Validate the iss authorization-response parameter (RFC 9207 / SEP-2468) (#2921)

.github/actions/conformance/client.py

@@ -43,7 +43,7 @@ from mcp.client.auth.extensions.client_credentials import (
 )
 from mcp.client.context import ClientRequestContext
 from mcp.client.streamable_http import streamable_http_client
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
+from mcp.shared.auth import AuthorizationCodeResult, OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
 
 # Set up logging to stderr (stdout is for conformance test output)
 logging.basicConfig(
@@ -119,6 +119,7 @@ class ConformanceOAuthCallbackHandler:
     def __init__(self) -> None:
         self._auth_code: str | None = None
         self._state: str | None = None
+        self._iss: str | None = None
 
     async def handle_redirect(self, authorization_url: str) -> None:
         """Fetch the authorization URL and extract the auth code from the redirect."""
@@ -140,6 +141,8 @@ class ConformanceOAuthCallbackHandler:
                         self._auth_code = query_params["code"][0]
                         state_values = query_params.get("state")
                         self._state = state_values[0] if state_values else None
+                        iss_values = query_params.get("iss")
+                        self._iss = iss_values[0] if iss_values else None
                         logger.debug(f"Got auth code from redirect: {self._auth_code[:10]}...")
                         return
                     else:
@@ -149,15 +152,15 @@ class ConformanceOAuthCallbackHandler:
             else:
                 raise RuntimeError(f"Expected redirect response, got {response.status_code} from {authorization_url}")
 
-    async def handle_callback(self) -> tuple[str, str | None]:
-        """Return the captured auth code and state."""
+    async def handle_callback(self) -> AuthorizationCodeResult:
+        """Return the captured auth code, state, and iss."""
         if self._auth_code is None:
             raise RuntimeError("No authorization code available - was handle_redirect called?")
-        auth_code = self._auth_code
-        state = self._state
+        result = AuthorizationCodeResult(code=self._auth_code, state=self._state, iss=self._iss)
         self._auth_code = None
         self._state = None
-        return auth_code, state
+        self._iss = None
+        return result
 
 
 # --- Scenario Handlers ---

.github/actions/conformance/expected-failures.2026-07-28.yml

@@ -44,6 +44,15 @@ client:
   - auth/token-endpoint-auth-post
   - auth/token-endpoint-auth-none
   - auth/offline-access-not-supported
+  # SEP-2468 (authorization response iss parameter) is implemented, but these
+  # 2026-introduced scenarios reach DCR and so still fail the application_type
+  # check above; they unblock with SEP-837, not SEP-2468.
+  - auth/iss-supported
+  - auth/iss-not-advertised
+  - auth/iss-supported-missing
+  - auth/iss-wrong-issuer
+  - auth/iss-unexpected
+  - auth/iss-normalized
 
   # --- Auth scenarios cut short by the 2026 connection lifecycle ---
   # The auth fixture flow drives the 2025 stateful lifecycle; the 2026-mode
@@ -65,14 +74,6 @@ client:
   - http-invalid-tool-headers
   # SEP-2106 (JSON Schema $ref handling): client still dereferences network $refs.
   - json-schema-ref-no-deref
-  # SEP-2468 (authorization response iss parameter): not implemented in the client.
-  - auth/iss-supported
-  - auth/iss-not-advertised
-  - auth/iss-supported-missing
-  - auth/iss-wrong-issuer
-  - auth/iss-unexpected
-  - auth/iss-normalized
-  - auth/metadata-issuer-mismatch
   # SEP-2352 (authorization server migration): client does not re-register when
   # PRM authorization_servers changes.
   - auth/authorization-server-migration

.github/actions/conformance/expected-failures.yml

@@ -24,20 +24,21 @@ client:
   - http-invalid-tool-headers
   # SEP-2106 (JSON Schema $ref handling): client still dereferences network $refs.
   - json-schema-ref-no-deref
-  # SEP-2468 (authorization response iss parameter): not implemented in the client.
+  # SEP-2352 (authorization server migration): client does not re-register when
+  # PRM authorization_servers changes.
+  - auth/authorization-server-migration
+  # SEP-837 (application_type during DCR): the check fires on every non-legacy
+  # spec version (the default LATEST is 2026-07-28). The client omits
+  # application_type during Dynamic Client Registration, so every scenario that
+  # reaches DCR fails it. SEP-2468 iss validation is implemented, so these now
+  # fail only on the application_type check, not on iss.
+  - auth/offline-access-not-supported
   - auth/iss-supported
   - auth/iss-not-advertised
   - auth/iss-supported-missing
   - auth/iss-wrong-issuer
   - auth/iss-unexpected
   - auth/iss-normalized
-  - auth/metadata-issuer-mismatch
-  # SEP-2352 (authorization server migration): client does not re-register when
-  # PRM authorization_servers changes.
-  - auth/authorization-server-migration
-  # SEP-837 (application_type during DCR): the check only fires on draft-version
-  # runs; this draft scenario is the one place the client still hits it.
-  - auth/offline-access-not-supported
 
