From 9a056127051282cd30fc5e73f4e24c6427927bdd Mon Sep 17 00:00:00 2001
From: Leonardo Araujo <leonardo.aa88@gmail.com>
Date: Sat, 11 Apr 2026 18:48:59 -0300
Subject: [PATCH 1/2] feat(tools/mcp): stdio MCP client with retries and
 registry wiring
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Add JSON-RPC newline transport, initialize handshake, tools/call,
result parsing to §13.2-style maps, and CallStdio with retry/backoff
from ToolSpec.retry.

Wire Registry type mcp + transport stdio to mcp.CallStdio. Include
testdata/mockmcp subprocess and integration tests (issue #19).

Made-with: Cursor
---
 internal/tools/doc.go                       |   2 +-
 internal/tools/mcp/call.go                  | 106 +++++++++++++
 internal/tools/mcp/client.go                |  73 +++++++++
 internal/tools/mcp/doc.go                   |   4 +
 internal/tools/mcp/errors.go                |  16 ++
 internal/tools/mcp/mcp_test.go              |  50 ++++++
 internal/tools/mcp/testdata/mockmcp/main.go |  62 ++++++++
 internal/tools/mcp/transport_stdio.go       | 160 ++++++++++++++++++++
 internal/tools/registry.go                  |  14 +-
 internal/tools/tools_test.go                |  44 ++++++
 10 files changed, 528 insertions(+), 3 deletions(-)
 create mode 100644 internal/tools/mcp/call.go
 create mode 100644 internal/tools/mcp/client.go
 create mode 100644 internal/tools/mcp/doc.go
 create mode 100644 internal/tools/mcp/errors.go
 create mode 100644 internal/tools/mcp/mcp_test.go
 create mode 100644 internal/tools/mcp/testdata/mockmcp/main.go
 create mode 100644 internal/tools/mcp/transport_stdio.go

