Add prior-knowledge connection path for modern protocol clients

[halter73]

Summary

Add a C# client API for 2026-07-28+ connections that lets callers skip the server/discover round trip when they already have trusted prior knowledge about the server. This should be distinct from McpClientOptions.ProtocolVersion, so exact version pinning can continue to mean "require this version / disable fallback" rather than "skip discovery."

Motivation

Today, setting McpClientOptions.ProtocolVersion = "2026-07-28" disables legacy fallback, but the client still sends server/discover before adopting the modern protocol. That is safe, but it leaves no zero-round-trip path for known modern servers or for clients that have cached a previous DiscoverResult.

A separate prior-knowledge API would improve startup latency and align C# with the stronger ergonomics in the other Tier 1 SDKs while preserving C#'s current safe defaults.

Current C# behavior

• ProtocolVersion = null: default modern auto path; sends server/discover and falls back to legacy initialize when appropriate.
• ProtocolVersion = "2026-07-28": sends server/discover and requires a modern result; does not fall back to legacy.
• Legacy exact versions such as "2025-11-25": skip server/discover and use legacy initialize.
• No public API was found for providing a cached DiscoverResult or directly adopting a known modern protocol version.

Relevant C# files:

• src/ModelContextProtocol.Core/Client/McpClientOptions.cs
• src/ModelContextProtocol.Core/Client/McpClientImpl.cs
• src/ModelContextProtocol.Core/Protocol/DiscoverResult.cs

Tier 1 SDK references

TypeScript SDK

TypeScript exposes the strongest explicit prior-knowledge shape:

• ConnectOptions.prior?: DiscoverResult lets callers provide a previously obtained discovery result.
• _connectFromPrior(...) bypasses version negotiation and server/discover, adopts capabilities/server info/instructions, and sets the negotiated protocol version.
• getDiscoverResult() returns a reusable DiscoverResult.

References:

• https://github.com/modelcontextprotocol/typescript-sdk/blob/main/packages/client/src/client/client.ts
• Look for ConnectOptions.prior, _connectFromPrior, and getDiscoverResult.

Python SDK

Python supports both an explicit cached-discovery path and a simple direct-modern mode:

• Client(mode="auto") probes with server/discover and falls back.
• Client(mode="2026-07-28") adopts the modern protocol directly without sending server/discover.
• prior_discover can provide a cached DiscoverResult.
• ClientSession.adopt() installs an InitializeResult or DiscoverResult without wire traffic.
• ClientSession.discover_result exposes the round-trippable discovery result for reuse.

References:

• https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/client.py
• https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/session.py
• Look for ConnectMode, prior_discover, adopt, and discover_result.

Go SDK

Go has solid 2026-07-28 plumbing but does not appear to expose a public prior-knowledge client API:

• Client.Connect defaults to latest 2026-07-28, sends server/discover, negotiates, and falls back to legacy initialize when needed.
• ClientSessionOptions.protocolVersion is unexported and documented as a testing override.
• Server discover advertises transport-filtered supported versions.
• Go also implements modern per-request _meta, standard HTTP headers, subscriptions/listen, multi-round-trip helpers, and TTL caches for modern list/read results.

References:

• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/client.go
• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/server.go
• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/cache.go
• https://github.com/modelcontextprotocol/go-sdk/blob/main/docs/protocol.md

Proposed C# API direction

Add a distinct opt-in prior-knowledge path, for example:

• McpClientOptions.KnownDiscoverResult or McpClientOptions.PriorDiscoverResult: adopt a trusted discovery result and skip server/discover.
• Optionally, a direct-modern option such as AssumeProtocolVersion = "2026-07-28" for callers that only know the protocol version but do not have server info/capabilities yet.
• Keep ProtocolVersion semantics unchanged: exact modern version should still probe and require a compatible result unless the new prior-knowledge option is used.

Acceptance criteria

• A caller can connect to a known 2026-07-28+ server without sending server/discover.
• A caller can retrieve or construct the prior-knowledge payload needed for a later zero-round-trip connection.
• Existing default auto-negotiation and exact ProtocolVersion behavior remain backward compatible.
• Per-request _meta and HTTP headers continue to be populated correctly after prior-knowledge adoption.
• Tests cover default auto, exact modern pin, cached prior discovery, and invalid/incompatible prior discovery.