feat!: add server discovery and negotiation (SEP-2575)#973
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DaleSeo
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Jul 14, 2026
| /// The request used a protocol version the server does not support. | ||
| pub const UNSUPPORTED_PROTOCOL_VERSION: Self = Self(-32022); | ||
| /// Processing the request requires a client capability that was not declared. | ||
| pub const MISSING_REQUIRED_CLIENT_CAPABILITY: Self = Self(-32021); |
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I initially set them to -32004 and -32003 following the SEP, but it failed the conformance suite. Then, I noticed that the error codes in the SEP differ from the draft schema. I reported this issue in modelcontextprotocol/modelcontextprotocol/issues/3091
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alexhancock
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Jul 15, 2026
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| /// Select the first client-preferred protocol version supported by the server. | ||
| /// | ||
| /// Returns `None` when no version is shared. |
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What happens in this case? When a client only expresses a preference the server doesn't yet support?
Shouldn't it fall back to the newest one the server does support?
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Motivation and Context
This implements the discovery and negotiation portion of SEP-2575. It adds the
server/discoverRPC, per-request protocol version negotiation, and typed unsupported-version errors so clients can learn a server's versions and capabilities without initializing a session, while preserving the legacy initialization flow.All six protocol union enums generated by
ts_union!are now#[non_exhaustive]. This requires downstream exhaustive matches to add a wildcard arm once, but allows future protocol variants to be added without causing the same Rust API break.How Has This Been Tested?
Added tests
Breaking Changes
Yes.
ClientRequest,ClientNotification,ClientResult,ServerRequest,ServerNotification, andServerResultare now#[non_exhaustive], so downstream exhaustive matches must add a wildcard arm.DiscoverRequestandDiscoverResultalso add variants toClientRequestandServerResult; subsequent variants can be added without another exhaustive-match break.Types of changes
Checklist