fix(mcp-auth): negotiate protocol version instead of rejecting - #381
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Add 2025-06-18 to the default supported protocol versions and negotiate downwards on initialize instead of returning JSON-RPC -32602 for an unsupported version. Per the MCP lifecycle spec the server echoes the requested version when supported, otherwise responds with a version it does support. We pick the highest supported version that is not newer than the requested one, because @modelcontextprotocol/sdk <= 1.16.0 rejects any negotiated version outside its own accepted set. Only when no supported version is old enough do we fall back to -32602.
The McpAuth construct test asserted the old default env value; update it to include 2025-06-18 to match the new default.
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Summary
The MCP server rejected
initializerequests whoseprotocolVersionwas not in the configured set with JSON-RPC-32602, and2025-06-18was missing from the defaults even though the adjacent2025-03-26and2025-11-25revisions were present.This changes
handleInitializeto negotiate a version rather than error:-32602.2025-06-18is added to the default supported set (['2025-11-25', '2025-06-18', '2025-03-26', '2024-11-05']).Why negotiate downwards
Per the MCP lifecycle spec, when the client requests an unsupported version the server responds with a version it supports and lets the client decide. We never answer with a version newer than requested, because
@modelcontextprotocol/sdk<= 1.16.0 hardcodesprotocolVersion: LATEST("2025-06-18") and rejects any negotiated version outside its own accepted set. Answering with our latest (2025-11-25) would fail those clients too.Version strings are
YYYY-MM-DD, so lexicographic comparison matches chronological order.Tests
Added regression coverage in
test/mcp-auth/mcp-server.test.ts: echo of2025-06-18, downgrade to the highest not-newer supported version, downgrade for an unknown future version, and the-32602fallback when no version is old enough. All 25 tests pass; lint clean.Related
Companion fix in taimos/nornkeep, whose runtime MCP server (
@taimos/mcp-cognito-auth) carries an identical copy of this logic. Found while adding the Nornkeep server to MetaMCP (SDK 1.16.0).