   # --- Pre-existing scenarios that fail on checks added after conformance 0.1.15 ---
   # SEP-2350 (scope step-up): WARNING-only; the expected-failures evaluator

README.v2.md

@@ -2323,7 +2323,7 @@ import httpx
 from pydantic import AnyUrl
 
 from mcp import ClientSession
-from mcp.client.auth import OAuthClientProvider, TokenStorage
+from mcp.client.auth import AuthorizationCodeResult, OAuthClientProvider, TokenStorage
 from mcp.client.streamable_http import streamable_http_client
 from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
 
@@ -2356,10 +2356,14 @@ async def handle_redirect(auth_url: str) -> None:
     print(f"Visit: {auth_url}")
 
 
-async def handle_callback() -> tuple[str, str | None]:
+async def handle_callback() -> AuthorizationCodeResult:
     callback_url = input("Paste callback URL: ")
     params = parse_qs(urlparse(callback_url).query)
-    return params["code"][0], params.get("state", [None])[0]
+    return AuthorizationCodeResult(
+        code=params["code"][0],
+        state=params.get("state", [None])[0],
+        iss=params.get("iss", [None])[0],
+    )
 
 
 async def main():

docs/migration.md

@@ -62,6 +62,35 @@ async with http_client:
 
 v1's internal client set `follow_redirects=True`; set it explicitly when supplying your own `httpx.AsyncClient` to preserve that behavior.
 
+### OAuth `callback_handler` returns `AuthorizationCodeResult`
+
+The `callback_handler` passed to `OAuthClientProvider` now returns an `AuthorizationCodeResult` instead of a `tuple[str, str | None]` of `(code, state)`. The new object adds an `iss` field so the client can validate the RFC 9207 authorization-response issuer (SEP-2468): when the redirect carries an `iss` query parameter it must match the authorization server's issuer, and a missing `iss` is rejected when the server advertised `authorization_response_iss_parameter_supported`.
+
+**Before (v1):**
+
+```python
+async def callback_handler() -> tuple[str, str | None]:
+    params = parse_qs(urlparse(await wait_for_redirect()).query)
+    return params["code"][0], params.get("state", [None])[0]
+```
+
+**After (v2):**
+
+```python
+from mcp.client.auth import AuthorizationCodeResult
+
+
+async def callback_handler() -> AuthorizationCodeResult:
+    params = parse_qs(urlparse(await wait_for_redirect()).query)
+    return AuthorizationCodeResult(
+        code=params["code"][0],
+        state=params.get("state", [None])[0],
+        iss=params.get("iss", [None])[0],
+    )
+```
+
+Forward the `iss` query parameter from the redirect so the validation can run: omitting it makes the flow fail with `OAuthFlowError` against servers that advertise `authorization_response_iss_parameter_supported`, and silently skips the check for servers that send `iss` without advertising it.
+
 ### `get_session_id` callback removed from `streamable_http_client`
 
 The `get_session_id` callback (third element of the returned tuple) has been removed from `streamable_http_client`. The function now returns a 2-tuple `(read_stream, write_stream)` instead of a 3-tuple.

examples/clients/simple-auth-client/mcp_simple_auth_client/main.py

@@ -19,7 +19,7 @@ from urllib.parse import parse_qs, urlparse
 
 import httpx
 from mcp.client._transport import ReadStream, WriteStream
-from mcp.client.auth import OAuthClientProvider, TokenStorage
+from mcp.client.auth import AuthorizationCodeResult, OAuthClientProvider, TokenStorage
 from mcp.client.session import ClientSession
 from mcp.client.sse import sse_client
 from mcp.client.streamable_http import streamable_http_client
@@ -69,6 +69,7 @@ class CallbackHandler(BaseHTTPRequestHandler):
         if "code" in query_params:
             self.callback_data["authorization_code"] = query_params["code"][0]
             self.callback_data["state"] = query_params.get("state", [None])[0]
+            self.callback_data["iss"] = query_params.get("iss", [None])[0]
             self.send_response(200)
             self.send_header("Content-type", "text/html")
             self.end_headers()
@@ -112,7 +113,7 @@ class CallbackServer:
         self.port = port
         self.server = None
         self.thread = None
-        self.callback_data = {"authorization_code": None, "state": None, "error": None}
+        self.callback_data = {"authorization_code": None, "state": None, "iss": None, "error": None}
 
     def _create_handler_with_data(self):
         """Create a handler class with access to callback data."""
@@ -156,10 +157,16 @@ class CallbackServer:
             time.sleep(0.1)
         raise Exception("Timeout waiting for OAuth callback")
 
-    def get_state(self):
-        """Get the received state parameter."""
+    @property
+    def state(self):
+        """The received state parameter."""
         return self.callback_data["state"]
 
