feat(cli): migrate exp mcp stdio server to official MCP Go SDK (#28057)

## Stack Context

PR 2 of 6 in a stack that migrates every Coder MCP surface from the
archived `github.com/mark3labs/mcp-go` library to the official
`github.com/modelcontextprotocol/go-sdk` v1.7.0.

Stack: #28056 -> #28057 -> #28058 -> #28059 -> #28060 -> #28061

## Why

`coder exp mcp server` (stdio) now uses the official SDK server with
`mcp.IOTransport` over the invocation's stdin/stdout, and reuses the
shared `coderd/mcp.RegisterSDKTool` helper from PR #28056 so both
servers register tools identically.

- A `nopWriteCloser` prevents the SDK from closing the invocation's
stdout.
- Tests send spec-compliant initialize params and
`notifications/initialized` before `tools/list` because the official SDK
enforces the protocol lifecycle.

> Mux created this PR on Mike's behalf.

cli/exp_mcp.go

@@ -1,10 +1,10 @@
 package cli
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
+	"io"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -13,8 +13,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/mark3labs/mcp-go/mcp"
-	"github.com/mark3labs/mcp-go/server"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
 	"github.com/spf13/afero"
 	"golang.org/x/xerrors"
 
@@ -23,6 +22,7 @@ import (
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/cliui"
 	"github.com/coder/coder/v2/cli/cliutil"
+	coderdmcp "github.com/coder/coder/v2/coderd/mcp"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/codersdk/agentsdk"
 	"github.com/coder/coder/v2/codersdk/toolsdk"
@@ -692,11 +692,12 @@ func (s *mcpServer) startServer(ctx context.Context, inv *serpent.Invocation, in
 		cliui.Infof(inv.Stderr, "Allowed Tools  : %v", allowedTools)
 	}
 
-	mcpSrv := server.NewMCPServer(
-		"Coder Agent",
-		buildinfo.Version(),
-		server.WithInstructions(instructions),
-	)
+	mcpSrv := mcp.NewServer(&mcp.Implementation{
+		Name:    "Coder Agent",
+		Version: buildinfo.Version(),
+	}, &mcp.ServerOptions{
+		Instructions: instructions,
+	})
 
 	// If neither the user client nor the agent socket is available, there
 	// are no tools we can enable.
@@ -751,14 +752,16 @@ func (s *mcpServer) startServer(ctx context.Context, inv *serpent.Invocation, in
 			continue
 		}
 
-		mcpSrv.AddTools(mcpFromSDK(tool, toolDeps))
+		coderdmcp.RegisterSDKTool(mcpSrv, tool, toolDeps)
 	}
 
-	srv := server.NewStdioServer(mcpSrv)
 	done := make(chan error)
 	go func() {
 		defer close(done)
-		srvErr := srv.Listen(ctx, inv.Stdin, inv.Stdout)
+		srvErr := mcpSrv.Run(ctx, &mcp.IOTransport{
+			Reader: io.NopCloser(inv.Stdin),
+			Writer: nopWriteCloser{inv.Stdout},
+		})
 		done <- srvErr
 	}()
 
@@ -772,6 +775,14 @@ func (s *mcpServer) startServer(ctx context.Context, inv *serpent.Invocation, in
 	return nil
 }
 
