diff --git a/examples/clients/typescript/everything-client.ts b/examples/clients/typescript/everything-client.ts
index 3f86339..4314fd8 100644
--- a/examples/clients/typescript/everything-client.ts
+++ b/examples/clients/typescript/everything-client.ts
@@ -147,6 +147,8 @@ registerScenarios(
     'auth/token-endpoint-auth-none',
     // Resource mismatch (client should error when PRM resource doesn't match)
     'auth/resource-mismatch',
+    // Issuer mismatch (client should error when AS metadata issuer doesn't match, RFC 8414 §3.3)
+    'auth/issuer-mismatch',
     // SEP-2207: Offline access / refresh token guidance (draft)
     'auth/offline-access-scope',
     'auth/offline-access-not-supported'
diff --git a/src/scenarios/client/auth/discovery-metadata.ts b/src/scenarios/client/auth/discovery-metadata.ts
index 3dd64b7..bf64554 100644
--- a/src/scenarios/client/auth/discovery-metadata.ts
+++ b/src/scenarios/client/auth/discovery-metadata.ts
@@ -22,8 +22,8 @@ interface MetadataScenarioConfig {
   prmLocation: string;
   inWwwAuth: boolean;
   oauthMetadataLocation: string;
-  /** Route prefix for the auth server (e.g., '/tenant1') */
-  authRoutePrefix?: string;
+  /** Issuer path component for the auth server (e.g., '/tenant1' for multi-tenant) */
+  authIssuerPath?: string;
   /** If true, add a trap for root PRM requests */
   trapRootPrm?: boolean;
 }
@@ -57,14 +57,14 @@ const SCENARIO_CONFIGS: MetadataScenarioConfig[] = [
     prmLocation: '/.well-known/oauth-protected-resource',
     inWwwAuth: false,
     oauthMetadataLocation: '/.well-known/oauth-authorization-server/tenant1',
-    authRoutePrefix: '/tenant1'
+    authIssuerPath: '/tenant1'
   },
   {
     name: 'metadata-var3',
     prmLocation: '/custom/metadata/location.json',
     inWwwAuth: true,
     oauthMetadataLocation: '/tenant1/.well-known/openid-configuration',
-    authRoutePrefix: '/tenant1'
+    authIssuerPath: '/tenant1'
   }
 ];
 
@@ -76,7 +76,7 @@ function createMetadataScenario(config: MetadataScenarioConfig): Scenario {
   const server = new ServerLifecycle();
   let checks: ConformanceCheck[] = [];
 
-  const routePrefix = config.authRoutePrefix || '';
+  const issuerPath = config.authIssuerPath || '';
   const isOpenIdConfiguration = config.oauthMetadataLocation.includes(
     'openid-configuration'
   );
@@ -100,11 +100,11 @@ function createMetadataScenario(config: MetadataScenarioConfig): Scenario {
       const authApp = createAuthServer(checks, authServer.getUrl, {
         metadataPath: config.oauthMetadataLocation,
         isOpenIdConfiguration,
-        ...(routePrefix && { routePrefix })
+        ...(issuerPath && { issuerPath })
       });
 
       // If path-based OAuth metadata, trap root requests
-      if (routePrefix) {
+      if (issuerPath) {
         authApp.get('/.well-known/oauth-authorization-server', (req, res) => {
           checks.push({
             id: 'authorization-server-metadata-wrong-path',
@@ -127,8 +127,8 @@ function createMetadataScenario(config: MetadataScenarioConfig): Scenario {
 
       await authServer.start(authApp);
 