+    @property
+    def iss(self):
+        """The received iss parameter."""
+        return self.callback_data["iss"]
+
 
 class SimpleAuthClient:
     """Simple MCP client with auth support."""
@@ -183,12 +190,12 @@ class SimpleAuthClient:
             callback_server = CallbackServer(port=3030)
             callback_server.start()
 
-            async def callback_handler() -> tuple[str, str | None]:
-                """Wait for OAuth callback and return auth code and state."""
+            async def callback_handler() -> AuthorizationCodeResult:
+                """Wait for OAuth callback and return auth code, state, and iss."""
                 print("⏳ Waiting for authorization callback...")
                 try:
                     auth_code = callback_server.wait_for_callback(timeout=300)
-                    return auth_code, callback_server.get_state()
+                    return AuthorizationCodeResult(code=auth_code, state=callback_server.state, iss=callback_server.iss)
                 finally:
                     callback_server.stop()
 

examples/snippets/clients/oauth_client.py

@@ -13,7 +13,7 @@ import httpx
 from pydantic import AnyUrl
 
 from mcp import ClientSession
-from mcp.client.auth import OAuthClientProvider, TokenStorage
+from mcp.client.auth import AuthorizationCodeResult, OAuthClientProvider, TokenStorage
 from mcp.client.streamable_http import streamable_http_client
 from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
 
@@ -46,10 +46,14 @@ async def handle_redirect(auth_url: str) -> None:
     print(f"Visit: {auth_url}")
 
 
-async def handle_callback() -> tuple[str, str | None]:
+async def handle_callback() -> AuthorizationCodeResult:
     callback_url = input("Paste callback URL: ")
     params = parse_qs(urlparse(callback_url).query)
-    return params["code"][0], params.get("state", [None])[0]
+    return AuthorizationCodeResult(
+        code=params["code"][0],
+        state=params.get("state", [None])[0],
+        iss=params.get("iss", [None])[0],
+    )
 
 
 async def main():

src/mcp/client/auth/__init__.py

@@ -9,8 +9,10 @@ from mcp.client.auth.oauth2 import (
     PKCEParameters,
     TokenStorage,
 )
+from mcp.shared.auth import AuthorizationCodeResult
 
 __all__ = [
+    "AuthorizationCodeResult",
     "OAuthClientProvider",
     "OAuthFlowError",
     "OAuthRegistrationError",

src/mcp/client/auth/extensions/client_credentials.py

@@ -18,7 +18,7 @@ import jwt
 from pydantic import BaseModel, Field
 
 from mcp.client.auth import OAuthClientProvider, OAuthFlowError, OAuthTokenError, TokenStorage
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata
+from mcp.shared.auth import AuthorizationCodeResult, OAuthClientInformationFull, OAuthClientMetadata
 
 
 class ClientCredentialsOAuthProvider(OAuthClientProvider):
@@ -405,7 +405,7 @@ class RFC7523OAuthClientProvider(OAuthClientProvider):
         client_metadata: OAuthClientMetadata,
         storage: TokenStorage,
         redirect_handler: Callable[[str], Awaitable[None]] | None = None,
-        callback_handler: Callable[[], Awaitable[tuple[str, str | None]]] | None = None,
+        callback_handler: Callable[[], Awaitable[AuthorizationCodeResult]] | None = None,
         timeout: float = 300.0,
         jwt_parameters: JWTParameters | None = None,
     ) -> None:

src/mcp/client/auth/oauth2.py

@@ -35,9 +35,12 @@ from mcp.client.auth.utils import (
     handle_token_response_scopes,
     is_valid_client_metadata_url,
     should_use_client_metadata_url,
+    validate_authorization_response_iss,
+    validate_metadata_issuer,
 )
 from mcp.client.streamable_http import MCP_PROTOCOL_VERSION
 from mcp.shared.auth import (
+    AuthorizationCodeResult,
     OAuthClientInformationFull,
     OAuthClientMetadata,
     OAuthMetadata,
@@ -97,7 +100,7 @@ class OAuthContext:
     client_metadata: OAuthClientMetadata
     storage: TokenStorage
     redirect_handler: Callable[[str], Awaitable[None]] | None
-    callback_handler: Callable[[], Awaitable[tuple[str, str | None]]] | None
+    callback_handler: Callable[[], Awaitable[AuthorizationCodeResult]] | None
     timeout: float = 300.0
     client_metadata_url: str | None = None
 
@@ -227,7 +230,7 @@ class OAuthClientProvider(httpx.Auth):
         client_metadata: OAuthClientMetadata,
         storage: TokenStorage,
         redirect_handler: Callable[[str], Awaitable[None]] | None = None,
-        callback_handler: Callable[[], Awaitable[tuple[str, str | None]]] | None = None,
+        callback_handler: Callable[[], Awaitable[AuthorizationCodeResult]] | None = None,
         timeout: float = 300.0,
         client_metadata_url: str | None = None,
         validate_resource_url: Callable[[str, str | None], Awaitable[None]] | None = None,
@@ -356,16 +359,19 @@ class OAuthClientProvider(httpx.Auth):
         await self.context.redirect_handler(authorization_url)
 
