diff --git a/clients/tui/__tests__/App.test.tsx b/clients/tui/__tests__/App.test.tsx
index b8efdb7c..dcf3be47 100644
--- a/clients/tui/__tests__/App.test.tsx
+++ b/clients/tui/__tests__/App.test.tsx
@@ -983,9 +983,12 @@ describe("App (OAuth flows)", () => {
     const r = await mount(oneHttp());
     await press(r, ["c"]);
     await waitUntil(() => h.cb.opts !== null);
-    await h.cb.opts!.onCallback({ code: "abc" });
+    await h.cb.opts!.onCallback({ code: "abc", iss: "https://as.example" });
     await tick();
-    expect(h.clientSpies.completeOAuthFlow).toHaveBeenCalledWith("abc");
+    expect(h.clientSpies.completeOAuthFlow).toHaveBeenCalledWith(
+      "abc",
+      "https://as.example",
+    );
   });
 
   it("reports an OAuth callback error during a 401 connect", async () => {
diff --git a/clients/web/src/App.tsx b/clients/web/src/App.tsx
index 289fc371..92ad40b2 100644
--- a/clients/web/src/App.tsx
+++ b/clients/web/src/App.tsx
@@ -2326,6 +2326,7 @@ function App() {
         connectStartRef.current = Date.now();
         await client.resumeAfterOAuth(params.code, {
           remoteSessionId: resumeSnapshot?.remoteSessionId,
+          iss: params.iss,
         });
       } catch (err) {
         connectStartRef.current = undefined;
diff --git a/clients/web/src/test/core/auth/ema/emaFlow.test.ts b/clients/web/src/test/core/auth/ema/emaFlow.test.ts
index 25ff3b78..2b71f484 100644
--- a/clients/web/src/test/core/auth/ema/emaFlow.test.ts
+++ b/clients/web/src/test/core/auth/ema/emaFlow.test.ts
@@ -381,6 +381,7 @@ describe("emaFlow", () => {
     const tokens = await completeEmaIdpAuthorizationAndMint(
       config,
       "auth-code",
+      IDP_ISSUER,
     );
 
     expect(tokens.access_token).toBe("resource-access-token");
@@ -415,7 +416,7 @@ describe("emaFlow", () => {
     await startEmaIdpAuthorization(config);
 
     await expect(
-      completeEmaIdpAuthorizationAndMint(config, "auth-code"),
+      completeEmaIdpAuthorizationAndMint(config, "auth-code", IDP_ISSUER),
     ).rejects.toThrow(/EMA legs 2–3 \(resource token mint\)/);
   });
 
@@ -480,7 +481,7 @@ describe("emaFlow", () => {
     });
 
     await expect(
-      completeEmaIdpAuthorizationAndMint(config, "auth-code"),
+      completeEmaIdpAuthorizationAndMint(config, "auth-code", IDP_ISSUER),
     ).rejects.toThrow(
       "EMA leg 1 (IdP authorization code exchange): leg1-non-error",
     );
diff --git a/clients/web/src/test/core/auth/ema/idpOidc.test.ts b/clients/web/src/test/core/auth/ema/idpOidc.test.ts
index 1c6d73f3..1f9fec62 100644
--- a/clients/web/src/test/core/auth/ema/idpOidc.test.ts
+++ b/clients/web/src/test/core/auth/ema/idpOidc.test.ts
@@ -390,6 +390,7 @@ describe("completeIdpOidcAuthorization", () => {
     const result = await completeIdpOidcAuthorization({
       idp,
       authorizationCode: "auth-code-xyz",
+      iss: IDP_ISSUER,
       redirectUrl: "http://127.0.0.1:3000/oauth/callback",
       storage,
       fetchFn,
@@ -465,6 +466,7 @@ describe("completeIdpOidcAuthorization", () => {
     const result = await completeIdpOidcAuthorization({
       idp,
       authorizationCode: "code",
+      iss: IDP_ISSUER,
       redirectUrl: "http://127.0.0.1:3000/oauth/callback",
       storage,
       fetchFn,
@@ -498,10 +500,67 @@ describe("completeIdpOidcAuthorization", () => {
       completeIdpOidcAuthorization({
         idp,
         authorizationCode: "code",
+        iss: IDP_ISSUER,
         redirectUrl: "http://127.0.0.1:3000/oauth/callback",
         storage,
         fetchFn,
       }),
     ).rejects.toThrow("IdP token response did not include an ID Token");
   });
+
+  // Regression: the IdP metadata advertises
+  // `authorization_response_iss_parameter_supported`, so the SDK enforces
+  // RFC 9207 §2.4 and rejects the exchange unless the callback `iss` is
+  // forwarded. Dropping `iss` anywhere between the callback and
+  // `exchangeAuthorization` breaks EMA leg 1 against every real IdP.
+  it("forwards the callback iss to the token exchange (RFC 9207)", async () => {
+    storage = buildStorage();
+    const idToken = jwtWithExp(Math.floor(Date.now() / 1000) + 3600);
+    const fetchFn = vi.fn(
+      async () =>
+        new Response(
+          JSON.stringify({
+            id_token: idToken,
+            access_token: "at",
+            token_type: "Bearer",
+          }),
+        ),
+    );
+
+    await expect(
+      completeIdpOidcAuthorization({
+        idp,
+        authorizationCode: "code",
+        redirectUrl: "http://127.0.0.1:3000/oauth/callback",
+        storage,
+        fetchFn,
+      }),
+    ).rejects.toThrow(/Issuer mismatch/);
+
+    const result = await completeIdpOidcAuthorization({
+      idp,
+      authorizationCode: "code",
+      iss: IDP_ISSUER,
+      redirectUrl: "http://127.0.0.1:3000/oauth/callback",
+      storage,
+      fetchFn,
+    });
+    expect(result.idToken).toBe(idToken);
+  });
+
+  it("rejects a callback iss from a different issuer (mix-up defense)", async () => {
+    storage = buildStorage();
+    const fetchFn = vi.fn();
+    await expect(
+      completeIdpOidcAuthorization({
+        idp,
+        authorizationCode: "code",
+        iss: "https://evil.example.com",
+        redirectUrl: "http://127.0.0.1:3000/oauth/callback",
+        storage,
+        fetchFn,
+      }),
+    ).rejects.toThrow(/Issuer mismatch/);
+    expect(fetchFn).not.toHaveBeenCalled();
+  });
 });
diff --git a/clients/web/src/test/core/auth/runner-interactive-oauth.test.ts b/clients/web/src/test/core/auth/runner-interactive-oauth.test.ts
index 1dbf740f..d572e0bb 100644
--- a/clients/web/src/test/core/auth/runner-interactive-oauth.test.ts
+++ b/clients/web/src/test/core/auth/runner-interactive-oauth.test.ts
@@ -56,15 +56,21 @@ function createMockCallbackServer(handlers: {
   } as unknown as OAuthCallbackServer;
 }
 