-      const getAuthServerUrl = routePrefix
-        ? () => `${authServer.getUrl()}${routePrefix}`
+      const getAuthServerUrl = issuerPath
+        ? () => `${authServer.getUrl()}${issuerPath}`
         : authServer.getUrl;
 
       const app = createServer(checks, server.getUrl, getAuthServerUrl, {
diff --git a/src/scenarios/client/auth/helpers/createAuthServer.ts b/src/scenarios/client/auth/helpers/createAuthServer.ts
index c8c4ecd..5d5ef42 100644
--- a/src/scenarios/client/auth/helpers/createAuthServer.ts
+++ b/src/scenarios/client/auth/helpers/createAuthServer.ts
@@ -33,7 +33,18 @@ export interface AuthServerOptions {
   metadataPath?: string;
   isOpenIdConfiguration?: boolean;
   loggingEnabled?: boolean;
-  routePrefix?: string;
+  /**
+   * Path component of the issuer identifier (e.g., '/tenant1' for multi-tenant).
+   * Per RFC 8414, this must match the path used to construct the metadata URL.
+   * OAuth endpoints (/authorize, /token, /register) are mounted under this path.
+   */
+  issuerPath?: string;
+  /**
+   * Override the issuer value in the metadata response. For negative testing
+   * of RFC 8414 §3.3 issuer validation — clients MUST reject when the issuer
+   * in the response doesn't match the one used to construct the metadata URL.
+   */
+  issuerOverride?: string;
   scopesSupported?: string[];
   grantTypesSupported?: string[];
   tokenEndpointAuthMethodsSupported?: string[];
@@ -78,7 +89,8 @@ export function createAuthServer(
     metadataPath = '/.well-known/oauth-authorization-server',
     isOpenIdConfiguration = false,
     loggingEnabled = true,
-    routePrefix = '',
+    issuerPath = '',
+    issuerOverride,
     scopesSupported,
     grantTypesSupported = ['authorization_code', 'refresh_token'],
     tokenEndpointAuthMethodsSupported = ['none'],
@@ -98,9 +110,9 @@ export function createAuthServer(
   let storedCodeChallenge: string | undefined;
 
   const authRoutes = {
-    authorization_endpoint: `${routePrefix}/authorize`,
-    token_endpoint: `${routePrefix}/token`,
-    registration_endpoint: `${routePrefix}/register`
+    authorization_endpoint: `${issuerPath}/authorize`,
+    token_endpoint: `${issuerPath}/token`,
+    registration_endpoint: `${issuerPath}/register`
   };
 
   const app = express();
@@ -134,7 +146,7 @@ export function createAuthServer(
     });
 
     const metadata: any = {
-      issuer: `${getAuthBaseUrl()}${routePrefix}`,
+      issuer: issuerOverride ?? `${getAuthBaseUrl()}${issuerPath}`,
       authorization_endpoint: `${getAuthBaseUrl()}${authRoutes.authorization_endpoint}`,
       token_endpoint: `${getAuthBaseUrl()}${authRoutes.token_endpoint}`,
       ...(!disableDynamicRegistration && {
diff --git a/src/scenarios/client/auth/index.test.ts b/src/scenarios/client/auth/index.test.ts
index e18a467..32195de 100644
--- a/src/scenarios/client/auth/index.test.ts
+++ b/src/scenarios/client/auth/index.test.ts
@@ -22,14 +22,19 @@ beforeAll(() => {
 });
 
 const skipScenarios = new Set<string>([
-  // Add scenarios that should be skipped here
+  // TS SDK does not yet validate that AS metadata issuer matches the issuer
+  // used to construct the metadata URL (RFC 8414 §3.3). Unskip once
+  // typescript-sdk implements validation.
+  'auth/issuer-mismatch'
 ]);
 
 const allowClientErrorScenarios = new Set<string>([
   // Client is expected to give up (error) after limited retries, but check should pass
   'auth/scope-retry-limit',
   // Client is expected to error when PRM resource doesn't match server URL
-  'auth/resource-mismatch'
+  'auth/resource-mismatch',
+  // Client is expected to error when AS metadata issuer doesn't match (RFC 8414 §3.3)
+  'auth/issuer-mismatch'
 ]);
 
