modelcontextprotocol
diff --git a/docs/migration.md b/docs/migration.md
@@ -1445,6 +1445,37 @@ client_metadata = OAuthClientMetadata(
 
 Under OIDC, omitting `application_type` defaults to `"web"`, which an authorization server may reject for the `localhost` redirect URIs native clients use; sending `"native"` avoids that. Non-OIDC servers ignore the parameter.
 
+### Identity Assertion Authorization Grant for enterprise IdP flows (SEP-990)
+
+The SDK now supports SEP-990's enterprise identity-provider policy controls. The client presents an Identity Assertion Authorization Grant (ID-JAG) - a signed JWT issued by the enterprise IdP - to the MCP authorization server using the RFC 7523 jwt-bearer grant (`grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer`, the ID-JAG as `assertion`), and receives an MCP access token. This matches the SEP-990 normative profile and interoperates with the other MCP SDKs. (Leg 1 - exchanging the user's IdP ID token for the ID-JAG against the IdP - is deployment-specific and out of scope for the SDK.) This is additive and opt-in on both sides; existing flows are unchanged.
+
+On the client, `IdentityAssertionOAuthProvider` (in `mcp.client.auth.extensions.identity_assertion`) is an `httpx.Auth` that posts the jwt-bearer request. The ID-JAG is supplied lazily through an async `assertion_provider(audience, resource)` callback - `audience` is the authorization server's issuer (the ID-JAG `aud`) and `resource` is the MCP server's identifier (the ID-JAG `resource` claim):
+
+```python
+from mcp.client.auth.extensions.identity_assertion import IdentityAssertionOAuthProvider
+
+
+async def fetch_id_jag(audience: str, resource: str) -> str:
+    # The ID-JAG must carry `audience` as `aud` and `resource` as its `resource` claim.
+    return await my_idp.issue_id_jag(audience=audience, resource=resource)
+
+
+provider = IdentityAssertionOAuthProvider(
+    server_url="https://mcp.example.com/mcp",
+    storage=my_token_storage,
+    client_id="enterprise-mcp-client",
+    client_secret="enterprise-mcp-secret",
+    issuer="https://auth.example.com",
+    assertion_provider=fetch_id_jag,
+)
+```
+
+SEP-990 §5.1 requires the client to authenticate; this SDK currently requires a shared secret, so `client_secret` is mandatory (`token_endpoint_auth_method` chooses `client_secret_post` (default) or `client_secret_basic`; the spec also permits `private_key_jwt`). The authorization server is configuration, not discovery: `issuer` is the AS the client is provisioned for, authorization-server metadata is fetched from that issuer's RFC 8414 well-known, and the resource server is never asked which AS to use - so a hostile resource server cannot redirect the ID-JAG or secret.
+
+On the authorization server, set `AuthSettings(identity_assertion_enabled=True)` (or pass `identity_assertion_enabled=True` to `create_auth_routes`) and implement `exchange_identity_assertion` on your `OAuthAuthorizationServerProvider`. The method receives an `IdentityAssertionParams` (the ID-JAG `assertion`, requested scopes, and request `resource`) and returns a plain RFC 6749 `OAuthToken`. The flag gates both metadata advertisement and the token endpoint: when off, `/token` rejects the grant with `unsupported_grant_type` even if the provider implements the hook. When on, the metadata advertises the jwt-bearer grant and the `urn:ietf:params:oauth:grant-profile:id-jag` profile in `authorization_grant_profiles_supported` (the discovery mechanism per ext-auth §6).
+
+The implementation is responsible for validating the assertion per RFC 7523 §3 and SEP-990 §5.1 - verify the signature/`iss`/`exp`/`typ`, require `aud` to be this AS, require the ID-JAG's `client_id` claim to match the authenticated client, audience-restrict the issued token to the ID-JAG's `resource` claim (not the client-controlled request `resource`), and derive scopes from the ID-JAG rather than granting the request verbatim. See `examples/snippets/servers/identity_assertion_server.py`, which fails closed. Two hardening points are enforced by the SDK: the handler rejects clients without a stored secret before calling the hook (and `ClientAuthenticator` itself now refuses a secret-based auth method registered without a secret), and Dynamic Client Registration refuses the jwt-bearer grant so the ID-JAG flow requires a pre-registered confidential client.
+
 ### 2025-11-25 and 2026-07-28 protocol fields modeled
 
 `mcp_types` models the 2025-11-25 and 2026-07-28 protocol fields (e.g. `resultType`, `ttlMs`/`cacheScope` on cacheable results, `inputResponses`/`requestState` on retried requests), so inbound payloads carrying these keys parse into typed fields and round-trip. `ttlMs`/`cacheScope` default to `0`/`"private"` (immediately stale, not shared-cacheable); `resultType` defaults to `"complete"` on concrete results (`None` on `EmptyResult`); the server strips all of them from the wire at pre-2026 versions.

