internal/mcp: update protocol versions

Update protocol versions to reflect that 2025-03-26 is supported, and
add server logic to negotiate the supported version with the client.

Update the client to set the protocol version, fixing what was probably
a spec violation.

+tests

Change-Id: I564b591b3daf018dd54f5c7e5d8f2d2b1e4ff502
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682516
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

mcp/client.go

@@ -101,9 +101,13 @@ func (c *Client) Connect(ctx context.Context, t Transport) (cs *ClientSession, e
 	}
 
 	params := &InitializeParams{
-		ClientInfo:   &implementation{Name: c.name, Version: c.version},
-		Capabilities: caps,
+		ClientInfo:      &implementation{Name: c.name, Version: c.version},
+		Capabilities:    caps,
+		ProtocolVersion: "2025-03-26",
 	}
+	// TODO(rfindley): handle protocol negotiation gracefully. If the server
+	// responds with 2024-11-05, surface that failure to the caller of connect,
+	// so that they can choose a different transport.
 	res, err := handleSend[*InitializeResult](ctx, cs, methodInitialize, params)
 	if err != nil {
 		_ = cs.Close()

mcp/conformance_go124_test.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.24 && goexperiment.synctest
+
+package mcp
+
+import (
+	"testing"
+	"testing/synctest"
+)
+
+func runSyncTest(t *testing.T, f func(t *testing.T)) {
+	synctest.Run(func() { f(t) })
+}

mcp/conformance_go125_test.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.25
+
+package mcp
+
+import (
+	"testing"
+	"testing/synctest"
+)
+
+func runSyncTest(t *testing.T, f func(t *testing.T)) {
+	synctest.Test(t, f)
+}

mcp/conformance_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.24 && goexperiment.synctest
+//go:build (go1.24 && goexperiment.synctest) || go1.25
 
 package mcp
 
@@ -85,7 +85,7 @@ func TestServerConformance(t *testing.T) {
 			// By comparison, gopls has a complicated framework based on progress
 			// reporting and careful accounting to detect when all 'expected' work
 			// on the server is complete.
-			synctest.Run(func() { runServerTest(t, test) })
+			runSyncTest(t, func(t *testing.T) { runServerTest(t, test) })
 
 			// TODO: in 1.25, use the following instead:
 			// synctest.Test(t, func(t *testing.T) {

mcp/server.go

@@ -634,9 +634,16 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 		ss.mu.Unlock()
 	}()
 
+	version := "2025-03-26" // preferred version
+	switch v := params.ProtocolVersion; v {
+	case "2024-11-05", "2025-03-26":
+		version = v
+	}
+
 	return &InitializeResult{
-		// TODO(rfindley): support multiple protocol versions.
-		ProtocolVersion: "2024-11-05",
+		// TODO(rfindley): alter behavior when falling back to an older version:
+		// reject unsupported features.
+		ProtocolVersion: version,
 		Capabilities: &serverCapabilities{
 			Prompts: &promptCapabilities{
 				ListChanged: true,

mcp/streamable_test.go

@@ -122,7 +122,7 @@ func TestStreamableServerTransport(t *testing.T) {
 			Resources: &resourceCapabilities{ListChanged: true},
 			Tools:     &toolCapabilities{ListChanged: true},
 		},
-		ProtocolVersion: "2024-11-05",
+		ProtocolVersion: "2025-03-26",
 		ServerInfo:      &implementation{Name: "testServer", Version: "v1.0.0"},
 	}, nil)
 	initializedMsg := req(0, "initialized", &InitializedParams{})

mcp/testdata/conformance/server/version-latest.txtar

@@ -0,0 +1,40 @@
+Check that when the client sends an arbitrary later version, the server
+response with its latest supported version.
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2029-01-01",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"_meta": {},
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2025-03-26",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}

mcp/testdata/conformance/server/version-older.txtar

@@ -0,0 +1,40 @@
+Check that when the client sends an older version, the server indicates
+support.
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"_meta": {},
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2024-11-05",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}