         # Wait for callback
-        auth_code, returned_state = await self.context.callback_handler()
+        result = await self.context.callback_handler()
 
-        if returned_state is None or not secrets.compare_digest(returned_state, state):
-            raise OAuthFlowError(f"State parameter mismatch: {returned_state} != {state}")
+        if result.state is None or not secrets.compare_digest(result.state, state):
+            raise OAuthFlowError(f"State parameter mismatch: {result.state} != {state}")
 
-        if not auth_code:
+        # RFC 9207: validate the authorization-response issuer
+        validate_authorization_response_iss(result.iss, self.context.oauth_metadata)
+
+        if not result.code:
             raise OAuthFlowError("No authorization code received")
 
         # Return auth code and code verifier for token exchange
-        return auth_code, pkce_params.code_verifier
+        return result.code, pkce_params.code_verifier
 
     def _get_token_endpoint(self) -> str:
         if self.context.oauth_metadata and self.context.oauth_metadata.token_endpoint:
@@ -570,6 +576,9 @@ class OAuthClientProvider(httpx.Auth):
                         if not ok:
                             break
                         if ok and asm:
+                            # SEP-2468: metadata issuer must match the discovery issuer
+                            if self.context.auth_server_url is not None:
+                                validate_metadata_issuer(asm, self.context.auth_server_url)
                             self.context.oauth_metadata = asm
                             break
                         else:

src/mcp/client/auth/utils.py

@@ -4,7 +4,7 @@ from urllib.parse import urljoin, urlparse
 from httpx import Request, Response
 from pydantic import AnyUrl, ValidationError
 
-from mcp.client.auth import OAuthRegistrationError, OAuthTokenError
+from mcp.client.auth import OAuthFlowError, OAuthRegistrationError, OAuthTokenError
 from mcp.client.streamable_http import MCP_PROTOCOL_VERSION
 from mcp.shared.auth import (
     OAuthClientInformationFull,
@@ -211,6 +211,45 @@ async def handle_auth_metadata_response(response: Response) -> tuple[bool, OAuth
     return True, None
 
 
+def validate_authorization_response_iss(iss: str | None, oauth_metadata: OAuthMetadata | None) -> None:
+    """Validate the RFC 9207 `iss` authorization-response parameter.
+
+    Per RFC 9207 section 2.4, the client compares `iss` against the issuer of the
+    authorization server the request was sent to, using simple string comparison
+    (RFC 3986 section 6.2.1, i.e. without URL normalization), and rejects on mismatch.
+    A response that omits `iss` is rejected only when the server advertised support via
+    `authorization_response_iss_parameter_supported`.
+
+    Raises:
+        OAuthFlowError: If `iss` is present and does not match, or is absent when the
+            authorization server advertised support.
+    """
+    expected = str(oauth_metadata.issuer) if oauth_metadata else None
+
+    if iss is not None:
+        if iss != expected:
+            raise OAuthFlowError(f"Authorization response iss mismatch: {iss} != {expected}")
+        return
+
+    if oauth_metadata is not None and oauth_metadata.authorization_response_iss_parameter_supported:
+        raise OAuthFlowError("Authorization response missing iss parameter advertised by the authorization server")
+
+
+def validate_metadata_issuer(oauth_metadata: OAuthMetadata, expected_issuer: str) -> None:
+    """Validate that authorization server metadata `issuer` matches the discovery issuer.
+
+    Per RFC 8414 section 3.3 / SEP-2468, the `issuer` in the metadata must match the issuer
+    used to construct the well-known URL, compared as a simple string (RFC 3986 section 6.2.1).
+
+    Raises:
+        OAuthFlowError: If the metadata issuer does not match `expected_issuer`.
+    """
+    if str(oauth_metadata.issuer) != expected_issuer:
+        raise OAuthFlowError(
+            f"Authorization server metadata issuer mismatch: {oauth_metadata.issuer} != {expected_issuer}"
+        )
+
+
 def create_oauth_metadata_request(url: str) -> Request:
     return Request("GET", url, headers={MCP_PROTOCOL_VERSION: LATEST_PROTOCOL_VERSION})
 

src/mcp/shared/auth.py

@@ -22,6 +22,18 @@ class OAuthToken(BaseModel):
         return v  # pragma: no cover
 
