app/schemas/invoice.schema.js b/app/schemas/invoice.schema.js
index 1e05d13..450d643 100644
--- a/app/schemas/invoice.schema.js
+++ b/app/schemas/invoice.schema.js
@@ -12,6 +12,25 @@ const isoDate = z.string().regex(/^\d{4}-\d{2}-\d{2}$/, {
     message: 'Must be an ISO 8601 date (YYYY-MM-DD).',
 });
 
+// Cross-field refinement: an invoice's due date must be on or
+// after its issue date. Mirrors the teEndedAt >= teStartedAt
+// constraint from #130 — a due-before-issue invoice is operator
+// error worth surfacing at the boundary instead of papering over.
+// Equality (due same day as issue) is allowed: zero-day-net is a
+// real billing term ("Due on Receipt").
+//
+// String comparison is safe here because `isoDate` is the strict
+// `YYYY-MM-DD` regex above; lexicographic order on that shape
+// matches chronological order for any valid input.
+function refineDueDateAfterIssue(data) {
+    if (!data.invDate || !data.invDueDate) return true;
+    return data.invDueDate >= data.invDate;
+}
+const DUE_BEFORE_ISSUE = {
+    message: 'invDueDate must be on or after invDate.',
+    path: ['invDueDate'],
+};
+
 const createInvoiceBody = z.object({
     invCustId: z.coerce.number().int().positive(),
     invDate: isoDate,
@@ -19,7 +38,7 @@ const createInvoiceBody = z.object({
     invPaid: z.boolean().optional(),
 }).strict({
     message: 'Unexpected field in body. Whitelist: invCustId, invDate, invDueDate, invPaid.',
-});
+}).refine(refineDueDateAfterIssue, DUE_BEFORE_ISSUE);
 
 const updateInvoiceBody = z.object({
     invDate: isoDate.optional(),
@@ -27,7 +46,7 @@ const updateInvoiceBody = z.object({
     invPaid: z.boolean().optional(),
 }).strict({
     message: 'Unexpected field in body. Whitelist: invDate, invDueDate, invPaid.',
-});
+}).refine(refineDueDateAfterIssue, DUE_BEFORE_ISSUE);
 
 const listByCustomerQuery = z.object({
     limit: z.coerce.number().int().positive().max(500).optional(),
diff --git a/tests/api/invoice.test.js b/tests/api/invoice.test.js
index d682dc5..6043c00 100644
--- a/tests/api/invoice.test.js
+++ b/tests/api/invoice.test.js
@@ -56,4 +56,67 @@ describe('Invoice body validation', () => {
         const res = await request(app).post('/v1/invoice').set('authKey', 'any').send({ invCustId: 1, invDate: 'tomorrow', invDueDate: '2026-02-01' });
         expect(res.status).toBe(400);
     });
+
+    test('POST rejects invDueDate strictly before invDate', async () => {
+        // Inverted range — due date *before* issue date is nonsense. Pin
+        // the refinement so a future schema refactor can't accidentally
+        // drop the check.
+        const res = await request(app).post('/v1/invoice').set('authKey', 'any').send({
+            invCustId: 1,
+            invDate: '2026-05-15',
+            invDueDate: '2026-05-01',
+        });
+        expect(res.status).toBe(400);
+        const issue = res.body.issues && res.body.issues.find((i) => i.path === 'invDueDate');
+        expect(issue).toBeDefined();
+        expect(issue.message).toMatch(/on or after invDate/i);
+    });
+
+    test('POST accepts invDueDate equal to invDate (zero-day-net / "Due on Receipt")', async () => {
+        // Equality is a legitimate billing term, not a bug. Schema must
+        // not 400 — auth/controller decides the final status from there.
+        const res = await request(app).post('/v1/invoice').set('authKey', 'any').send({
+            invCustId: 1,
+            invDate: '2026-05-15',
+            invDueDate: '2026-05-15',
+        });
+        expect(res.status).not.toBe(400);
+    });
+
+    test('PATCH rejects inverted range when both bounds are sent', async () => {
+        const res = await request(app).patch('/v1/invoice/1').set('authKey', 'any').send({
+            invDate: '2026-05-15',
+            invDueDate: '2026-05-01',
+        });
+        expect(res.status).toBe(400);
+        const issue = res.body.issues && res.body.issues.find((i) => i.path === 'invDueDate');
+        expect(issue).toBeDefined();
+    });
+
+    test('PATCH with only invDueDate is not blocked by the schema', async () => {
+        // The cross-field refinement can't validate a single-bound PATCH
+        // without seeing the existing row; the schema must not reject
+        // it. Controller-layer enforcement against the existing invDate
+        // is a separate follow-up.
+        const res = await request(app).patch('/v1/invoice/1').set('authKey', 'any').send({
+            invDueDate: '2026-05-01',
+        });
+        expect(res.status).not.toBe(400);
+    });
+
+    test('bulk POST rejects an inverted-range entry inside the batch', async () => {
+        // The bulk path validates each element through createInvoiceBody,
+        // so the refinement must fire there too — anything else would
+        // let an attacker bypass the check by wrapping the bad entry in
+        // a bulk envelope.
+        const res = await request(app).post('/v1/invoice/bulk').set('authKey', 'any').send({
+            invoices: [
+                { invCustId: 1, invDate: '2026-05-15', invDueDate: '2026-05-01' },
+            ],
+        });
+        expect(res.status).toBe(400);
+        // Path on a bulk entry's issue: `invoices.0.invDueDate`.
+        const issue = res.body.issues && res.body.issues.find((i) => i.path.endsWith('invDueDate'));
+        expect(issue).toBeDefined();
+    });
 });