mcp: Implement support for SEP-2575 on client side (#975)
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
GitHub
parent
d54751a8ee
commit
c60a318d22
1 changed files with 1010 additions and 96 deletions
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@ -6,6 +6,7 @@ package mcp
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import (
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"context"
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"encoding/json"
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"errors"
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"fmt"
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"io"
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@ -102,6 +103,23 @@ func (s *fakeStreamableServer) ServeHTTP(w http.ResponseWriter, req *http.Reques
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resp, ok := s.responses[key]
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if !ok {
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if key.jsonrpcMethod == "server/discover" {
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// Return MethodNotFound to trigger fallback to legacy initialize.
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w.Header().Set("Content-Type", "application/json")
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w.WriteHeader(http.StatusOK)
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respErr := &jsonrpc.Error{
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Code: jsonrpc.CodeMethodNotFound,
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Message: `method not found: "server/discover"`,
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}
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var id jsonrpc.ID
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if jsonrpcReq != nil {
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id = jsonrpcReq.ID
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}
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respMsg, _ := jsonrpc2.NewResponse(id, nil, respErr)
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data, _ := jsonrpc2.EncodeMessage(respMsg)
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w.Write(data)
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return
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}
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s.t.Errorf("missing response for %v", key)
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http.Error(w, "no response", http.StatusInternalServerError)
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return
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@ -1212,3 +1230,408 @@ func TestStreamableClientOAuth_RetrieveError(t *testing.T) {
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t.Fatalf("client.Connect() error = %v, want %v", err, errTestAuthorizeFailed)
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}
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}
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// discoverResult is the canned successful DiscoverResult returned by
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// fakeStreamableServer setups in the tests below.
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var discoverResult = &DiscoverResult{
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SupportedVersions: []string{protocolVersion20260630},
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Capabilities: &ServerCapabilities{
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Tools: &ToolCapabilities{ListChanged: true},
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},
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ServerInfo: &Implementation{Name: "discoverServer", Version: "v1.0.0"},
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Instructions: "test discover",
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}
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// TestStreamableClientConnect_DiscoverSuccess verifies that Client.Connect on
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// the streamable transport:
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// - sends a POST server/discover with Mcp-Protocol-Version: 2026-06-30 and
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// the SEP-2575 per-request _meta triple in the body,
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// - on a successful DiscoverResult, skips the legacy initialize handshake
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// entirely, and
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// - seeds ClientSession.InitializeResult() from the discover response,
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// picking a mutually-supported protocol version.
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func TestStreamableClientConnect_DiscoverSuccess(t *testing.T) {
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ctx := context.Background()
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// Temporarily enable 2026-06-30 support in the SDK for this test
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oldSupported := supportedProtocolVersions
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supportedProtocolVersions = append([]string{protocolVersion20260630}, supportedProtocolVersions...)
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t.Cleanup(func() {
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supportedProtocolVersions = oldSupported
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})
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var (
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gotDiscoverMu sync.Mutex
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gotDiscover *jsonrpc.Request
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)
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "sess-1",
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},
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wantProtocolVersion: protocolVersion20260630,
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responseFunc: func(r *jsonrpc.Request) (string, int) {
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gotDiscoverMu.Lock()
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gotDiscover = r
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gotDiscoverMu.Unlock()
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return jsonBody(t, resp(1, discoverResult, nil)), http.StatusOK
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},
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},
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{"DELETE", "sess-1", "", ""}: {optional: true},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{Endpoint: httpServer.URL}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, &ClientSessionOptions{protocolVersion: protocolVersion20260630})
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if err != nil {
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t.Fatalf("Connect: %v", err)
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}
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defer session.Close()
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if missing := fake.missingRequests(); len(missing) > 0 {
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t.Errorf("missing expected requests: %v", missing)
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}
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gotDiscoverMu.Lock()
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defer gotDiscoverMu.Unlock()
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if gotDiscover == nil {
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t.Fatal("server did not receive server/discover")
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}
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// Inspect the discover request body for the SEP-2575 _meta triple.
