OAuth token refresh sends RFC 8707 resource parameter that Entra ID v2.0 rejects (AADSTS9010010)

Problem

The MCP Python SDK sends an RFC 8707 resource parameter on all token requests — including refresh_token grants. Microsoft Entra ID v2.0 rejects this with AADSTS9010010 (The resource parameter provided in the request doesn't match with the requested scopes).

This causes MCP servers using Entra ID OAuth to lose authentication after ~1 hour when the access token expires and the SDK attempts a silent refresh.

Root Cause

Two compounding issues:

1. Entra v2.0 does not support resource on refresh

Entra's v2.0 token endpoint expects scope, not resource. The resource parameter is a v1.0 concept. Since March 2026, Entra strictly validates and rejects resource on token refresh (previously it was silently ignored).

2. Pydantic v2 AnyHttpUrl trailing-slash normalization

ProtectedResourceMetadata.resource is typed as AnyHttpUrl (shared/auth.py:143). When str() is called on a bare-domain URL, Pydantic v2 adds a trailing slash:

>>> str(AnyHttpUrl("https://mcp-server.example.com"))
'https://mcp-server.example.com/'   # trailing slash added

In get_resource_url() (client/auth/oauth2.py:155), this trailing-slash version is used:

prm_resource = str(self.protected_resource_metadata.resource)  # adds trailing slash

But the Entra app registration has the audience as https://mcp-server.example.com (no slash), so the resource and scope audience don't match.

Affected Code

src/mcp/client/auth/oauth2.py:

async def _refresh_token(self) -> httpx.Request:
    refresh_data = {
        "grant_type": "refresh_token",
        "refresh_token": self.context.current_tokens.refresh_token,
        "client_id": self.context.client_info.client_id,
    }
    # This sends 'resource' on refresh — Entra v2.0 rejects it
    if self.context.should_include_resource_param(self.context.protocol_version):
        refresh_data["resource"] = self.context.get_resource_url()  # RFC 8707

The same issue exists in the TypeScript SDK (packages/client/src/client/auth.ts), where WHATWG URL also normalizes bare-domain URLs with a trailing slash.

Suggested Fix

Option A: Strip trailing slash in get_resource_url()

def get_resource_url(self) -> str:
    resource = resource_url_from_server_url(self.server_url)
    if self.protected_resource_metadata and self.protected_resource_metadata.resource:
        prm_resource = str(self.protected_resource_metadata.resource).rstrip('/')
        if check_resource_allowed(requested_resource=resource, configured_resource=prm_resource):
            resource = prm_resource
    return resource

Option B: Include scope alongside resource on refresh

Entra v2.0 tolerates resource if scope is also present and consistent:

refresh_data["scope"] = " ".join(self.context.scopes)

Option C: Make resource on refresh configurable

Allow servers to signal whether the resource parameter should be included on refresh grants, since not all authorization servers support RFC 8707.

Related Issues

• https://github.com/anthropics/claude-code/issues/52871 — same trailing-slash + AADSTS9010010 bug
• https://github.com/microsoft/powerbi-modeling-mcp/issues/68 — same Entra v2.0 incompatibility

Environment

• MCP Python SDK: v1.27.0
• Authorization server: Microsoft Entra ID v2.0
• MCP server: Azure Container Apps with custom EntraTokenVerifier
• MCP spec version: 2025-06-18 (mandates RFC 8707 resource)

Current Workaround

Server-side: set resource_server_url=None in AuthSettings and do NOT serve /.well-known/oauth-protected-resource metadata. Without PRM, should_include_resource_param() returns False and resource is omitted from refresh requests. Initial auth still works via the WWW-Authenticate header fallback.

Start in src/mcp/client/auth/oauth2.py by tracing _refresh_token() and get_resource_url(), then inspect shared/auth.py and packages/client/src/client/auth.ts for the corresponding TypeScript flow. Reproduce the refresh request with an Entra ID v2.0 setup and check existing OAuth tests or request fixtures. Done means refresh behavior handles Entra's rejected parameters and URL normalization consistently across the affected SDKs.