 
+class AuthorizationCodeResult(BaseModel):
+    """Authorization-code-grant redirect parameters returned by a callback handler.
+
+    `iss` carries the RFC 9207 authorization-response issuer when the authorization server
+    includes it in the redirect; the client validates it against the expected issuer.
+    """
+
+    code: str
+    state: str | None = None
+    iss: str | None = None
+
+
 class InvalidScopeError(Exception):
     def __init__(self, message: str):
         self.message = message
@@ -149,6 +161,7 @@ class OAuthMetadata(BaseModel):
     introspection_endpoint_auth_signing_alg_values_supported: list[str] | None = None
     code_challenge_methods_supported: list[str] | None = None
     client_id_metadata_document_supported: bool | None = None
+    authorization_response_iss_parameter_supported: bool | None = None
 
 
 class ProtectedResourceMetadata(BaseModel):

tests/client/auth/extensions/test_client_credentials.py

@@ -13,7 +13,13 @@ from mcp.client.auth.extensions.client_credentials import (
     SignedJWTParameters,
     static_assertion_provider,
 )
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata, OAuthToken
+from mcp.shared.auth import (
+    AuthorizationCodeResult,
+    OAuthClientInformationFull,
+    OAuthClientMetadata,
+    OAuthMetadata,
+    OAuthToken,
+)
 
 
 class MockTokenStorage:
@@ -57,9 +63,9 @@ def rfc7523_oauth_provider(client_metadata: OAuthClientMetadata, mock_storage: M
         """Mock redirect handler."""
         pass
 
-    async def callback_handler() -> tuple[str, str | None]:  # pragma: no cover
+    async def callback_handler() -> AuthorizationCodeResult:  # pragma: no cover
         """Mock callback handler."""
-        return "test_auth_code", "test_state"
+        return AuthorizationCodeResult(code="test_auth_code", state="test_state")
 
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", DeprecationWarning)

tests/client/test_auth.py

@@ -25,10 +25,13 @@ from mcp.client.auth.utils import (
     handle_registration_response,
     is_valid_client_metadata_url,
     should_use_client_metadata_url,
+    validate_authorization_response_iss,
+    validate_metadata_issuer,
 )
 from mcp.server.auth.routes import build_metadata
 from mcp.server.auth.settings import ClientRegistrationOptions, RevocationOptions
 from mcp.shared.auth import (
+    AuthorizationCodeResult,
     OAuthClientInformationFull,
     OAuthClientMetadata,
     OAuthMetadata,
@@ -89,9 +92,9 @@ def oauth_provider(client_metadata: OAuthClientMetadata, mock_storage: MockToken
         """Mock redirect handler."""
         pass  # pragma: no cover
 
-    async def callback_handler() -> tuple[str, str | None]:
+    async def callback_handler() -> AuthorizationCodeResult:
         """Mock callback handler."""
-        return "test_auth_code", "test_state"  # pragma: no cover
+        return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
     return OAuthClientProvider(
         server_url="https://api.example.com/v1/mcp",
@@ -272,8 +275,8 @@ class TestOAuthFlow:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com",
@@ -1380,8 +1383,8 @@ class TestAuthFlow:
         oauth_provider.context.redirect_handler = capture_redirect
 
         # Mock callback
-        async def mock_callback() -> tuple[str, str | None]:
-            return "auth_code", captured_state
+        async def mock_callback() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="auth_code", state=captured_state)
 
         oauth_provider.context.callback_handler = mock_callback
 
@@ -1517,8 +1520,8 @@ class TestLegacyServerFallback:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         # Simulate a legacy server like Linear
         provider = OAuthClientProvider(
@@ -1616,8 +1619,8 @@ class TestLegacyServerFallback:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -1721,8 +1724,8 @@ class TestSEP985Discovery:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -1756,8 +1759,8 @@ class TestSEP985Discovery:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -1857,8 +1860,8 @@ class TestSEP985Discovery:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2071,8 +2074,8 @@ class TestCIMD:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2092,8 +2095,8 @@ class TestCIMD:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         with pytest.raises(ValueError) as exc_info:
             OAuthClientProvider(
@@ -2115,8 +2118,8 @@ class TestCIMD:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2206,8 +2209,8 @@ class TestCIMD:
         async def redirect_handler(url: str) -> None:
             pass  # pragma: no cover
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", "test_state"  # pragma: no cover
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2439,8 +2442,8 @@ class TestSEP2207OfflineAccessScope:
             params = parse_qs(parsed.query)
             captured_state = params.get("state", [None])[0]
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", captured_state
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state=captured_state)
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2548,8 +2551,8 @@ class TestSEP2207OfflineAccessScope:
             params = parse_qs(parsed.query)
             captured_state = params.get("state", [None])[0]
 