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var body struct {
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Meta map[string]any `json:"_meta"`
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}
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if err := json.Unmarshal(gotDiscover.Params, &body); err != nil {
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t.Fatalf("decoding discover params: %v", err)
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}
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if v, _ := body.Meta[MetaKeyProtocolVersion].(string); v != protocolVersion20260630 {
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t.Errorf("_meta[%s] = %q, want %q", MetaKeyProtocolVersion, v, protocolVersion20260630)
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}
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if _, ok := body.Meta[MetaKeyClientInfo]; !ok {
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t.Errorf("_meta[%s] missing", MetaKeyClientInfo)
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}
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if _, ok := body.Meta[MetaKeyClientCapabilities]; !ok {
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t.Errorf("_meta[%s] missing", MetaKeyClientCapabilities)
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}
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ir := session.InitializeResult()
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if ir == nil {
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t.Fatal("InitializeResult is nil after Connect")
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}
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if got, want := ir.ProtocolVersion, protocolVersion20260630; got != want {
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t.Errorf("InitializeResult.ProtocolVersion = %q, want %q", got, want)
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}
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if ir.ServerInfo == nil || ir.ServerInfo.Name != "discoverServer" {
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t.Errorf("InitializeResult.ServerInfo = %+v, want Name=discoverServer", ir.ServerInfo)
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}
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if ir.Instructions != "test discover" {
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t.Errorf("InitializeResult.Instructions = %q, want %q", ir.Instructions, "test discover")
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}
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}
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// TestStreamableClientConnect_DiscoverMethodNotFound verifies that Client.Connect
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// falls back to the legacy initialize handshake when the server responds to
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// server/discover with a JSON-RPC "Method not found" error.
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func TestStreamableClientConnect_DiscoverMethodNotFound(t *testing.T) {
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ctx := context.Background()
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// Each request gets a fresh jsonrpc2 ID from the same client connection.
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// Use responseFunc to echo the request's ID back so the client matches
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// the response to the in-flight call regardless of ordering.
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echoErr := func(err error) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Error: err.(*jsonrpc.Error)}), http.StatusOK
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}
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}
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echoResult := func(result any) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Result: mustMarshal(result)}), http.StatusOK
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}
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}
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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header: header{"Content-Type": "application/json"},
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wantProtocolVersion: protocolVersion20260630,
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responseFunc: echoErr(&jsonrpc.Error{
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Code: jsonrpc.CodeMethodNotFound,
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Message: "method not found",
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}),
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},
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{"POST", "", methodInitialize, ""}: {
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "fallback",
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},
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responseFunc: echoResult(initResult),
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},
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{"POST", "fallback", notificationInitialized, ""}: {
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status: http.StatusAccepted,
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wantProtocolVersion: latestProtocolVersion,
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},
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{"GET", "fallback", "", ""}: {
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header: header{"Content-Type": "text/event-stream"},
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wantProtocolVersion: latestProtocolVersion,
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optional: true,
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},
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{"DELETE", "fallback", "", ""}: {optional: true},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{Endpoint: httpServer.URL}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, nil)
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if err != nil {
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t.Fatalf("Connect: %v", err)
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}
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defer session.Close()
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if got := session.InitializeResult().ProtocolVersion; got != latestProtocolVersion {
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t.Errorf("InitializeResult.ProtocolVersion = %q, want %q (initialize fallback)", got, latestProtocolVersion)
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}
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}
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// TestStreamableClientConnect_DiscoverUnsupportedVersion verifies that
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// Client.Connect falls back to the legacy initialize handshake when the
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// server responds to server/discover with the SEP-2575
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// UnsupportedProtocolVersionError JSON-RPC code.
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func TestStreamableClientConnect_DiscoverUnsupportedVersion(t *testing.T) {
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ctx := context.Background()
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echoErr := func(err error) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Error: err.(*jsonrpc.Error)}), http.StatusOK
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}
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}
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echoResult := func(result any) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Result: mustMarshal(result)}), http.StatusOK
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}
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}
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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header: header{"Content-Type": "application/json"},
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wantProtocolVersion: protocolVersion20260630,
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responseFunc: echoErr(&jsonrpc.Error{
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Code: CodeUnsupportedProtocolVersion,
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Message: "unsupported protocol version",
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}),
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},
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{"POST", "", methodInitialize, ""}: {
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "fallback",
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},
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responseFunc: echoResult(initResult),
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},
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{"POST", "fallback", notificationInitialized, ""}: {
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status: http.StatusAccepted,
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wantProtocolVersion: latestProtocolVersion,
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},
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{"GET", "fallback", "", ""}: {
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header: header{"Content-Type": "text/event-stream"},
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wantProtocolVersion: latestProtocolVersion,
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optional: true,
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},
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{"DELETE", "fallback", "", ""}: {optional: true},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{Endpoint: httpServer.URL}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, nil)
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if err != nil {
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t.Fatalf("Connect: %v", err)
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}
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defer session.Close()
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if got := session.InitializeResult().ProtocolVersion; got != latestProtocolVersion {
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t.Errorf("InitializeResult.ProtocolVersion = %q, want %q (initialize fallback)", got, latestProtocolVersion)
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}
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}
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// TestStreamableClientConnect_DiscoverPropagatesOtherErrors verifies that
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// Client.Connect does NOT fall back to initialize when server/discover
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// returns an unrelated JSON-RPC error (here, CodeInternalError). The Connect
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// call should fail with the propagated error rather than masking it.