-/** Drive the loopback callback the way a browser redirect would. */
+const CALLBACK_ISS = "https://as.example";
+
+/**
+ * Drive the loopback callback the way a browser redirect would, including the
+ * RFC 9207 `iss` that real authorization servers send.
+ */
 async function simulateCallback(
   handlers: MockCallbackHandlers,
   code = "auth-code-123",
+  iss: string | undefined = CALLBACK_ISS,
 ): Promise<void> {
   if (!handlers.onCallback) {
     throw new Error("onCallback not registered");
   }
-  await handlers.onCallback({ code });
+  await handlers.onCallback({ code, iss });
 }
 
 describe("runRunnerInteractiveOAuth", () => {
@@ -145,7 +151,10 @@ describe("runRunnerInteractiveOAuth", () => {
     });
 
     expect(result).toEqual({ kind: "success" });
-    expect(client.completeOAuthFlow).toHaveBeenCalledWith("auth-code-123");
+    expect(client.completeOAuthFlow).toHaveBeenCalledWith(
+      "auth-code-123",
+      CALLBACK_ISS,
+    );
     expect(redirectUrlProvider.redirectUrl).toBe(
       "http://127.0.0.1:6276/oauth/callback",
     );
@@ -179,7 +188,10 @@ describe("runRunnerInteractiveOAuth", () => {
       authorizationUrl,
     );
     expect(client.authenticate).not.toHaveBeenCalled();
-    expect(client.completeOAuthFlow).toHaveBeenCalledWith("step-up-code");
+    expect(client.completeOAuthFlow).toHaveBeenCalledWith(
+      "step-up-code",
+      CALLBACK_ISS,
+    );
   });
 
   it("returns insufficient_scope when post-step-up check fails", async () => {
diff --git a/clients/web/src/test/core/mcp/oauthManager.test.ts b/clients/web/src/test/core/mcp/oauthManager.test.ts
index 064c9301..b26538dd 100644
--- a/clients/web/src/test/core/mcp/oauthManager.test.ts
+++ b/clients/web/src/test/core/mcp/oauthManager.test.ts
@@ -1525,11 +1525,12 @@ describe("OAuthManager", () => {
       );
       expect(outcome.kind).toBe("interactive");
 