 describe('Client Auth Scenarios', () => {
@@ -68,6 +73,9 @@ describe('Client Back-compat Scenarios', () => {
 describe('Client Draft Scenarios', () => {
   for (const scenario of draftScenariosList) {
     test(`${scenario.name} passes`, async () => {
+      if (skipScenarios.has(scenario.name)) {
+        return;
+      }
       const clientFn = getHandler(scenario.name);
       if (!clientFn) {
         throw new Error(`No handler registered for scenario: ${scenario.name}`);
diff --git a/src/scenarios/client/auth/index.ts b/src/scenarios/client/auth/index.ts
index 7f65aa0..03a1296 100644
--- a/src/scenarios/client/auth/index.ts
+++ b/src/scenarios/client/auth/index.ts
@@ -22,6 +22,7 @@ import {
   ClientCredentialsBasicScenario
 } from './client-credentials';
 import { ResourceMismatchScenario } from './resource-mismatch';
+import { IssuerMismatchScenario } from './issuer-mismatch';
 import { PreRegistrationScenario } from './pre-registration';
 import { CrossAppAccessCompleteFlowScenario } from './cross-app-access';
 import {
@@ -60,6 +61,7 @@ export const extensionScenariosList: Scenario[] = [
 // Draft scenarios (informational - not scored for tier assessment)
 export const draftScenariosList: Scenario[] = [
   new ResourceMismatchScenario(),
+  new IssuerMismatchScenario(),
   new OfflineAccessScopeScenario(),
   new OfflineAccessNotSupportedScenario()
 ];
diff --git a/src/scenarios/client/auth/issuer-mismatch.ts b/src/scenarios/client/auth/issuer-mismatch.ts
new file mode 100644
index 0000000..1211846
--- /dev/null
+++ b/src/scenarios/client/auth/issuer-mismatch.ts
@@ -0,0 +1,101 @@
+import type { Scenario, ConformanceCheck } from '../../../types.js';
+import { ScenarioUrls, SpecVersion } from '../../../types.js';
+import { createAuthServer } from './helpers/createAuthServer.js';
+import { createServer } from './helpers/createServer.js';
+import { ServerLifecycle } from './helpers/serverLifecycle.js';
+import { SpecReferences } from './spec-references.js';
+
+/**
+ * Scenario: Authorization Server Issuer Mismatch Detection
+ *
+ * Tests that clients correctly detect and reject when the Authorization
+ * Server metadata response contains an `issuer` value that doesn't match
+ * the issuer identifier used to construct the metadata URL.
+ *
+ * Per RFC 8414 §3.3, clients MUST validate that the issuer in the metadata
+ * response matches the issuer used to construct the well-known metadata URL.
+ * Failing to do so enables mix-up attacks where a malicious AS impersonates
+ * another.
+ *
+ * Setup:
+ * - PRM advertises authorization server at http://localhost:<port> (root issuer)
+ * - Client constructs metadata URL /.well-known/oauth-authorization-server
+ * - AS responds with issuer: "https://evil.example.com" (mismatch)
+ *
+ * Expected behavior:
+ * - Client should NOT proceed with authorization
+ * - Client should abort due to issuer mismatch
+ * - Test passes if client does NOT make an authorization request
+ */
+export class IssuerMismatchScenario implements Scenario {
+  name = 'auth/issuer-mismatch';
+  specVersions: SpecVersion[] = ['draft'];
+  description =
+    'Tests that client rejects when AS metadata issuer does not match the issuer used to construct the metadata URL (RFC 8414 §3.3)';
+  allowClientError = true;
+
+  private authServer = new ServerLifecycle();
+  private server = new ServerLifecycle();
+  private checks: ConformanceCheck[] = [];
+  private authorizationRequestMade = false;
+
+  async start(): Promise<ScenarioUrls> {
+    this.checks = [];
+    this.authorizationRequestMade = false;
+
+    const authApp = createAuthServer(this.checks, this.authServer.getUrl, {
+      // Root issuer: metadata at /.well-known/oauth-authorization-server,
+      // so the expected issuer is just the base URL. Override it to a
