diff --git a/src/commands/cloud.ts b/src/commands/cloud.ts
index 8eb2106..bedb952 100644
--- a/src/commands/cloud.ts
+++ b/src/commands/cloud.ts
@@ -35,7 +35,11 @@ import {
   isIosMatrixConfig,
 } from '../types/domain/device.types.js';
 import { resolveAuth } from '../utils/auth.js';
-import { matrixIsIos, parseDeviceMatrix } from '../utils/device-matrix.js';
+import {
+  assertMatrixSupported,
+  matrixIsIos,
+  parseDeviceMatrix,
+} from '../utils/device-matrix.js';
 import { detectCiContext, isCI } from '../utils/ci.js';
 import {
   CliError,
@@ -822,6 +826,10 @@ export const cloudCommand = defineCommand({
       // there is no matrix, and tolerant of older APIs that lack the endpoint.
       if (deviceMatrix.length > 0) {
         const estimate = await ApiGateway.estimateMatrix(apiUrl, auth, fields);
+        // A null estimate means the API predates the matrix (404/405). It would
+        // silently strip deviceMatrix and run one device — refuse rather than
+        // hand back a green single-device run the user reads as a matrix.
+        assertMatrixSupported(deviceMatrix, estimate);
         if (estimate) {
           const osPrefix = matrixIsIos(deviceMatrix) ? 'iOS' : 'API';
           const rows = ui.fields([
diff --git a/src/utils/device-matrix.ts b/src/utils/device-matrix.ts
index 8e9e5f9..7f171ae 100644
--- a/src/utils/device-matrix.ts
+++ b/src/utils/device-matrix.ts
@@ -4,6 +4,34 @@ import {
 } from '../types/domain/device.types.js';
 import { CliError } from './cli.js';
 
+/**
+ * Refuse to submit a device matrix to an API that cannot honour it.
+ *
+ * The estimate endpoint is only called when a matrix was actually requested, so
+ * a null estimate (the gateway maps 404/405 to null) means the API predates the
+ * feature. That API would **silently strip** the unknown `deviceMatrix` field —
+ * its ValidationPipe runs `whitelist: true, forbidNonWhitelisted: false` — and
+ * run every flow on a single default device, exiting 0. The user would believe
+ * they had tested N devices when they tested one: the exact silent
+ * under-testing the device matrix exists to prevent. Fail loudly instead.
+ *
+ * @throws CliError when a matrix was requested but the API does not support it.
+ */
+export function assertMatrixSupported(
+  deviceMatrix: DeviceMatrixConfig[],
+  estimate: unknown | null,
+): void {
+  if (deviceMatrix.length === 0 || estimate) return;
+
+  throw new CliError(
+    'This DeviceCloud API does not support device matrices, so ' +
+      '--ios-device-matrix / --android-device-matrix cannot be honoured. ' +
+      'Submitting anyway would silently run every flow on a single default ' +
+      'device and report success. Upgrade the API, or drop the matrix flags ' +
+      'and use --ios-device / --android-device for a single-device run.',
+  );
+}
+
 /**
  * Parse repeated `--ios-device-matrix <device>:<version>` and
  * `--android-device-matrix <device>:<apiLevel>[:play]` flags into an explicit device
diff --git a/test/unit/device-matrix.test.ts b/test/unit/device-matrix.test.ts
index 8a4cb11..20476a1 100644
--- a/test/unit/device-matrix.test.ts
+++ b/test/unit/device-matrix.test.ts
@@ -2,6 +2,7 @@ import { expect } from 'chai';
 
 import { CliError } from '../../src/utils/cli.js';
 import {
+  assertMatrixSupported,
   matrixIsIos,
   parseDeviceMatrix,
 } from '../../src/utils/device-matrix.js';
@@ -59,3 +60,36 @@ describe('parseDeviceMatrix', () => {
     );
   });
 });
+
+/**
+ * An API that predates the matrix silently STRIPS the unknown deviceMatrix
+ * field (its ValidationPipe is whitelist:true / forbidNonWhitelisted:false) and
+ * runs every flow on one default device, exiting 0. Submitting into that is the
+ * worst outcome the feature can produce, so it must be refused, not tolerated.
+ */
+describe('assertMatrixSupported', () => {
+  const matrix = [{ iOSDevice: 'iphone-16', iOSVersion: '18' }];
+
+  it('throws when a matrix was requested but the API has no estimate endpoint', () => {
+    expect(() => assertMatrixSupported(matrix, null)).to.throw(
+      CliError,
+      /does not support device matrices/i,
+    );
+  });
+
+  it('explains that submitting anyway would silently run a single device', () => {
+    expect(() => assertMatrixSupported(matrix, null)).to.throw(
+      /silently run every flow on a single default device/i,
+    );
+  });
+
+  it('passes when the API returned an estimate', () => {
+    expect(() =>
+      assertMatrixSupported(matrix, { cellCount: 2, totalCost: 0.16 }),
+    ).to.not.throw();
+  });
+
+  it('is a no-op when no matrix was requested (legacy single-device runs)', () => {
+    expect(() => assertMatrixSupported([], null)).to.not.throw();
+  });
+});