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func TestStreamableClientConnect_DiscoverPropagatesOtherErrors(t *testing.T) {
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ctx := context.Background()
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var sawInitialize atomic.Bool
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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header: header{"Content-Type": "application/json"},
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wantProtocolVersion: protocolVersion20260630,
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responseFunc: func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{
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ID: r.ID,
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Error: &jsonrpc.Error{
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Code: jsonrpc.CodeInternalError,
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Message: "boom",
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},
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}), http.StatusOK
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},
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},
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{"POST", "", methodInitialize, ""}: {
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responseFunc: func(r *jsonrpc.Request) (string, int) {
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sawInitialize.Store(true)
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Result: mustMarshal(initResult)}), http.StatusOK
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},
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "fallback",
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},
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optional: true,
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},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{Endpoint: httpServer.URL}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, &ClientSessionOptions{protocolVersion: protocolVersion20260630})
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if err == nil {
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_ = session.Close()
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t.Fatal("Connect succeeded; want propagated error")
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}
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if sawInitialize.Load() {
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t.Error("server received initialize; Connect should have aborted on the discover error")
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}
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}
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// TestStreamableClientConnect_DiscoverMethodNotFoundVPre verifies that
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// Client.Connect falls back to the legacy initialize handshake when a
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// pre-SEP-2575 (vPre) server rejects server/discover.
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func TestStreamableClientConnect_DiscoverMethodNotFoundVPre(t *testing.T) {
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ctx := context.Background()
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echoResult := func(result any) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Result: mustMarshal(result)}), http.StatusOK
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}
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}
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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wantProtocolVersion: protocolVersion20260630,
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// Reproduce the exact body a vPre server produces via
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// http.Error(w, err.Error(), 400) where err comes from
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// checkRequest. http.Error appends a trailing newline.
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body: "JSON RPC not handled: \"server/discover\" unsupported\n",
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status: http.StatusBadRequest,
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header: header{"Content-Type": "text/plain; charset=utf-8"},
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},
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{"POST", "", methodInitialize, ""}: {
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "fallback",
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},
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responseFunc: echoResult(initResult),
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},
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{"POST", "fallback", notificationInitialized, ""}: {
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status: http.StatusAccepted,
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wantProtocolVersion: latestProtocolVersion,
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},
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{"DELETE", "fallback", "", ""}: {optional: true},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{
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Endpoint: httpServer.URL,
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DisableStandaloneSSE: true,
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}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, &ClientSessionOptions{protocolVersion: protocolVersion20260630})
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if err != nil {
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t.Fatalf("Connect: %v", err)
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}
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defer session.Close()
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if got := session.InitializeResult().ProtocolVersion; got != latestProtocolVersion {
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t.Errorf("InitializeResult.ProtocolVersion = %q, want %q (initialize fallback)", got, latestProtocolVersion)
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}
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}
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// TestStreamableClientConnect_DiscoverUnsupportedProtocolVersion verifies that
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// Client.Connect falls back to the legacy initialize handshake when a
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// server rejects server/discover with a plain HTTP 400 containing "Unsupported protocol version".
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func TestStreamableClientConnect_DiscoverUnsupportedVersionVPre(t *testing.T) {
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ctx := context.Background()
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echoResult := func(result any) func(*jsonrpc.Request) (string, int) {
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return func(r *jsonrpc.Request) (string, int) {
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return jsonBody(t, &jsonrpc.Response{ID: r.ID, Result: mustMarshal(result)}), http.StatusOK
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}
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}
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fake := &fakeStreamableServer{
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t: t,
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responses: fakeResponses{
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{"POST", "", methodDiscover, ""}: {
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wantProtocolVersion: protocolVersion20260630,
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body: "Bad Request: Unsupported protocol version\n",
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status: http.StatusBadRequest,
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header: header{"Content-Type": "text/plain; charset=utf-8"},
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},
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{"POST", "", methodInitialize, ""}: {
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header: header{
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"Content-Type": "application/json",
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sessionIDHeader: "fallback",
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},
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responseFunc: echoResult(initResult),
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},
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{"POST", "fallback", notificationInitialized, ""}: {
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status: http.StatusAccepted,
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wantProtocolVersion: latestProtocolVersion,
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},
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{"DELETE", "fallback", "", ""}: {optional: true},
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},
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}
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httpServer := httptest.NewServer(fake)
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defer httpServer.Close()
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transport := &StreamableClientTransport{
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Endpoint: httpServer.URL,
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DisableStandaloneSSE: true,
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}
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client := NewClient(testImpl, nil)
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session, err := client.Connect(ctx, transport, &ClientSessionOptions{protocolVersion: protocolVersion20260630})
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if err != nil {
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t.Fatalf("Connect: %v", err)
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}
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defer session.Close()
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if got := session.InitializeResult().ProtocolVersion; got != latestProtocolVersion {
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t.Errorf("InitializeResult.ProtocolVersion = %q, want %q (initialize fallback)", got, latestProtocolVersion)
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}
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}
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