diff --git a/src/scenarios/authorization-server/authorization-server-metadata.test.ts b/src/scenarios/authorization-server/authorization-server-metadata.test.ts
index b20b47e..1cf862b 100644
--- a/src/scenarios/authorization-server/authorization-server-metadata.test.ts
+++ b/src/scenarios/authorization-server/authorization-server-metadata.test.ts
@@ -8,27 +8,32 @@ vi.mock('undici', () => ({
 
 const mockedRequest = vi.mocked(request);
 
-describe('AuthorizationServerMetadataEndpointScenario (SUCCESS only)', () => {
+const SERVER_URL = 'https://example.com';
+const AUTHORIZATION_ENDPOINT = `${SERVER_URL}/auth`;
+const TOKEN_ENDPOINT = `${SERVER_URL}/token`;
+
+const validMetadata = {
+  issuer: SERVER_URL,
+  authorization_endpoint: AUTHORIZATION_ENDPOINT,
+  token_endpoint: TOKEN_ENDPOINT,
+  response_types_supported: ['code'],
+  code_challenge_methods_supported: ['plain', 'S256']
+};
+
+function mockMetadataResponse(body: Record<string, unknown>) {
+  mockedRequest.mockResolvedValue({
+    statusCode: 200,
+    headers: { 'content-type': 'application/json' },
+    body: { json: async () => body }
+  } as any);
+}
+
+describe('AuthorizationServerMetadataEndpointScenario', () => {
   it('returns SUCCESS for valid authorization server metadata', async () => {
     const scenario = new AuthorizationServerMetadataEndpointScenario();
-    const serverUrl = 'https://example.com';
-
-    mockedRequest.mockResolvedValue({
-      statusCode: 200,
-      headers: {
-        'content-type': 'application/json'
-      },
-      body: {
-        json: async () => ({
-          issuer: 'https://example.com',
-          authorization_endpoint: 'https://example.com/auth',
-          token_endpoint: 'https://example.com/token',
-          response_types_supported: ['code']
-        })
-      }
-    } as any);
-
-    const checks = await scenario.run(serverUrl);
+    mockMetadataResponse(validMetadata);
+
+    const checks = await scenario.run(SERVER_URL);
 
     expect(checks).toHaveLength(1);
 
@@ -37,15 +42,50 @@ describe('AuthorizationServerMetadataEndpointScenario (SUCCESS only)', () => {
     expect(check.errorMessage).toBeUndefined();
     expect(check.details).toBeDefined();
     expect(check.details!.contentType).toContain('application/json');
-    expect((check.details!.body as any).issuer).toBe('https://example.com');
+    expect((check.details!.body as any).issuer).toBe(SERVER_URL);
     expect((check.details!.body as any).authorization_endpoint).toBe(
-      'https://example.com/auth'
-    );
-    expect((check.details!.body as any).token_endpoint).toBe(
-      'https://example.com/token'
+      AUTHORIZATION_ENDPOINT
     );
+    expect((check.details!.body as any).token_endpoint).toBe(TOKEN_ENDPOINT);
     expect((check.details!.body as any).response_types_supported).toEqual([
       'code'
     ]);
+    expect(
+      (check.details!.body as any).code_challenge_methods_supported
+    ).toEqual(['plain', 'S256']);
+  });
+
+  it('returns FAILURE when code_challenge_methods_supported is missing', async () => {
+    const scenario = new AuthorizationServerMetadataEndpointScenario();
+    mockMetadataResponse({
+      issuer: validMetadata.issuer,
+      authorization_endpoint: validMetadata.authorization_endpoint,
+      token_endpoint: validMetadata.token_endpoint,
+      response_types_supported: validMetadata.response_types_supported
+    });
+
+    const checks = await scenario.run(SERVER_URL);
+
+    expect(checks).toHaveLength(1);
+
+    const check = checks[0];
+    expect(check.status).toBe('FAILURE');
+    expect(check.errorMessage).toContain('code_challenge_methods_supported');
+  });
+
+  it('returns FAILURE when code_challenge_methods_supported does not include S256', async () => {
+    const scenario = new AuthorizationServerMetadataEndpointScenario();
+    mockMetadataResponse({
+      ...validMetadata,
+      code_challenge_methods_supported: ['plain']
+    });
+
+    const checks = await scenario.run(SERVER_URL);
+
+    expect(checks).toHaveLength(1);
+
+    const check = checks[0];
+    expect(check.status).toBe('FAILURE');
+    expect(check.errorMessage).toContain('code_challenge_methods_supported');
   });
 });
diff --git a/src/scenarios/authorization-server/authorization-server-metadata.ts b/src/scenarios/authorization-server/authorization-server-metadata.ts
index b386090..96b3669 100644
--- a/src/scenarios/authorization-server/authorization-server-metadata.ts
+++ b/src/scenarios/authorization-server/authorization-server-metadata.ts
@@ -142,6 +142,15 @@ export class AuthorizationServerMetadataEndpointScenario implements ClientScenar
       );
     }
 
+    if (
+      !Array.isArray(body.code_challenge_methods_supported) ||
+      !body.code_challenge_methods_supported.includes('S256')
+    ) {
+      errors.push(
+        'Response body does not include valid "code_challenge_methods_supported" claim'
+      );
+    }
+
     if (body.issuer !== serverUrl) {
       errors.push(`Invalid issuer: ${body.issuer ?? '(missing)'}`);
     }