feat: add automatic MCP transport selection for 2024-11-05 / 2025-03-26 / 2025-06-18 vs 2025-11-25

[edk4971]

Is your feature request related to a problem? Please describe.
I'm coming from llama.cpp - localai fails on all my existing MCP servers. It appears the localai implementation of v1.5.0 of the mcp/go-sdk does not consider all permutations of transport/schema, currently hard-codes StreamableClientTransport and fails on 2025-06-18 stateless servers and SSE servers.

Describe the solution you'd like
Probe initialise, pick StreamableClientTransport{Stateless:false} for session-based 2025-11-25+, StreamableClientTransport{Stateless:true} for 2024-11-05/2025-03-26/2025-06-18, and fall back to SSEClientTransport

Describe alternatives you've considered
cleaner than a proxy - as should only be a single function (and the sdk is already in go.mod)

Additional context

[edk4971]

proposed solution

   func ChooseTransport(ctx context.Context, url string) mcp.Transport {
       // probe with the latest version we support
       body := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","clientInfo":{"name":"probe","version":"1"},"capabilities":{}}}`
       client := httpclient.New(httpclient.WithNoRedirects())
       req, _ := http.NewRequestWithContext(ctx, http.MethodPost, url, strings.NewReader(body))
       req.Header.Set("Content-Type", "application/json")
       req.Header.Set("Accept", "application/json,text/event-stream")

       resp, err := client.Do(req)
       if err != nil || resp == nil || resp.StatusCode != http.StatusOK {
           log.Warn("mcp probe failed, falling back to SSE", "url", url, "err", err)
           return &mcp.SSEClientTransport{Endpoint: url}
       }
       defer resp.Body.Close()

       data, _ := io.ReadAll(resp.Body)
       var probe struct {
           Result struct {
               ProtocolVersion string `json:"protocolVersion"`
           } `json:"result"`
       }
       _ = json.Unmarshal(data, &probe)

       hasSession := resp.Header.Get("Mcp-Session-Id") != ""
       ver := probe.Result.ProtocolVersion

       // session based 2025-11-25+
       if hasSession && (ver == "2025-11-25" || ver == "2026-07-28") {
           log.Info("using session streamable", "url", url, "version", ver)
           return &mcp.StreamableClientTransport{
               Endpoint:   url,
               HTTPClient: httpclient.New(httpclient.WithNoRedirects()),
               Stateless:  false,
           }
       }

       // 2024-11-05 / 2025-03-26 / 2025-06-18 → stateless streamable
       if ver == "2024-11-05" || ver == "2025-03-26" || ver == "2025-06-18" {
           log.Info("using stateless streamable", "url", url, "version", ver)
           return &mcp.StreamableClientTransport{
               Endpoint:   url,
               HTTPClient: httpclient.New(httpclient.WithNoRedirects()),
               Stateless:  true,
           }
       }

       log.Warn("unknown version, falling back to SSE", "url", url, "version", ver)
       return &mcp.SSEClientTransport{Endpoint: url}
   }

[localai-org-maint-bot]

I am picking this up. Latest master hard-codes Streamable HTTP in both remote MCP connection paths. I will add and test legacy SSE fallback while preserving authentication and discovery timeouts, and verify stateless and session-based Streamable HTTP with the pinned SDK. The SDK already derives session handling from the server response, so transport selection should not depend on a protocol-version lookup table.