OIDCProxy forwards MCP `resource` indicator to upstream IdP, causing `invalid_target` errors

[aminabedi-bentley]

Summary

When an MCP client (e.g. VS Code Copilot, Copilot CLI) sends an RFC 8707 resource parameter in the OAuth authorization request, OIDCProxy forwards it verbatim to the upstream Identity Provider. Upstream IdPs that don't recognise the MCP server URL as a valid resource reject the request with invalid_target.

Steps to reproduce

1. Configure OIDCProxy as a third-party auth proxy in front of an IdP (e.g. Bentley IMS / PingFederate)
2. Connect from an MCP client that sends a resource parameter (recent VS Code Copilot, Copilot CLI)
3. The upstream IdP returns:

   400 - invalid_target: The specified resource(s) did not match any
   access token manager for the selected client and authentication context
   

Root cause

In consent.py, _build_upstream_authorize_url forwards the resource from the transaction to the upstream authorize URL:

# Forward resource indicator if present in transaction
if resource := transaction.get("resource"):
    query_params["resource"] = resource

The resource value is the MCP server's own URL (e.g. http://localhost:5000/mcp), set by the MCP client per the MCP OAuth spec. This makes sense for the proxy's own local validation (lines 727-745 of proxy.py), but should not be forwarded upstream — the upstream IdP has its own resource model and doesn't know about the downstream MCP server URL.

Expected behaviour

OIDCProxy should:

• ✅ Validate the client's resource parameter locally (already done)
• ✅ Use it as the audience for the downstream JWT it issues (already done)
• ❌ Not forward it to the upstream IdP's authorize or token endpoints

Workaround

Subclass OIDCProxy and strip resource from the upstream authorize URL:

class _FixedOIDCProxy(OIDCProxy):
    def _build_upstream_authorize_url(self, txn_id, transaction):
        url = super()._build_upstream_authorize_url(txn_id, transaction)
        parsed = urlparse(url)
        params = parse_qs(parsed.query, keep_blank_values=True)
        if "resource" in params:
            del params["resource"]
            return urlunparse(parsed._replace(query=urlencode(params, doseq=True)))
        return url

Environment

• fastmcp 3.1.1 (also present in 3.0.2)
• Upstream IdP: Bentley IMS (PingFederate-based)
• MCP clients: VS Code Copilot, GitHub Copilot CLI

[marvin-context-protocol]

Found 3 possible duplicate issues:

1. Option to not forward the resource parameter in the OAuth provider #2810: Option to not forward the resource parameter in the OAuth provider
2. with AzureProvider AADSTS901002: The 'resource' request parameter is not supported. #2152: with AzureProvider AADSTS901002: The 'resource' request parameter is not supported.
3. Microsoft Entra OAuth Error: The 'resource' request parameter is not supported. #1846: Microsoft Entra OAuth Error: The 'resource' request parameter is not supported.

This issue will be automatically closed as a duplicate in 3 days.

• If your issue is a duplicate, please close it and 👍 the existing issue instead
• To prevent auto-closure, add a comment or 👎 this comment

[aminabedi-bentley]

Already resolved in #2810