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MCP Go SDK was rejecting spec-legal Base64-wrapped Mcp-Name headers as a mismatch
modelcontextprotocol/go-sdk PR #1242 merged 2026-09-06 at 10:06 UTC. The 2026-07-28 transport spec lets a client Base64-wrap a name or resource URI that is not header-safe using the `=?base64?...?=` form and requires the server to decode before comparing; validateMcpHeaders decoded Mcp-Param-* values that way but compared Mcp-Name raw, so a legal encoded name was refused as a header mismatch. The fix mirrors the Mcp-Param-* path and rejects an invalid Base64 wrapper with a clear error, with table tests covering both cases. This is invisible to any release sweep since the Go SDK has cut no release since.
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