Six MCP SDKs Lack the Dual-Persona Auth Enterprises Need, and FastMCP Extensions Close the Gap

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"Zero-Trust Authorization and Discovery for Enterprise MCP" (arXiv 2609.22573, 18 Sep 2026) runs a gap analysis across the Python, TypeScript, Go, Rust, C# and Swift MCP SDKs and names three structural shortcomings: credential extraction bound to a single Authorization header, no pre-authentication tool discovery, and no fine-grained per-tool authorization in the base SDKs. The first bites hardest for a server that must serve human users over corporate SSO and automated agents over service-account credentials on a different header, which today requires custom middleware. The paper ships composable FastMCP extensions adding cross-header credential normalization, cached token verification across heterogeneous IdPs, and an unauthenticated metadata endpoint for credential-free registry discovery.