diff --git a/examples/snippets/clients/identity_assertion_client.py b/examples/snippets/clients/identity_assertion_client.py
@@ -0,0 +1,82 @@
+"""Client side of SEP-990 (enterprise IdP policy controls).
+
+`IdentityAssertionOAuthProvider` presents an Identity Assertion Authorization Grant (ID-JAG) issued
+by the enterprise IdP to the MCP authorization server using the RFC 7523 jwt-bearer grant
+(`grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer`, ID-JAG as `assertion`), and receives an
+MCP access token. No browser redirect or dynamic client registration is involved.
+
+Obtaining the ID-JAG (logging into the IdP and the leg-1 exchange against it) is deployment-specific
+and out of scope for the SDK; supply it through the `assertion_provider` callback. The callback
+receives the authorization server's issuer (the ID-JAG `aud`) and the MCP server's resource
+identifier (the ID-JAG `resource` claim). SEP-990 requires a confidential client, so a client secret
+is mandatory, and `issuer` is the authorization server the credentials are provisioned for - the
+provider fetches metadata from that issuer's well-known and never asks the resource server which AS
+to use.
+"""
+
+import asyncio
+
+import httpx
+
+from mcp import ClientSession
+from mcp.client.auth.extensions.identity_assertion import IdentityAssertionOAuthProvider
+from mcp.client.streamable_http import streamable_http_client
+from mcp.shared.auth import OAuthClientInformationFull, OAuthToken
+
+
+class InMemoryTokenStorage:
+    """Demo in-memory token storage."""
+
+    def __init__(self) -> None:
+        self.tokens: OAuthToken | None = None
+        self.client_info: OAuthClientInformationFull | None = None
+
+    async def get_tokens(self) -> OAuthToken | None:
+        return self.tokens
+
+    async def set_tokens(self, tokens: OAuthToken) -> None:
+        self.tokens = tokens
+
+    async def get_client_info(self) -> OAuthClientInformationFull | None:
+        return self.client_info
+
+    async def set_client_info(self, client_info: OAuthClientInformationFull) -> None:
+        self.client_info = client_info
+
+
+async def fetch_id_jag(audience: str, resource: str) -> str:
+    """Return the ID-JAG to present.
+
+    `audience` is the MCP authorization server's issuer (the ID-JAG `aud` claim); `resource` is the
+    MCP server's RFC 9728 identifier (the ID-JAG `resource` claim, which the AS audience-restricts
+    the issued token against). In production this exchanges the user's IdP ID token for an ID-JAG
+    against the enterprise identity provider.
+    """
+    raise NotImplementedError("Obtain the ID-JAG from your enterprise identity provider")
+
+
+async def main() -> None:
+    oauth_auth = IdentityAssertionOAuthProvider(
+        server_url="http://localhost:8001/mcp",
+        storage=InMemoryTokenStorage(),
+        client_id="enterprise-mcp-client",
+        client_secret="enterprise-mcp-secret",
+        issuer="http://localhost:8001",
+        assertion_provider=fetch_id_jag,
+        scope="user",
+    )
+
+    async with httpx.AsyncClient(auth=oauth_auth, follow_redirects=True) as http_client:
+        async with streamable_http_client("http://localhost:8001/mcp", http_client=http_client) as (read, write):
+            async with ClientSession(read, write) as session:
+                await session.initialize()
+                tools = await session.list_tools()
+                print(f"Available tools: {[tool.name for tool in tools.tools]}")
+
+
+def run() -> None:
+    asyncio.run(main())
+
+
+if __name__ == "__main__":
+    run()
diff --git a/examples/snippets/pyproject.toml b/examples/snippets/pyproject.toml
@@ -21,4 +21,5 @@ completion-client = "clients.completion_client:main"
 direct-execution-server = "servers.direct_execution:main"
 display-utilities-client = "clients.display_utilities:main"
 oauth-client = "clients.oauth_client:run"
+identity-assertion-client = "clients.identity_assertion_client:run"
 elicitation-client = "clients.url_elicitation_client:run"
diff --git a/examples/snippets/servers/identity_assertion_server.py b/examples/snippets/servers/identity_assertion_server.py
@@ -0,0 +1,103 @@
+"""Authorization-server side of SEP-990 (enterprise IdP policy controls).
+
+An authorization server enables the Identity Assertion Authorization Grant by setting
+`identity_assertion_enabled=True` and implementing `exchange_identity_assertion` on its provider.
