feat(mcp): identify the authorization-server issuer per RFC 9207

Advertise authorization_response_iss_parameter_supported in the
authorization-server metadata and echo the iss parameter on every
authorization response — both the success redirect carrying the code and
the access_denied error redirect. A client can now pin the response to
this server's issuer and reject a code substituted by a look-alike
authorization server in a multi-server deployment.

Additive and backward-compatible: existing clients that ignore iss are
unaffected; PKCE S256-only, audience binding, redirect-URI exact match
and the rate limits are untouched.

src/app/api/mcp/oauth/__tests__/authorize.test.ts

@@ -286,6 +286,8 @@ describe("POST /authorize — decision", () => {
     const code = new URL(loc).searchParams.get("code");
     expect(code).toMatch(/^hlac_/);
     expect(new URL(loc).searchParams.get("state")).toBe("xyz");
+    // RFC 9207 — the issuer identifier is echoed on the success redirect.
+    expect(new URL(loc).searchParams.get("iss")).toBe("https://health.example");
   });
 
   it("302s with access_denied on deny", async () => {
@@ -305,5 +307,7 @@ describe("POST /authorize — decision", () => {
     expect(res.status).toBe(302);
     const loc = res.headers.get("location") ?? "";
     expect(new URL(loc).searchParams.get("error")).toBe("access_denied");
+    // RFC 9207 — the issuer is echoed on the error redirect too.
+    expect(new URL(loc).searchParams.get("iss")).toBe("https://health.example");
   });
 });

src/app/api/mcp/oauth/authorize/route.ts

@@ -414,10 +414,16 @@ export async function POST(request: NextRequest): Promise<Response> {
     }
     const { v } = result;
 
+    // RFC 9207 — the authorization-server issuer identifier. Echoed on EVERY
+    // authorization response (success AND error) so the client can pin the
+    // response to this AS and reject a code from a substituted one.
+    const iss = resolveBaseOrigin(request.url);
+
     if (params.decision !== "allow") {
       annotate({ action: { name: "mcp.oauth.authorize.denied" } });
       return redirectBack(v.redirectUri, {
         error: "access_denied",
+        iss,
         ...(v.state !== undefined ? { state: v.state } : {}),
       });
     }
@@ -450,6 +456,7 @@ export async function POST(request: NextRequest): Promise<Response> {
 
     return redirectBack(v.redirectUri, {
       code,
+      iss,
       ...(v.state !== undefined ? { state: v.state } : {}),
     });
   });

src/lib/mcp/oauth/__tests__/oauth-bridge.test.ts

@@ -230,6 +230,12 @@ describe("metadata + audience", () => {
     expect(as.grant_types_supported).toContain("refresh_token");
   });
 
+  it("AS metadata advertises the RFC 9207 iss parameter and a matching issuer", () => {
+    const as = authorizationServerMetadata();
+    expect(as.authorization_response_iss_parameter_supported).toBe(true);
+    expect(as.issuer).toBe("https://health.example");
+  });
+
   it("the WWW-Authenticate challenge points at the PRM", () => {
     expect(wwwAuthenticateChallenge()).toMatch(
       /resource_metadata="https:\/\/health\.example\/\.well-known\/oauth-protected-resource"/,

src/lib/mcp/oauth/metadata.ts

@@ -42,6 +42,7 @@ export interface AuthorizationServerMetadata {
   token_endpoint_auth_methods_supported: string[];
   code_challenge_methods_supported: string[];
   client_id_metadata_document_supported: boolean;
+  authorization_response_iss_parameter_supported: boolean;
 }
 
 /** RFC 9728 — the Protected Resource Metadata for the `/mcp` resource. */
@@ -76,6 +77,11 @@ export function authorizationServerMetadata(
     code_challenge_methods_supported: ["S256"],
     // CIMD is the preferred registration path (SEP-991); DCR is the fallback.
     client_id_metadata_document_supported: true,
+    // RFC 9207 — the authorization response carries an `iss` parameter the
+    // client validates against this metadata's `issuer`, so a code from a
+    // look-alike AS in a multi-AS deployment cannot be substituted (mix-up
+    // defence). The `/authorize` redirect always includes it.
+    authorization_response_iss_parameter_supported: true,
   };
 }