-        async def callback_handler() -> tuple[str, str | None]:
-            return "test_auth_code", captured_state
+        async def callback_handler() -> AuthorizationCodeResult:
+            return AuthorizationCodeResult(code="test_auth_code", state=captured_state)
 
         provider = OAuthClientProvider(
             server_url="https://api.example.com/v1/mcp",
@@ -2636,3 +2639,65 @@ class TestSEP2207OfflineAccessScope:
             await auth_flow.asend(final_response)
         except StopAsyncIteration:
             pass
+
+
+_ISSUER = "https://as.example.com"
+
+
+def _issuer_metadata(*, issuer: str = _ISSUER, iss_supported: bool | None = None) -> OAuthMetadata:
+    # Validate from string inputs so url_preserve_empty_path keeps the issuer as transmitted,
+    # matching the wire path (model_validate_json) rather than normalizing a bare authority.
+    return OAuthMetadata.model_validate(
+        {
+            "issuer": issuer,
+            "authorization_endpoint": f"{issuer}/authorize",
+            "token_endpoint": f"{issuer}/token",
+            "authorization_response_iss_parameter_supported": iss_supported,
+        }
+    )
+
+
+@pytest.mark.parametrize(
+    ("issuer", "iss", "iss_supported"),
+    [
+        pytest.param(_ISSUER, _ISSUER, True, id="advertised-and-correct"),
+        pytest.param(_ISSUER, None, None, id="not-advertised-and-omitted"),
+        pytest.param(_ISSUER, _ISSUER, None, id="not-advertised-but-correct"),
+        # An issuer that genuinely ends in a slash (e.g. Auth0) must match its own iss.
+        pytest.param("https://as.example.com/", "https://as.example.com/", True, id="trailing-slash-issuer"),
+    ],
+)
+def test_validate_authorization_response_iss_accepts(issuer: str, iss: str | None, iss_supported: bool | None):
+    """RFC 9207: a matching or legitimately absent iss is accepted."""
+    validate_authorization_response_iss(iss, _issuer_metadata(issuer=issuer, iss_supported=iss_supported))
+
+
+@pytest.mark.parametrize(
+    ("iss", "iss_supported", "match"),
+    [
+        pytest.param(None, True, "missing iss", id="advertised-but-omitted"),
+        pytest.param("https://evil.example.com", True, "iss mismatch", id="wrong-issuer"),
+        pytest.param("https://evil.example.com", None, "iss mismatch", id="unexpected-when-not-advertised"),
+        pytest.param(f"{_ISSUER}/", True, "iss mismatch", id="trailing-slash-not-normalized"),
+    ],
+)
+def test_validate_authorization_response_iss_rejects(iss: str | None, iss_supported: bool | None, match: str):
+    """RFC 9207: a mismatched iss, or one missing when advertised, is rejected via simple string compare."""
+    with pytest.raises(OAuthFlowError, match=match):
+        validate_authorization_response_iss(iss, _issuer_metadata(iss_supported=iss_supported))
+
+
+def test_validate_authorization_response_iss_without_metadata():
+    """With no AS metadata, a present iss is rejected and an absent one is accepted."""
+    validate_authorization_response_iss(None, None)
+    with pytest.raises(OAuthFlowError, match="iss mismatch"):
+        validate_authorization_response_iss(_ISSUER, None)
+
+
+def test_validate_metadata_issuer_accepts_match():
+    validate_metadata_issuer(_issuer_metadata(issuer=_ISSUER), _ISSUER)
+
+
+def test_validate_metadata_issuer_rejects_mismatch():
+    with pytest.raises(OAuthFlowError, match="metadata issuer mismatch"):
+        validate_metadata_issuer(_issuer_metadata(issuer="https://attacker.example.com"), _ISSUER)

tests/client/test_scope_bug_1630.py

@@ -11,7 +11,12 @@ import pytest
 from pydantic import AnyUrl
 
 from mcp.client.auth import OAuthClientProvider
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
+from mcp.shared.auth import (
+    AuthorizationCodeResult,
+    OAuthClientInformationFull,
+    OAuthClientMetadata,
+    OAuthToken,
+)
 
 
 class MockTokenStorage:
@@ -48,8 +53,8 @@ async def test_401_uses_www_auth_scope_not_resource_metadata_url():
     async def redirect_handler(url: str) -> None:
         pass  # pragma: no cover
 
-    async def callback_handler() -> tuple[str, str | None]:
-        return "test_auth_code", "test_state"  # pragma: no cover
+    async def callback_handler() -> AuthorizationCodeResult:
+        return AuthorizationCodeResult(code="test_auth_code", state="test_state")  # pragma: no cover
 
     client_metadata = OAuthClientMetadata(
         redirect_uris=[AnyUrl("http://localhost:3030/callback")],

