diff --git a/app/controllers/billingtypecontroller.js b/app/controllers/billingtypecontroller.js
index 1d0c656..00dbc2e 100644
--- a/app/controllers/billingtypecontroller.js
+++ b/app/controllers/billingtypecontroller.js
@@ -91,8 +91,13 @@ exports.getById = async (req, res) => {
     const isMaster = await IsMaster(authKey);
     if (!isMaster) {
         const companyId = await GetCompanyId(authKey);
+        // Cross-tenant access is reported as 404, not 403 — otherwise
+        // a scoped caller can enumerate which BillingType ids are
+        // populated across the whole tenant table by status code:
+        // 403 = "exists but not yours", 404 = "doesn't exist". Same
+        // secure-404 pattern landed for /v1/company in #174.
         if (companyId === -1 || billingType.btCompId !== companyId) {
-            return res.status(403).json({ message: "Invalid Authorization Key." });
+            return res.status(404).json({ message: "Not found." });
         }
     }
     return res.status(200).json({ message: "Found.", billingType });
@@ -168,8 +173,11 @@ exports.update = async (req, res) => {
     const isMaster = await IsMaster(authKey);
     if (!isMaster) {
         const companyId = await GetCompanyId(authKey);
+        // Secure-404 on PATCH for the same reason as GET — don't let
+        // a scoped caller distinguish "exists but not yours" from
+        // "doesn't exist".
         if (companyId === -1 || billingType.btCompId !== companyId) {
-            return res.status(403).json({ message: "Invalid Authorization Key." });
+            return res.status(404).json({ message: "Not found." });
         }
     }
 
@@ -211,8 +219,9 @@ exports.remove = async (req, res) => {
     const isMaster = await IsMaster(authKey);
     if (!isMaster) {
         const companyId = await GetCompanyId(authKey);
+        // Secure-404 on DELETE for the same reason as GET / PATCH.
         if (companyId === -1 || billingType.btCompId !== companyId) {
-            return res.status(403).json({ message: "Invalid Authorization Key." });
+            return res.status(404).json({ message: "Not found." });
         }
     }
 
diff --git a/tests/api/billingtype.test.js b/tests/api/billingtype.test.js
index 4841249..81a1040 100644
--- a/tests/api/billingtype.test.js
+++ b/tests/api/billingtype.test.js
@@ -67,6 +67,92 @@ describe('BillingType route mounting', () => {
     });
 });
 
+describe('BillingType tenant-enumeration defense (secure 404)', () => {
+    // Same approach as the company secure-404 unit tests: drive the
+    // controller directly with stubbed Model + spied auth helpers, so
+    // we don't have to wire every upstream middleware. The HTTP-level
+    // mock above makes findByPk resolve to null by default, which
+    // short-circuits to 404 before the cross-tenant branch is reached.
+    test('controller getById: existing-but-not-yours returns 404 to non-master', async () => {
+        const auth = require('../../app/middleware/auth.js');
+        const controller = require('../../app/controllers/billingtypecontroller.js');
+        const isMasterSpy = vi.spyOn(auth, 'isMaster').mockResolvedValue(false);
+        const getCompanyIdSpy = vi.spyOn(auth, 'getCompanyId').mockResolvedValue(7);
+        try {
+            const db = require('../../app/config/db.config.js');
+            db.BillingType.findByPk = vi.fn().mockResolvedValue({
+                btId: 99, btCompId: 99, btArch: false,
+            });
+            const req = { get: (h) => (h === 'authKey' ? 'scoped-to-7' : undefined), params: { id: 99 } };
+            let captured = null;
+            const res = {
+                status(code) { this._code = code; return this; },
+                json(body) { captured = { code: this._code, body }; return this; },
+            };
+            await controller.getById(req, res);
+            expect(captured.code).toBe(404);
+            expect(captured.body.message).toMatch(/not found/i);
+        } finally {
+            isMasterSpy.mockRestore();
+            getCompanyIdSpy.mockRestore();
+        }
+    });
+
+    test('controller update: existing-but-not-yours returns 404 to non-master', async () => {
+        const auth = require('../../app/middleware/auth.js');
+        const controller = require('../../app/controllers/billingtypecontroller.js');
+        const isMasterSpy = vi.spyOn(auth, 'isMaster').mockResolvedValue(false);
+        const getCompanyIdSpy = vi.spyOn(auth, 'getCompanyId').mockResolvedValue(7);
+        try {
+            const db = require('../../app/config/db.config.js');
+            db.BillingType.findByPk = vi.fn().mockResolvedValue({
+                btId: 99, btCompId: 99, btArch: false, update: vi.fn(),
+            });
+            const req = {
+                get: (h) => (h === 'authKey' ? 'scoped-to-7' : undefined),
+                params: { id: 99 },
+                body: { btName: 'X' },
+            };
+            let captured = null;
+            const res = {
+                status(code) { this._code = code; return this; },
+                json(body) { captured = { code: this._code, body }; return this; },
+            };
+            await controller.update(req, res);
+            expect(captured.code).toBe(404);
+            expect(captured.body.message).toMatch(/not found/i);
+        } finally {
+            isMasterSpy.mockRestore();
+            getCompanyIdSpy.mockRestore();
+        }
+    });
+
+    test('controller remove: existing-but-not-yours returns 404 to non-master', async () => {
+        const auth = require('../../app/middleware/auth.js');
+        const controller = require('../../app/controllers/billingtypecontroller.js');
+        const isMasterSpy = vi.spyOn(auth, 'isMaster').mockResolvedValue(false);
+        const getCompanyIdSpy = vi.spyOn(auth, 'getCompanyId').mockResolvedValue(7);
+        try {
+            const db = require('../../app/config/db.config.js');
+            db.BillingType.findByPk = vi.fn().mockResolvedValue({
+                btId: 99, btCompId: 99, btArch: false, update: vi.fn(),
+            });
+            const req = { get: (h) => (h === 'authKey' ? 'scoped-to-7' : undefined), params: { id: 99 } };
+            let captured = null;
+            const res = {
+                status(code) { this._code = code; return this; },
+                json(body) { captured = { code: this._code, body }; return this; },
+            };
+            await controller.remove(req, res);
+            expect(captured.code).toBe(404);
+            expect(captured.body.message).toMatch(/not found/i);
+        } finally {
+            isMasterSpy.mockRestore();
+            getCompanyIdSpy.mockRestore();
+        }
+    });
+});
+
 describe('BillingType body validation', () => {
     test('POST rejects unknown field with 400', async () => {
         const res = await request(app)