+      // different origin to trigger the mismatch.
+      issuerOverride: 'https://evil.example.com',
+      onAuthorizationRequest: () => {
+        // If we get here, the client incorrectly proceeded past issuer validation
+        this.authorizationRequestMade = true;
+      }
+    });
+    await this.authServer.start(authApp);
+
+    const app = createServer(
+      this.checks,
+      this.server.getUrl,
+      this.authServer.getUrl,
+      {
+        prmPath: '/.well-known/oauth-protected-resource/mcp'
+      }
+    );
+    await this.server.start(app);
+
+    return { serverUrl: `${this.server.getUrl()}/mcp` };
+  }
+
+  async stop() {
+    await this.authServer.stop();
+    await this.server.stop();
+  }
+
+  getChecks(): ConformanceCheck[] {
+    const timestamp = new Date().toISOString();
+
+    if (!this.checks.some((c) => c.id === 'issuer-mismatch-rejected')) {
+      const correctlyRejected = !this.authorizationRequestMade;
+      this.checks.push({
+        id: 'issuer-mismatch-rejected',
+        name: 'Client rejects mismatched issuer',
+        description: correctlyRejected
+          ? 'Client correctly rejected authorization when AS metadata issuer does not match the metadata URL'
+          : 'Client MUST validate that the issuer in AS metadata matches the issuer used to construct the metadata URL (RFC 8414 §3.3)',
+        status: correctlyRejected ? 'SUCCESS' : 'FAILURE',
+        timestamp,
+        specReferences: [SpecReferences.RFC_AUTH_SERVER_METADATA_VALIDATION],
+        details: {
+          metadataIssuer: 'https://evil.example.com',
+          expectedIssuer: this.authServer.getUrl(),
+          expectedBehavior: 'Client should NOT proceed with authorization',
+          authorizationRequestMade: this.authorizationRequestMade
+        }
+      });
+    }
+
+    return this.checks;
+  }
+}
diff --git a/src/scenarios/client/auth/march-spec-backcompat.ts b/src/scenarios/client/auth/march-spec-backcompat.ts
index 3bc857e..255318a 100644
--- a/src/scenarios/client/auth/march-spec-backcompat.ts
+++ b/src/scenarios/client/auth/march-spec-backcompat.ts
@@ -18,11 +18,12 @@ export class Auth20250326OAuthMetadataBackcompatScenario implements Scenario {
     this.checks = [];
     // Legacy server, so we create the auth server endpoints on the
     // same URL as the main server (rather than separating AS / RS).
+    // Metadata at root well-known → issuer is the root URL (no path).
+    // Test integrity against fallback-bypass is ensured by expectedSlugs
+    // requiring 'authorization-server-metadata'.
     const authApp = createAuthServer(this.checks, this.server.getUrl, {
       // Disable logging since the main server will already have logging enabled
-      loggingEnabled: false,
-      // Add a prefix to auth endpoints to avoid being caught by auth fallbacks
-      routePrefix: '/oauth'
+      loggingEnabled: false
     });
     const app = createServer(
       this.checks,
diff --git a/src/scenarios/client/auth/spec-references.ts b/src/scenarios/client/auth/spec-references.ts
index 768dd65..89ecc55 100644
--- a/src/scenarios/client/auth/spec-references.ts
+++ b/src/scenarios/client/auth/spec-references.ts
@@ -9,6 +9,10 @@ export const SpecReferences: { [key: string]: SpecReference } = {
     id: 'RFC-8414-metadata-request',
     url: 'https://www.rfc-editor.org/rfc/rfc8414.html#section-3.1'
   },
+  RFC_AUTH_SERVER_METADATA_VALIDATION: {
+    id: 'RFC-8414-metadata-validation',
+    url: 'https://www.rfc-editor.org/rfc/rfc8414.html#section-3.3'
+  },
   LEGACY_2025_03_26_AUTH_DISCOVERY: {
     id: 'MCP-2025-03-26-Authorization-metadata-discovery',
     url: 'https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#server-metadata-discovery'