fix(auth): add issuer to pre-registration context (#423)

examples/clients/typescript/everything-client.ts

@@ -741,6 +741,10 @@ export async function runPreRegistration(serverUrl: string): Promise<void> {
     redirect_uris: ['http://localhost:3000/callback']
   });
 
+  // Associate the pre-registered credentials with the AS that issued them,
+  // keyed by its issuer (Authorization Server Binding).
+  provider.bindIssuer(ctx.issuer);
+
   // Use the provider-based middleware
   const oauthFetch = withOAuthRetryWithProvider(
     provider,

src/scenarios/client/auth/index.test.ts

@@ -38,6 +38,8 @@ import { runClient as dpopClient } from '../../../../examples/clients/typescript
 import { getHandler } from '../../../../examples/clients/typescript/everything-client';
 import { setLogLevel } from '../../../../examples/clients/typescript/helpers/logger';
 import { DRAFT_PROTOCOL_VERSION } from '../../../types';
+import { testScenarioContext } from '../../../mock-server/testing';
+import { ClientConformanceContextSchema } from '../../../schemas/context';
 
 beforeAll(() => {
   setLogLevel('error');
@@ -112,6 +114,37 @@ describe('Client Draft Scenarios', () => {
   }
 });
 
+describe('auth/pre-registration context', () => {
+  // Authorization Server Binding: the context issuer is only usable as a
+  // binding key if it equals the issuer the mock AS publishes in its metadata.
+  test('supplies the issuer the mock AS publishes in its metadata', async () => {
+    const scenario = authScenariosList.find(
+      (s) => s.name === 'auth/pre-registration'
+    );
+    if (!scenario) {
+      throw new Error('auth/pre-registration scenario not found');
+    }
+    const urls = await scenario.start(testScenarioContext());
+    try {
+      const context = ClientConformanceContextSchema.parse({
+        name: 'auth/pre-registration',
+        ...urls.context
+      });
+      if (context.name !== 'auth/pre-registration') {
+        throw new Error(`Unexpected context variant: ${context.name}`);
+      }
+      const res = await fetch(
+        `${context.issuer}/.well-known/oauth-authorization-server`
+      );
+      expect(res.ok).toBe(true);
+      const metadata = (await res.json()) as { issuer?: string };
+      expect(metadata.issuer).toBe(context.issuer);
+    } finally {
+      await scenario.stop();
+    }
+  });
+});
+
 describe('Negative tests', () => {
   test('bad client requests root PRM location', async () => {
     const runner = new InlineClientRunner(badPrmClient);

src/scenarios/client/auth/pre-registration.ts

@@ -13,7 +13,12 @@ const PRE_REGISTERED_CLIENT_SECRET = 'pre-registered-secret';
  * Scenario: Pre-registration (static client credentials)
  *
  * Tests OAuth flow where the server does NOT support Dynamic Client Registration.
- * Clients must use pre-registered credentials passed via context.
+ * Clients must use pre-registered credentials passed via context. The context
+ * also carries the mock AS's `issuer` identifier: clients targeting the
+ * 2026-07-28 spec MUST associate pre-registered credentials with the
+ * authorization server that issued them, keyed by that issuer
+ * (Authorization Server Binding):
+ * https://modelcontextprotocol.io/specification/2026-07-28/basic/authorization/client-registration#authorization-server-binding
  *
  * This tests the pre-registration approach described in the MCP spec:
  * https://modelcontextprotocol.io/specification/draft/basic/authorization#preregistration
@@ -123,7 +128,11 @@ export class PreRegistrationScenario implements Scenario {
       serverUrl: `${this.server.getUrl()}/mcp`,
       context: {
         client_id: PRE_REGISTERED_CLIENT_ID,
-        client_secret: PRE_REGISTERED_CLIENT_SECRET
+        client_secret: PRE_REGISTERED_CLIENT_SECRET,
+        // The `issuer` the mock AS publishes in its metadata — the scenario
+        // sets neither `metadataIssuer` nor `routePrefix`, so createAuthServer
+        // resolves the published issuer to exactly this URL.
+        issuer: this.authServer.getUrl()
       }
     };
   }

src/schemas/context.ts

@@ -21,7 +21,8 @@ export const ClientConformanceContextSchema = z.discriminatedUnion('name', [
   z.object({
     name: z.literal('auth/pre-registration'),
     client_id: z.string(),
-    client_secret: z.string()
+    client_secret: z.string(),
+    issuer: z.string()
   }),
   z.object({
     name: z.literal('auth/enterprise-managed-authorization'),