tests/interaction/_requirements.py

@@ -3279,16 +3279,10 @@ REQUIREMENTS: dict[str, Requirement] = {
         source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-metadata-discovery",
         behavior=(
             "The client rejects authorization-server metadata whose issuer does not match the URL the "
-            "metadata was retrieved from (RFC 8414 section 3.3)."
+            "metadata was retrieved from (RFC 8414 section 3.3 / SEP-2468)."
         ),
         transports=("streamable-http",),
         note="OAuth is HTTP-only.",
-        divergence=Divergence(
-            note=(
-                "The SDK parses authorization-server metadata without comparing issuer to the discovery "
-                "URL; a mismatched issuer is accepted and the flow proceeds."
-            ),
-        ),
     ),
     "client-auth:authorize:error-surfaces": Requirement(
         source=f"{SPEC_BASE_URL}/basic/authorization#authorization-flow-steps",
@@ -3492,6 +3486,16 @@ REQUIREMENTS: dict[str, Requirement] = {
         transports=("streamable-http",),
         note="OAuth is HTTP-only.",
     ),
+    "client-auth:authorization-response:iss-verify": Requirement(
+        source=f"{SPEC_BASE_URL}/basic/authorization#authorization-server-metadata-discovery",
+        behavior=(
+            "The client validates the RFC 9207 iss authorization-response parameter against the "
+            "authorization server issuer (simple string comparison) and rejects a mismatch, or a "
+            "missing iss when the server advertises support (SEP-2468)."
+        ),
+        transports=("streamable-http",),
+        note="OAuth is HTTP-only.",
+    ),
     "client-auth:token-endpoint-auth-method": Requirement(
         source="sdk",
         behavior="The client authenticates to the token endpoint using the auth method established at registration.",

tests/interaction/auth/_harness.py

@@ -26,7 +26,7 @@ from mcp.client.streamable_http import streamable_http_client
 from mcp.server import Server
 from mcp.server.auth.provider import AccessToken, ProviderTokenVerifier
 from mcp.server.auth.settings import AuthSettings, ClientRegistrationOptions, RevocationOptions
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
+from mcp.shared.auth import AuthorizationCodeResult, OAuthClientInformationFull, OAuthClientMetadata, OAuthToken
 from tests.interaction._connect import BASE_URL, NO_DNS_REBINDING_PROTECTION
 from tests.interaction.auth._provider import InMemoryAuthorizationServerProvider
 from tests.interaction.transports._bridge import StreamingASGITransport
@@ -136,16 +136,21 @@ class HeadlessOAuth:
 
     `state_override`: when set, `callback_handler` returns this value as the state instead of
     the one parsed from the redirect, so tests can drive the state-mismatch path.
+
+    `iss_override`: when set, `callback_handler` returns this value as the RFC 9207 issuer
+    instead of the one parsed from the redirect, so tests can drive the iss-mismatch path.
     """
 
-    def __init__(self, *, state_override: str | None = None) -> None:
+    def __init__(self, *, state_override: str | None = None, iss_override: str | None = None) -> None:
         self.authorize_url: str | None = None
         self.authorize_urls: list[str] = []
         self.error: str | None = None
         self._state_override = state_override
+        self._iss_override = iss_override
         self._http: httpx.AsyncClient | None = None
         self._code: str = ""
         self._state: str | None = None
+        self._iss: str | None = None
 
     def bind(self, http_client: httpx.AsyncClient) -> None:
         self._http = http_client
@@ -161,10 +166,15 @@ class HeadlessOAuth:
         params = parse_qs(urlsplit(response.headers["location"]).query)
         self._code = params.get("code", [""])[0]
         self._state = params.get("state", [None])[0]
+        self._iss = params.get("iss", [None])[0]
         self.error = params.get("error", [None])[0]
 
-    async def callback_handler(self) -> tuple[str, str | None]:
-        return self._code, self._state_override if self._state_override is not None else self._state
+    async def callback_handler(self) -> AuthorizationCodeResult:
+        return AuthorizationCodeResult(
+            code=self._code,
+            state=self._state_override if self._state_override is not None else self._state,
+            iss=self._iss_override if self._iss_override is not None else self._iss,
+        )
 
 
 def auth_settings(

tests/interaction/auth/_provider.py

@@ -21,6 +21,7 @@ from mcp.server.auth.provider import (
     construct_redirect_uri,
 )
 from mcp.shared.auth import OAuthClientInformationFull, OAuthToken
+from tests.interaction._connect import BASE_URL
 
 _TOKEN_LIFETIME_SECONDS = 3600
 
@@ -53,9 +54,14 @@ class InMemoryAuthorizationServerProvider(
         issue_expired_first: bool = False,
         fail_next_refresh: bool = False,
         reject_all_tokens: bool = False,
+        issuer: str | None = None,
     ) -> None:
         self._default_scopes = list(default_scopes) if default_scopes is not None else ["mcp"]
-        self._issuer = "http://127.0.0.1:8000"
+        # The authorization-response iss must equal the AS metadata issuer the client recorded
+        # (RFC 9207 simple string comparison). `real_asm` builds the issuer from an AnyHttpUrl
+        # object, so it carries the trailing slash; the redirect iss matches it. Path-issuer
+        # tests pass the recorded issuer explicitly.
+        self._issuer = issuer if issuer is not None else f"{BASE_URL}/"
         self._deny_authorize = deny_authorize
         self._issue_expired_first = issue_expired_first
         self._fail_next_refresh = fail_next_refresh

tests/interaction/auth/test_authorize_token.py

@@ -187,6 +187,24 @@ async def test_a_mismatched_state_on_the_callback_aborts_the_flow() -> None:
             await connect_with_oauth(server, provider=provider, headless=headless).__aenter__()
 
