[Bugs] Copilot App fails to start MCP OAuth login for github-spaces from the Setting...

**Mood:** 😊
**Category:** Bugs

Copilot App fails to start MCP OAuth login for github-spaces from the Settings “Sign in” flow.

Observed behavior

In Copilot App Settings, github-spaces shows Sign in.
Clicking Sign in immediately returns Request Error.
App log shows:
start_mcp_oauth_login is received
then Request session.mcp.oauth.login failed with message: Failed to register OAuth client
server name: github-spaces
Why this looks like an App-side OAuth flow issue

Running the bundled CLI binary directly (...github-copilot-sdk-1.0.49\copilot.exe) does not reproduce this exact failure.
Direct CLI can call github-spaces MCP and reaches a later-stage API response (twirp error not_found) instead of failing at OAuth client registration.
So the failure appears specific to the Copilot App “Sign in” path (OAuth client registration step), not basic MCP connectivity.
Relevant config/log context

MCP server config is in ~/.copilot/mcp-config.json (not ~/.copilot/mcp.json in this environment).
github-spaces config:
URL: https://api.githubcopilot.com/mcp/
Header: X-MCP-Toolsets: default,copilot_spaces
App log evidence includes:
failed to start MCP OAuth login ... Failed to register OAuth client server_name=github-spaces
repeated on multiple attempts.

---
| Field | Value |
| --- | --- |
| App version | 0.2.7 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 7 (Week 1) |