Drop-in OAuth Provider for Personal FastMCP Servers on All Claude Platforms

What This Solves

A developer built a personal memory server using mem0 that Claude can read and write to, creating a personal knowledge vault. While getting it working on Claude Code was straightforward, making it function across Claude.ai web, mobile, and Desktop platforms presented authentication challenges.

The core problem: Claude.ai custom connectors require full OAuth 2.1 compliance. FastMCP provides either an in-memory test provider (unsuitable for production) or a proxy requiring setup of external identity providers like Google, GitHub, or Auth0. For a server intended only for personal use, setting up Auth0 was undesirable overhead.

The Solution

The developer created a single-file authentication provider written in Python that handles the entire OAuth flow without external services. Key features include:

• Dynamic Client Registration (DCR)
• PKCE (Proof Key for Code Exchange)
• Token persistence
• Complete OAuth 2.1 implementation

The provider restricts redirect URIs to claude.ai and localhost only, preventing unauthorized access even though client registration remains open (a Claude.ai requirement that took time to understand).

Implementation Details and Gotchas

The developer encountered several undocumented issues during implementation:

• FastAPI's BaseHTTPMiddleware silently breaks streaming responses, requiring workarounds
• Tool naming conflicts: If tools are named generically (like "add_memory" or "search"), Claude will use its own built-in memory functions instead of calling your server tools. The solution is to prefix tool names distinctively
• Serverless database connections: Neon Postgres (and likely other serverless databases) drop idle connections. Creating a single connection at startup causes tools to randomly fail after a few minutes
• DCR configuration: Dynamic Client Registration is disabled by default in FastMCP. Without enabling it, the /register endpoint returns 404, and Claude.ai silently fails to connect without any error messages

Availability

The solution is available on GitHub at github.com/crumrine/fastmcp-personal-auth as a single Python file under MIT license. This provides a practical alternative for developers who want their personal FastMCP servers to work across all Claude platforms without the complexity of external identity providers.