MCP OAuth with Microsoft Entra fails when protected-resource metadata has a no-trailing-slash resource

Summary

codex mcp login cannot authenticate against the Microsoft Fabric Core MCP server because the emitted Microsoft Entra authorization URL sends a resource parameter with a trailing slash, while the requested Fabric scope identifies the same resource without one. Microsoft Entra rejects the request before the user can grant consent:

AADSTS9010010: The resource parameter provided in the request doesn't match with the requested scopes.

This is distinct from #17265, #13891, and #33403. Those reports concern missing/omitted resource or scope propagation; this report concerns an extra trailing slash in an emitted resource value.

Environment

• Codex CLI: 0.145.0
• OS: Windows
• MCP server: https://api.fabric.microsoft.com/v1/mcp/core
• OAuth client: pre-registered Microsoft Entra public client with PKCE
• Codex callback: configured local localhost callback (registered successfully)
• No bearer token, client secret, certificate, service principal, or proxy

Fabric OAuth protected-resource metadata

{
  "issuer": "https://login.microsoftonline.com/organizations/v2.0",
  "authorization_servers": [
    "https://login.microsoftonline.com/organizations/v2.0"
  ],
  "resource": "https://api.fabric.microsoft.com",
  "scopes_supported": [
    "https://api.fabric.microsoft.com/.default"
  ]
}

The resource is deliberately advertised without a trailing slash.

Codex configuration

mcp_oauth_callback_port = 19877
mcp_oauth_callback_url = "http://localhost:19877/codex-mcp"

[mcp_servers.fabric-core]
url = "https://api.fabric.microsoft.com/v1/mcp/core"
scopes = ["https://api.fabric.microsoft.com/.default"]
enabled = true

[mcp_servers.fabric-core.oauth]
client_id = "<redacted-public-client-id>"

There is no oauth_resource configuration.

Reproduction

1. Run codex mcp login fabric-core.
2. Inspect the authorization URL printed by Codex.
3. It contains the following effective values (state, PKCE challenge, client ID, and redirect URI redacted):

scope=https://api.fabric.microsoft.com/.default
resource=https://api.fabric.microsoft.com/

1. Microsoft Entra immediately returns AADSTS9010010.

Expected behavior

Codex must preserve the canonical protected-resource value from metadata and emit exactly one resource parameter:

scope=https://api.fabric.microsoft.com/.default
resource=https://api.fabric.microsoft.com

No trailing slash should be introduced.

The MCP authorization specification recommends a no-trailing-slash canonical resource form when the slash is not semantically significant:
https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization

Microsoft Fabric documents the same .default scope:
https://learn.microsoft.com/en-us/rest/api/fabric/articles/mcp-servers/core-remote/get-started-core

Suspected implementation area

In rust-v0.145.0, perform_oauth_login.rs delegates authorization URL construction to rmcp::AuthorizationManager; the subsequent Codex helper only appends an explicit oauth_resource value rather than replacing an existing query value:
https://github.com/openai/codex/blob/rust-v0.145.0/codex-rs/rmcp-client/src/perform_oauth_login.rs

This suggests the protected-resource URI is parsed/serialized as a URL and normalized to https://api.fabric.microsoft.com/, while the advertised scope remains https://api.fabric.microsoft.com/.default.

Requested fix

1. Preserve the raw resource value returned by protected-resource metadata, including its trailing-slash semantics.
2. Ensure the authorization URL contains exactly one resource parameter.
3. If oauth_resource is configured, replace any existing resource parameter instead of appending another one.
4. Forward the same exact resource value in authorization, token exchange, and refresh-token requests.
5. Add a regression test with:

• metadata resource: https://api.fabric.microsoft.com
• scope: https://api.fabric.microsoft.com/.default
• assertion: exactly one resource, no trailing slash.

This is blocking direct Fabric Core MCP authentication in both Codex CLI and Codex Desktop, which share the Codex MCP OAuth path.