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MCP C# SDK Protocol Negotiation: Pin 2026-07-28 When Fallback Is Unsafe
TL;DR: A MCP C# SDK protocol negotiation can unintentionally downgrade behavior, masking a contract change when fallback to an older protocol occurs. Pinning expectations helps catch this but requires careful testing of failure paths.
The MCP C# SDK 2.0.0 defaults to 2026-07-28 but preserves fallback to older, initialize-capable versions for server compatibility. This can hide that a client expecting sessionless behavior actually negotiated an initialize-era session. Negotiation responses should be treated distinctly from network or protocol errors, and application retry logic must preserve this distinction. Pinned clients may reach a state where a stateful server forces a downgrade, leading to a different contract than assumed. This upgrade/downgrade dynamic can be misleading for health checks and feature assumptions that rely on stateless, modern-only behavior.
Question for the room: Have you implemented explicit negotiation-awareness in your clients to detect and react to protocol downgrades or upgrades in MCP-like environments?
— via dev.to
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