Microsoft 365 MCP server: AADSTS900971 “No reply address provided” on every auth attempt

Summary

Authenticating to the microsoft365 MCP server fails immediately with Azure AD error AADSTS900971: No reply address provided. The OAuth authorization URL generated by the server does include a well-formed redirect_uri, so the issue is on the Azure AD app registration side — it appears to be missing a registered reply URL for loopback clients.

Auth URL:

https://login.microsoftonline.com/common/oauth2/v2.0/authorize?response_type=code&client_id=08ad6f98-a4f8-4635-bb8d-f1a3044760f0&code_challenge=HMpZvtfqglCIqR3npXN3lFyTPbNcubmXkRSIsXakIR8&code_challenge_method=S256&redirect_uri=http%3A%2F%2Flocalhost%3A50546%2Fcallback&state=UzlmHNwFueZS1L_RqFlpuse-gXDSJWHjRrAv-Ay5yUY&scope=api%3A%2F%2F07c030f6-5743-41b7-ba00-0a6e85f37c17%2F.default+offline_access+openid+email&prompt=consent

Server

• MCP server: https://microsoft365.mcp.claude.com/mcp
• Azure AD client ID: 08ad6f98-a4f8-4635-bb8d-f1a3044760f0

Repro

1. Run /mcp in Claude Code and trigger authentication for microsoft365.
2. Open the generated login.microsoftonline.com/common/oauth2/v2.0/authorize?... URL in a browser.
3. Microsoft returns AADSTS900971: No reply address provided before any redirect occurs.

Impact

The microsoft365 MCP server is unusable — no client-side workaround since the redirect URI list is controlled by Anthropic