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mcp: Implement stateless server (SEP-2275) (#965)

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## Description

This PR lays the foundational server-side groundwork for the `>=
2026-06-30` sessionless and stateless feature introduced by
[SEP-2575](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2575)
and SEP-2567, tracked in
[design/stateless.md](614460a225/design/stateless.md).

### SEP-2575: Stateless MCP

*   **Per-request protocol detection in `ServerSession.handle()`**
Unmarshal `_meta` from the raw JSON-RPC params determines whether each
request follows the new protocol.

*   **Per-request typed accessors on `ServerRequest[P]`**
    ```go
    func (r *ServerRequest[P]) ProtocolVersion() string
    func (r *ServerRequest[P]) ClientInfo() *Implementation
    func (r *ServerRequest[P]) ClientCapabilities() *ClientCapabilities
    ```

* **Reject client->server `initialize`, `initialized` and `ping` for
new-protocol requests**

*   **Per-request `_meta` field name constants**
Three constants for the wire-protocol field names
(`MetaKeyProtocolVersion`, `MetaKeyClientInfo`,
`MetaKeyClientCapabilities`)

* **Stop synthesizing fake `InitializeParams` for new-protocol
requests**

Fixes: #966
This commit is contained in:
Guglielmo Colombo 2026-05-26 16:25:51 +02:00 • committed by GitHub
commit 4cbdd6a514
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1 changed files with 1113 additions and 62 deletions
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27
mcp/server.go
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@ -1450,13 +1450,32 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
initialized := ss.state.InitializeParams != nil
ss.mu.Unlock()
// From the spec:
// "The client SHOULD NOT send requests other than pings before the server
// has responded to the initialize request."
// Per-request protocol detection (SEP-2575): if the request carries
// `io.modelcontextprotocol/protocolVersion` in its `_meta` field, it
// follows the new sessionless protocol. The initialization gate is
// skipped for such requests.
validatedMeta, perRequestErr := validateRequestMeta(req)
if perRequestErr != nil {
return nil, perRequestErr
}
if !initialized && validatedMeta.usesNewProtocol && validatedMeta.initializeParams != nil {
ss.updateState(func(state *ServerSessionState) {
state.InitializeParams = validatedMeta.initializeParams
})
}
switch req.Method {
case methodInitialize, methodPing, notificationInitialized:
if validatedMeta.usesNewProtocol {
ss.server.opts.Logger.Error("method removed in the new protocol", "method", req.Method)
return nil, &jsonrpc.Error{
Code: jsonrpc.CodeMethodNotFound,
Message: fmt.Sprintf("%q is not supported in the new protocol", req.Method),
}
}
default:
if !initialized {
if !initialized && !validatedMeta.usesNewProtocol {
ss.server.opts.Logger.Error("method invalid during initialization", "method", req.Method)
return nil, fmt.Errorf("method %q is invalid during session initialization", req.Method)
}