+The client presents the IdP-issued ID-JAG using the RFC 7523 jwt-bearer grant; the provider
+validates the assertion and mints an MCP access token.
+
+Validating the ID-JAG is the provider's responsibility and is only stubbed here. A real
+implementation MUST, per RFC 7523 §3 and SEP-990 §5.1:
+
+- verify the JWT signature, `iss`, and `exp`, and that `typ` is `oauth-id-jag+jwt`;
+- require `aud` to identify this authorization server;
+- require the ID-JAG's `client_id` claim to match the authenticated client;
+- audience-restrict the issued token to the resource named in the ID-JAG's `resource` claim
+  (NOT the client-supplied `params.resource`);
+- derive the granted scopes from the ID-JAG and policy.
+
+`_decode_and_validate_id_jag` below raises `NotImplementedError` so this snippet fails closed and
+forces a real implementation. Wire the returned routes into a Starlette app with
+`create_auth_routes(..., identity_assertion_enabled=True)`, or set
+`AuthSettings(identity_assertion_enabled=True)` with `MCPServer`/`Server`.
+"""
+
+import secrets
+import time
+from dataclasses import dataclass
+
+from mcp.server.auth.provider import (
+    AccessToken,
+    AuthorizationCode,
+    IdentityAssertionParams,
+    OAuthAuthorizationServerProvider,
+    RefreshToken,
+)
+from mcp.shared.auth import OAuthClientInformationFull, OAuthToken
+
+
+@dataclass
+class IdJagClaims:
+    """The trusted claims extracted from a validated ID-JAG."""
+
+    subject: str  # the end user the ID-JAG was issued for
+    client_id: str  # the ID-JAG `client_id` claim; §5.1 requires it to match the authenticated client
+    resource: str  # the MCP server the issued token must be audience-restricted to
+    scopes: list[str]
+
+
+class IdentityAssertionProvider(OAuthAuthorizationServerProvider[AuthorizationCode, RefreshToken, AccessToken]):
+    """Authorization-server provider that accepts an ID-JAG via the RFC 7523 jwt-bearer grant."""
+
+    def __init__(self) -> None:
+        self.access_tokens: dict[str, AccessToken] = {}
+        # SEP-990 clients are pre-registered out of band (DCR refuses the grant) and must be
+        # confidential. `get_client` must return them, or the token endpoint 401s before the
+        # exchange runs. Real deployments load these from their own store.
+        self.clients: dict[str, OAuthClientInformationFull] = {
+            "enterprise-mcp-client": OAuthClientInformationFull(
+                client_id="enterprise-mcp-client",
+                client_secret="enterprise-mcp-secret",
+                redirect_uris=None,
+                grant_types=["urn:ietf:params:oauth:grant-type:jwt-bearer"],
+                token_endpoint_auth_method="client_secret_post",
+            )
+        }
+
+    async def get_client(self, client_id: str) -> OAuthClientInformationFull | None:
+        return self.clients.get(client_id)
+
+    async def exchange_identity_assertion(
+        self, client: OAuthClientInformationFull, params: IdentityAssertionParams
+    ) -> OAuthToken:
+        claims = self._decode_and_validate_id_jag(params.assertion, client)
+
+        access_token = f"access_{secrets.token_hex(16)}"
+        self.access_tokens[access_token] = AccessToken(
+            token=access_token,
+            client_id=claims.client_id,
+            scopes=claims.scopes,
+            expires_at=int(time.time()) + 3600,
+            # Bind to the resource from the validated ID-JAG, not the client-controlled request.
+            resource=claims.resource,
+            subject=claims.subject,
+        )
+        # No refresh token: SEP-990 relies on the IdP re-issuing ID-JAGs to control session lifetime.
+        return OAuthToken(
+            access_token=access_token,
+            token_type="Bearer",
+            expires_in=3600,
+            scope=" ".join(claims.scopes),
+        )
+
+    def _decode_and_validate_id_jag(self, assertion: str, client: OAuthClientInformationFull) -> IdJagClaims:
+        """Verify the ID-JAG and return its trusted claims, or reject the request.
+
+        Replace this stub with real RFC 7523 §3 / SEP-990 §5.1 validation. It fails closed - it
+        raises rather than trusting the assertion - so a copy of this example cannot accidentally
+        accept unverified tokens. RFC 7523 §3.1 / RFC 6749 §5.2 specify `invalid_grant` for a
+        rejected assertion.
+        """
+        raise NotImplementedError("Validate the ID-JAG (signature, iss/aud/exp/typ, client_id, resource)")
+
+    async def load_access_token(self, token: str) -> AccessToken | None:
+        return self.access_tokens.get(token)