Authorization header dropped on JSON-RPC tools/call after successful MCP initialize (Streamable HTTP + OAuth 2.1 / DCR)

[Raznak]

Describe the bug

Summary

After a successful OAuth 2.1 + DCR flow against an MCP server using Streamable HTTP, Copilot CLI sends the Authorization: Bearer … header on initialize but omits it on subsequent JSON-RPC requests (tools/list, tools/call).
The server-side session is shown as "connected" in /mcp, but every tool invocation fails with 401.

Environment

• copilot --version → 1.0.37 (latest at time of report)
• macOS (Darwin 25.4.0)
• MCP server: own implementation, Rust, Streamable HTTP via rmcp 1.4.0
• Auth: OAuth 2.1 + RFC 7591 (Dynamic Client Registration), public client (PKCE, token_endpoint_auth_method=none)
• AS metadata exposed at /.well-known/oauth-authorization-server (RFC 8414)
• Same server works correctly with Claude Desktop and Gemini CLI

Expected: Tool call succeeds (Bearer sent on the JSON-RPC tools/call HTTP request).

Observed: Tool call fails. Server logs show the JSON-RPC tools/call arrived with no Authorization header.

Server-side evidence

{
  "level": "WARN",
  "service": "backend",
  "target": "rmcp::service",
  "message": "response error",
  "id": "4",
  "error": "ErrorData { code: ErrorCode(-32600), message: \"UNAUTHORIZED: missing Authorization header\", data: None }"
}

JSON-RPC id 4 corresponds to a tools/call (ids 0–2 are the MCP handshake: initialize, notifications/initialized, tools/list).

The DCR registration of the Copilot CLI client succeeded:

field	value
client_name	GitHub Copilot CLI
client_type	public
grant_types	authorization_code, refresh_token
redirect_uris	http://127.0.0.1:<random-port>/
dynamically_registered	true

So the OAuth flow itself completes — the bug is downstream, in the MCP transport layer not propagating the Bearer to subsequent JSON-RPC HTTP requests.

Affected version

GitHub Copilot CLI 1.0.37.

Steps to reproduce the behavior

1. Configure the MCP server in ~/.copilot/mcp-config.json with "type": "http" pointing at a Streamable HTTP MCP that requires Bearer on every request.
2. Start Copilot CLI, /mcp lists the server as connected (browser opens, OAuth flow completes, refresh + access tokens stored).
3. Ask Copilot to invoke any tool exposed by the server.

Expected behavior

Tool call succeeds (Bearer sent on the JSON-RPC tools/call HTTP request).

Additional context

Cross-client check

The exact same server, with the exact same DCR + OAuth config, works correctly with:

• Claude Desktop — Bearer sent on every JSON-RPC request
• Gemini CLI MCP client — Bearer sent on every JSON-RPC request

So the server is compliant with the MCP Streamable HTTP spec (2025-03-26). The bug is specific to Copilot CLI's handling of the access token
after initialize.

Likely cause

Per the MCP Streamable HTTP spec, each JSON-RPC message is an independent HTTP POST to /mcp. The Authorization header must be attached to every request, not just to initialize or to the SSE long-lived stream.

It looks like Copilot CLI's OAuthProvider/transport binding only attaches the Bearer at session establishment (or to the SSE stream open), not on subsequent POSTs that carry the JSON-RPC payloads.