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

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75 changes: 22 additions & 53 deletions cli/exp_mcp.go
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Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
package cli

import (
"bytes"
"context"
"encoding/json"
"errors"
"io"
"net/url"
"os"
"path/filepath"
Expand All @@ -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"

Expand All @@ -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"
Expand Down Expand Up @@ -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.
Expand Down Expand Up @@ -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
}()

Expand All @@ -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
Expand Down Expand Up @@ -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
},
}
}
14 changes: 10 additions & 4 deletions cli/exp_mcp_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand Down Expand Up @@ -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()
Expand Down Expand Up @@ -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)

Expand Down Expand Up @@ -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

Expand Down Expand Up @@ -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

Expand Down
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