 
+@requirement("client-auth:authorization-response:iss-verify")
+async def test_a_mismatched_iss_on_the_callback_aborts_the_flow() -> None:
+    """A callback whose RFC 9207 iss does not match the authorization server issuer aborts the flow.
+
+    `iss_override` makes the headless callback return an issuer the AS never advertised; the SDK
+    compares it to `oauth_metadata.issuer` and raises `OAuthFlowError` before the token exchange.
+    """
+    provider = InMemoryAuthorizationServerProvider()
+    server = Server("guarded", on_list_tools=list_tools)
+    headless = HeadlessOAuth(iss_override="https://attacker.example.com")
+
+    with anyio.fail_after(5):
+        with pytest.RaisesGroup(
+            pytest.RaisesExc(OAuthFlowError, match="^Authorization response iss mismatch:"), flatten_subgroups=True
+        ):
+            await connect_with_oauth(server, provider=provider, headless=headless).__aenter__()
+
+
 @requirement("client-auth:resource-parameter")
 async def test_the_authorization_code_token_request_carries_grant_type_code_redirect_and_resource(
     recorded_oauth_flow: RecordedFlow,

tests/interaction/auth/test_discovery.py

@@ -236,7 +236,10 @@ async def test_as_metadata_discovery_falls_back_through_the_spec_endpoint_order(
     unrecognized field to prove the client's parser ignores unknown members (RFC 8414 §3.2).
     """
     recorded, on_request = record_requests()
-    provider = InMemoryAuthorizationServerProvider()
+    asm = real_asm()
+    asm.issuer = AnyHttpUrl(authorization_server)
+    # The redirect iss must equal the issuer the client records from this metadata.
+    provider = InMemoryAuthorizationServerProvider(issuer=str(asm.issuer))
     server = Server("guarded", on_list_tools=list_tools)
 
     prm = ProtectedResourceMetadata(
@@ -246,7 +249,7 @@ async def test_as_metadata_discovery_falls_back_through_the_spec_endpoint_order(
         not_found=not_found,
         serve={
             PRM_PATH_SUFFIXED: metadata_body(prm),
-            serve_at: metadata_body(real_asm(), x_unknown_extension="ignored"),
+            serve_at: metadata_body(asm, x_unknown_extension="ignored"),
         },
     )
 
@@ -311,23 +314,26 @@ async def test_as_metadata_advertises_authorize_token_registration_and_s256() ->
 
 
 @requirement("client-auth:as-metadata-discovery:issuer-validation")
-async def test_as_metadata_with_a_mismatched_issuer_is_accepted_and_the_flow_proceeds() -> None:
-    """Authorization-server metadata whose `issuer` does not match the discovery URL is accepted.
+async def test_as_metadata_with_a_mismatched_issuer_aborts_the_flow() -> None:
+    """Authorization-server metadata whose `issuer` does not match the discovery URL is rejected.
 
-    RFC 8414 §3.3 requires the client to reject the document; the SDK parses and uses it
-    without comparing `issuer` to the URL it was fetched from. See the divergence on the
-    requirement. The served body carries an unrecognized field as a fold-in proof of
-    unknown-field tolerance.
+    RFC 8414 §3.3 / SEP-2468 require the client to reject the document; the SDK compares `issuer`
+    to the URL the metadata was fetched from and raises `OAuthFlowError` before any authorize or
+    token request is made.
     """
+    recorded, on_request = record_requests()
     provider = InMemoryAuthorizationServerProvider()
     server = Server("guarded", on_list_tools=list_tools)
 
     metadata = real_asm()
     metadata.issuer = AnyHttpUrl(f"{BASE_URL}/wrong-issuer")
-    app_shim = shim(serve={ASM_ROOT: metadata_body(metadata, x_unknown_extension="ignored")})
+    app_shim = shim(serve={ASM_ROOT: metadata_body(metadata)})
 
     with anyio.fail_after(5):
-        async with connect_with_oauth(server, provider=provider, app_shim=app_shim) as (client, _):
-            result = await client.list_tools()
+        with pytest.RaisesGroup(
+            pytest.RaisesExc(OAuthFlowError, match="^Authorization server metadata issuer mismatch"),
+            flatten_subgroups=True,
+        ):
+            await connect_with_oauth(server, provider=provider, app_shim=app_shim, on_request=on_request).__aenter__()
 
-    assert result.tools[0].name == "probe"
+    assert [r.path for r in recorded if r.path in ("/authorize", "/token")] == []