-      await manager.completeOAuthFlow("auth-code");
+      await manager.completeOAuthFlow("auth-code", "https://idp.example.com");
 
       expect(mintSpy).toHaveBeenCalledWith(
         expect.objectContaining({ scope: "mcp tools:read weather:read" }),
         "auth-code",
+        "https://idp.example.com",
       );
       expect(storageOf(params).saveScope).toHaveBeenCalledWith(
         SERVER_URL,
diff --git a/clients/web/src/test/integration/mcp/ema-mock-servers.ts b/clients/web/src/test/integration/mcp/ema-mock-servers.ts
index 45ea16d0..d502cf9c 100644
--- a/clients/web/src/test/integration/mcp/ema-mock-servers.ts
+++ b/clients/web/src/test/integration/mcp/ema-mock-servers.ts
@@ -21,7 +21,15 @@ export const EMA_MOCK_IDP_CLIENT_SECRET = "ema-mock-idp-secret";
 export const EMA_MOCK_RESOURCE_CLIENT_ID = "ema-mock-resource-client";
 export const EMA_MOCK_RESOURCE_CLIENT_SECRET = "ema-mock-resource-secret";
 
-/** Fields required by SDK `OAuthMetadataSchema` for discovery in tests. */
+/**
+ * Fields required by SDK `OAuthMetadataSchema` for discovery in tests.
+ *
+ * `authorization_response_iss_parameter_supported` mirrors real-world IdPs
+ * (e.g. Okta/xaa.dev): once advertised, the SDK enforces RFC 9207 §2.4 and
+ * rejects a callback whose `iss` is missing. Keep it on so the mock stays at
+ * least as strict as production servers — with it absent the SDK silently takes
+ * its lenient path and a dropped `iss` goes unnoticed.
+ */
 export function minimalOAuthAsMetadata(
   baseUrl: string,
   authorizationEndpoint = `${baseUrl}/authorize`,
@@ -32,6 +40,7 @@ export function minimalOAuthAsMetadata(
     token_endpoint: `${baseUrl}/token`,
     response_types_supported: ["code"],
     token_endpoint_auth_methods_supported: ["client_secret_post"],
+    authorization_response_iss_parameter_supported: true,
   };
 }
 
diff --git a/core/auth/ema/emaFlow.ts b/core/auth/ema/emaFlow.ts
index b727fbf4..eb3f40f0 100644
--- a/core/auth/ema/emaFlow.ts
+++ b/core/auth/ema/emaFlow.ts
@@ -125,11 +125,13 @@ export async function startEmaIdpAuthorization(
 export async function completeEmaIdpAuthorizationAndMint(
   config: EmaFlowConfig,
   authorizationCode: string,
+  iss?: string,
 ): Promise<OAuthTokens> {
   try {
     await completeIdpOidcAuthorization({
       idp: config.idp,
       authorizationCode,
+      iss,
       redirectUrl: config.redirectUrl,
       storage: config.storage,
       fetchFn: config.fetchFn,
diff --git a/core/auth/ema/idpOidc.ts b/core/auth/ema/idpOidc.ts
index 78fc6978..60f3bdbb 100644
--- a/core/auth/ema/idpOidc.ts
+++ b/core/auth/ema/idpOidc.ts
@@ -97,6 +97,12 @@ export async function startIdpOidcAuthorization(params: {
 export async function completeIdpOidcAuthorization(params: {
   idp: EnterpriseManagedAuthIdpConfig;
   authorizationCode: string;
+  /**
+   * RFC 9207 `iss` from the IdP callback. Forwarded to the SDK, which rejects
+   * the exchange when the IdP metadata advertises
+   * `authorization_response_iss_parameter_supported` but no `iss` arrives.
+   */
+  iss?: string;
   redirectUrl: string;
   storage: OAuthStorage;
   fetchFn?: typeof fetch;
@@ -127,6 +133,7 @@ export async function completeIdpOidcAuthorization(params: {
     metadata,
     clientInformation,
     authorizationCode: params.authorizationCode,
+    iss: params.iss,
     codeVerifier,
     redirectUri: params.redirectUrl,
     fetchFn: params.fetchFn,
diff --git a/core/auth/mcpAuth.ts b/core/auth/mcpAuth.ts
index 45b7b1e6..4638605e 100644
--- a/core/auth/mcpAuth.ts
+++ b/core/auth/mcpAuth.ts
@@ -44,7 +44,7 @@ export type McpAuthResult = "AUTHORIZED" | "REDIRECT";
 export interface McpAuthOptions {
   serverUrl: string | URL;
   authorizationCode?: string;
-  /** RFC 9207 callback `iss` — forwarded on v2 upgrade; ignored by v1 SDK `auth()`. */
+  /** RFC 9207 callback `iss`, validated by the SDK against the metadata issuer. */
   iss?: string;
   scope?: string;
   resourceMetadataUrl?: URL;
@@ -74,6 +74,7 @@ export async function mcpAuth(
   return sdkAuth(provider, {
     serverUrl: options.serverUrl,
     authorizationCode: options.authorizationCode,
+    iss: options.iss,
     scope: options.scope,
     resourceMetadataUrl: options.resourceMetadataUrl,
     fetchFn: options.fetchFn,
diff --git a/core/auth/node/oauth-callback-server.ts b/core/auth/node/oauth-callback-server.ts
index 2b5149ec..37a0cce6 100644
--- a/core/auth/node/oauth-callback-server.ts
+++ b/core/auth/node/oauth-callback-server.ts
@@ -30,6 +30,8 @@ function escapeHtml(s: string): string {
 export type OAuthCallbackHandler = (params: {
   code: string;
   state?: string;
+  /** RFC 9207 `iss`, when the authorization server sends it. */
+  iss?: string;
 }) => Promise<void>;
 