+// nopWriteCloser adapts the invocation's stdout to the WriteCloser
+// the SDK transport requires without closing the underlying stream.
+type nopWriteCloser struct {
+	io.Writer
+}
+
+func (nopWriteCloser) Close() error { return nil }
+
 type ClaudeConfig struct {
 	ConfigPath       string
 	ProjectDirectory string
@@ -984,45 +995,3 @@ func indexOf(s, substr string) int {
 	}
 	return -1
 }
-
-// mcpFromSDK adapts a toolsdk.Tool to go-mcp's server.ServerTool.
-// It assumes that the tool responds with a valid JSON object.
-func mcpFromSDK(sdkTool toolsdk.GenericTool, tb toolsdk.Deps) server.ServerTool {
-	// NOTE: some clients will silently refuse to use tools if there is an issue
-	// with the tool's schema or configuration.
-	if sdkTool.Schema.Properties == nil {
-		panic("developer error: schema properties cannot be nil")
-	}
-	return server.ServerTool{
-		Tool: mcp.Tool{
-			Name:        sdkTool.Tool.Name,
-			Description: sdkTool.Description,
-			InputSchema: mcp.ToolInputSchema{
-				Type:       "object", // Default of mcp.NewTool()
-				Properties: sdkTool.Schema.Properties,
-				Required:   sdkTool.Schema.Required,
-			},
-			Annotations: mcp.ToolAnnotation{
-				ReadOnlyHint:    mcp.ToBoolPtr(sdkTool.MCPAnnotations.ReadOnlyHint),
-				DestructiveHint: mcp.ToBoolPtr(sdkTool.MCPAnnotations.DestructiveHint),
-				IdempotentHint:  mcp.ToBoolPtr(sdkTool.MCPAnnotations.IdempotentHint),
-				OpenWorldHint:   mcp.ToBoolPtr(sdkTool.MCPAnnotations.OpenWorldHint),
-			},
-		},
-		Handler: func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-			var buf bytes.Buffer
-			if err := json.NewEncoder(&buf).Encode(request.Params.Arguments); err != nil {
-				return nil, xerrors.Errorf("failed to encode request arguments: %w", err)
-			}
-			result, err := sdkTool.Handler(ctx, tb, buf.Bytes())
-			if err != nil {
-				return nil, err
-			}
-			return &mcp.CallToolResult{
-				Content: []mcp.Content{
-					mcp.NewTextContent(string(result)),
-				},
-			}, nil
-		},
-	}
-}

cli/exp_mcp_test.go

@@ -63,6 +63,12 @@ func TestExpMcpServer(t *testing.T) {
 			assert.NoError(t, err)
 		}()
 
+		// The SDK server enforces the MCP lifecycle, so complete the
+		// initialize handshake before listing tools.
+		stdin.WriteLine(`{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`)
+		_ = stdout.ReadLine(ctx)
+		stdin.WriteLine(`{"jsonrpc":"2.0","method":"notifications/initialized"}`)
+
 		// When: we send a tools/list request
 		toolsPayload := `{"jsonrpc":"2.0","id":2,"method":"tools/list"}`
 		stdin.WriteLine(toolsPayload)
@@ -140,7 +146,7 @@ func TestExpMcpServer(t *testing.T) {
 			assert.NoError(t, err)
 		}()
 
-		payload := `{"jsonrpc":"2.0","id":1,"method":"initialize"}`
+		payload := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`
 		stdin.WriteLine(payload)
 		output := stdout.ReadLine(ctx)
 		cancel()
@@ -588,7 +594,7 @@ func TestExpMcpServerOptionalUserToken(t *testing.T) {
 	}()
 
 	// Verify server starts by checking for a successful initialization
-	payload := `{"jsonrpc":"2.0","id":1,"method":"initialize"}`
+	payload := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`
 	stdin.WriteLine(payload)
 	output := stdout.ReadLine(ctx)
 
@@ -1005,7 +1011,7 @@ func TestExpMcpReporter(t *testing.T) {
 			}()
 
 			// Initialize.
-			payload := `{"jsonrpc":"2.0","id":1,"method":"initialize"}`
+			payload := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`
 			stdin.WriteLine(payload)
 			_ = stdout.ReadLine(ctx) // ignore init response
 
@@ -1112,7 +1118,7 @@ func TestExpMcpReporter(t *testing.T) {
 		clitest.Start(t, inv)
 
 		// Initialize.
-		payload := `{"jsonrpc":"2.0","id":1,"method":"initialize"}`
+		payload := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}`
 		stdin.WriteLine(payload)
 		_ = stdout.ReadLine(ctx) // ignore init response