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mcp: Implement support for SEP-2575 on client side (#975)

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Implements the client half of the SEP-2575 / `2026-06-30` protocol. This
CL is client-only; server-side `server/discover` is still a
`MethodNotFound` stub.
### Changes
- **`server/discover` probe in `Client.Connect()`** (`client.go`). When
the caller opts into `protocolVersion >= 2026-06-30`, the client probes
the server with `server/discover` before falling back to the legacy
`initialize`
  handshake.
- **Per-request `_meta` injection** (`shared.go`, `injectRequestMeta`).
For`>= 2026-06-30` sessions every outgoing request gets the SEP-2575
triple (`protocolVersion`, `clientInfo`, `clientCapabilities`).
- **Server-side fix**: `ServerSession.handle` no longer pre-populates
`state.InitializeParams` from a `server/discover` request's `_meta`, so
a subsequent fallback `initialize` is not rejected as a duplicate.
### Fallback rules
Client falls back to the legacy `initialize` handshake when discover
returns:
- `MethodNotFound` (-32601),
- `UnsupportedProtocolVersionError` (-32004),
- an HTTP `Bad Request` (substring-matched on the error message, to
interop
  with pre-2026-06-30 servers).

Fixes #966
This commit is contained in:
Guglielmo Colombo 2026-06-02 14:17:01 +02:00 • committed by GitHub
commit c60a318d22
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2 changed files with 1010 additions and 96 deletions
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2
internal/jsonrpc2/conn.go
View file

@ -720,7 +720,7 @@ func (c *Connection) write(ctx context.Context, msg Message) error {
// For cancelled or rejected requests, we don't set the writeErr (which would
// break the connection). They can just be returned to the caller.
if err != nil && ctx.Err() == nil && !errors.Is(err, ErrRejected) {
if err != nil && ctx.Err() == nil && !errors.Is(err, ErrRejected) && !errors.Is(err, ErrUnsupportedProtocolVersion) && !errors.Is(err, ErrMethodNotFound) {
// The call to Write failed, and since ctx.Err() is nil we can't attribute
// the failure (even indirectly) to Context cancellation. The writer appears
// to be broken, and future writes are likely to also fail.

4
internal/jsonrpc2/wire.go
View file

@ -31,7 +31,7 @@ var (
// ErrUnknown should be used for all non coded errors.
ErrUnknown = NewError(-32001, "unknown error")
// ErrServerClosing is returned for calls that arrive while the server is closing.
ErrServerClosing = NewError(-32004, "server is closing")
ErrServerClosing = NewError(-32006, "server is closing")
// ErrClientClosing is a dummy error returned for calls initiated while the client is closing.
ErrClientClosing = NewError(-32003, "client is closing")
@ -45,6 +45,8 @@ var (
// should be returned to the caller to indicate that the specific request is
// invalid in the current context.
ErrRejected = NewError(-32005, "rejected by transport")
// ErrUnsupportedProtocolVersion is returned when a server does not support the protocol version.
ErrUnsupportedProtocolVersion = NewError(-32004, "unsupported protocol version")
)
const wireVersion = "2.0"