 export type OAuthErrorHandler = (params: {
@@ -181,7 +183,7 @@ export class OAuthCallbackServer {
       this.handled = true;
       const cb = this.onCallback;
       if (cb) {
-        cb({ code: params.code, state })
+        cb({ code: params.code, state, iss: params.iss })
           .then(() => {
             send(200, SUCCESS_HTML);
             void this.stop();
diff --git a/core/auth/node/runner-interactive-oauth.ts b/core/auth/node/runner-interactive-oauth.ts
index f68a3280..edbb394d 100644
--- a/core/auth/node/runner-interactive-oauth.ts
+++ b/core/auth/node/runner-interactive-oauth.ts
@@ -9,7 +9,7 @@ import type { RunnerOAuthCallbackConfig } from "./runner-oauth-callback.js";
 export interface RunnerInteractiveOAuthClient {
   authenticate(): Promise<URL | undefined>;
   beginInteractiveAuthorization(authorizationUrl: URL): Promise<void>;
-  completeOAuthFlow(authorizationCode: string): Promise<void>;
+  completeOAuthFlow(authorizationCode: string, iss?: string): Promise<void>;
   checkAuthChallengeSatisfied(challenge: AuthChallenge): Promise<boolean>;
 }
 
@@ -67,7 +67,7 @@ export async function runRunnerInteractiveOAuth(
       path: options.callbackListen.pathname,
       onCallback: async (params) => {
         try {
-          await options.client.completeOAuthFlow(params.code);
+          await options.client.completeOAuthFlow(params.code, params.iss);
           flowResolve();
         } catch (err) {
           flowReject(err instanceof Error ? err : new Error(String(err)));
diff --git a/core/auth/types.ts b/core/auth/types.ts
index e40a7416..82b0bfb4 100644
--- a/core/auth/types.ts
+++ b/core/auth/types.ts
@@ -105,6 +105,12 @@ export type CallbackParams =
       successful: true;
       // The authorization code is generated by the authorization server.
       code: string;
+      // RFC 9207 `iss`. Present only when the authorization server includes it.
+      // Forwarded to the SDK, which validates it against the issuer in the
+      // validated metadata (RFC 9207 §2.4) to detect mix-up attacks. Servers
+      // advertising `authorization_response_iss_parameter_supported: true` MUST
+      // send it, and the SDK rejects the callback when it is missing.
+      iss?: string;
     }
   | {
       successful: false;
diff --git a/core/auth/utils.ts b/core/auth/utils.ts
index c15bb974..95ebccca 100644
--- a/core/auth/utils.ts
+++ b/core/auth/utils.ts
@@ -89,7 +89,10 @@ export const parseOAuthCallbackParams = (location: string): CallbackParams => {
 
   const code = params.get("code");
   if (code) {
-    return { successful: true, code };
+    const iss = params.get("iss");
+    return iss === null
+      ? { successful: true, code }
+      : { successful: true, code, iss };
   }
 
   const error = params.get("error");
diff --git a/core/mcp/inspectorClient.ts b/core/mcp/inspectorClient.ts
index 6d34e504..c2452dc3 100644
--- a/core/mcp/inspectorClient.ts
+++ b/core/mcp/inspectorClient.ts
@@ -3360,9 +3360,9 @@ export class InspectorClient extends InspectorClientEventTarget {
    */
   async resumeAfterOAuth(
     authorizationCode: string,
-    options?: { remoteSessionId?: string },
+    options?: { remoteSessionId?: string; iss?: string },
   ): Promise<void> {
-    await this.completeOAuthFlow(authorizationCode);
+    await this.completeOAuthFlow(authorizationCode, options?.iss);
 
     const remoteSessionId = options?.remoteSessionId;
     const transport = this.baseTransport;
diff --git a/core/mcp/oauthManager.ts b/core/mcp/oauthManager.ts
index 64b0b461..4297f70c 100644
--- a/core/mcp/oauthManager.ts
+++ b/core/mcp/oauthManager.ts
@@ -293,6 +293,7 @@ export class OAuthManager {
         const tokens = await completeEmaIdpAuthorizationAndMint(
           config,
           authorizationCode,
+          iss,
         );
         const requestedScope = this.pendingAuthorizationScope;
         const scopeToPersist = resolvePersistedScopeAfterGrant(