diff --git a/internal/tools/doc.go b/internal/tools/doc.go
index d01fe47..b8343b6 100644
--- a/internal/tools/doc.go
+++ b/internal/tools/doc.go
@@ -1,5 +1,5 @@
 // Package tools defines tool registries and integrations (MCP, HTTP, native).
 //
-// [Registry] resolves tool.<name>.<operation> uses strings and dispatches MVP native and mock tools.
+// [Registry] resolves tool.<name>.<operation> uses strings and dispatches MVP native, mock, and MCP stdio tools.
 // Responses use [ToolCallResponse] with output + meta per §13.2.
 package tools
diff --git a/internal/tools/mcp/call.go b/internal/tools/mcp/call.go
new file mode 100644
index 0000000..fc5a877
--- /dev/null
+++ b/internal/tools/mcp/call.go
@@ -0,0 +1,106 @@
+package mcp
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"time"
+
+	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
+)
+
+// ExecMeta is timing/cost metadata for an MCP call (§13.2 placeholders).
+type ExecMeta struct {
+	DurationMs int64
+	CostUSD    float64
+}
+
+// CallStdio runs one MCP tools/call over a fresh stdio subprocess, with optional retries on transport errors (§13.4).
+func CallStdio(ctx context.Context, cfg *spec.ToolMCP, retry *spec.ToolRetry, toolName string, arguments map[string]any) (map[string]any, ExecMeta, error) {
+	if cfg == nil {
+		return nil, ExecMeta{}, errors.New("mcp: nil mcp config")
+	}
+	if strings.ToLower(strings.TrimSpace(cfg.Transport)) != "stdio" {
+		return nil, ExecMeta{}, errors.New("mcp: only transport stdio is supported in MVP")
+	}
+	cmd := strings.TrimSpace(cfg.Command)
+	if cmd == "" {
+		return nil, ExecMeta{}, errors.New("mcp: empty command")
+	}
+
+	attempts := 1
+	if retry != nil && retry.MaxAttempts > 0 {
+		attempts = retry.MaxAttempts
+	}
+	backoff := ""
+	if retry != nil {
+		backoff = retry.Backoff
+	}
+
+	startAll := time.Now()
+	var lastErr error
+	for attempt := 0; attempt < attempts; attempt++ {
+		if attempt > 0 {
+			sleepBackoff(ctx, attempt, backoff)
+		}
+		out, err := oneStdioAttempt(ctx, cmd, cfg.Args, toolName, arguments)
+		if err == nil {
+			return out, ExecMeta{DurationMs: time.Since(startAll).Milliseconds(), CostUSD: 0}, nil
+		}
+		lastErr = err
+		if !retryableTransportErr(err) {
+			break
+		}
+	}
+	return nil, ExecMeta{DurationMs: time.Since(startAll).Milliseconds(), CostUSD: 0}, lastErr
+}
+
+func oneStdioAttempt(ctx context.Context, command string, args []string, toolName string, arguments map[string]any) (map[string]any, error) {
+	tr := NewStdioTransport(command, args)
+	if err := tr.Start(ctx); err != nil {
+		return nil, err
+	}
+	defer tr.Close()
+
+	if err := tr.Initialize(ctx); err != nil {
+		return nil, err
+	}
+	return tr.CallTool(ctx, toolName, arguments)
+}
+
+func retryableTransportErr(err error) bool {
+	if err == nil {
+		return false
+	}
+	if errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
+		return false
+	}
+	var re *rpcError
+	if errors.As(err, &re) {
+		return false
+	}
+	return true
+}
+
+func sleepBackoff(ctx context.Context, attempt int, kind string) {
+	if attempt <= 0 {
+		return
+	}
+	var d time.Duration
+	switch strings.ToLower(strings.TrimSpace(kind)) {
+	case "exponential":
+		shift := attempt
+		if shift > 8 {
+			shift = 8
+		}
+		d = time.Millisecond * time.Duration(50*(1<<shift))
+	case "fixed":
+		d = 100 * time.Millisecond
+	default:
+		d = 50 * time.Millisecond
+	}
+	select {
+	case <-ctx.Done():
+	case <-time.After(d):
+	}
+}
diff --git a/internal/tools/mcp/client.go b/internal/tools/mcp/client.go
new file mode 100644
index 0000000..07494f2
--- /dev/null
+++ b/internal/tools/mcp/client.go
@@ -0,0 +1,73 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+// Initialize performs the MCP initialize + notifications/initialized handshake.
+func (t *StdioTransport) Initialize(ctx context.Context) error {
+	params := map[string]any{
+		"protocolVersion": "2024-11-05",
+		"capabilities":    map[string]any{},
+		"clientInfo": map[string]any{
+			"name":    "agentic-control-plane",
+			"version": "0",
+		},
+	}
+	if _, err := t.RoundTrip(ctx, "initialize", params); err != nil {
+		return err
+	}
+	// Notification (no response).
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.writeMessage(map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "notifications/initialized",
+		"params":  map[string]any{},
+	})
+}
+
+// CallTool invokes tools/call and maps the MCP result into a plain map for §13.2 output.
+func (t *StdioTransport) CallTool(ctx context.Context, name string, arguments map[string]any) (map[string]any, error) {
+	if arguments == nil {
+		arguments = map[string]any{}
+	}
+	raw, err := t.RoundTrip(ctx, "tools/call", map[string]any{
+		"name":      name,
+		"arguments": arguments,
+	})
+	if err != nil {
+		return nil, err
+	}
+	return parseCallToolResult(raw)
+}
+
+func parseCallToolResult(raw json.RawMessage) (map[string]any, error) {
+	var envelope struct {
+		Content []struct {
+			Type string `json:"type"`
+			Text string `json:"text"`
+		} `json:"content"`
+		IsError bool `json:"isError"`
+	}
+	if err := json.Unmarshal(raw, &envelope); err != nil {
+		return nil, fmt.Errorf("mcp: decode tools/call result: %w", err)
+	}
+	if envelope.IsError {
+		return nil, rpcErrorf("tools/call isError=true: %s", string(raw))
+	}
+	if len(envelope.Content) == 0 {
+		return map[string]any{}, nil
+	}
+	first := envelope.Content[0]
+	if first.Type == "text" && first.Text != "" {
+		var obj map[string]any
+		if json.Unmarshal([]byte(first.Text), &obj) == nil {
+			return obj, nil
+		}
+		return map[string]any{"text": first.Text}, nil
+	}
+	return map[string]any{"content": envelope.Content}, nil
+}
diff --git a/internal/tools/mcp/doc.go b/internal/tools/mcp/doc.go
new file mode 100644
index 0000000..08bd2a3
--- /dev/null
+++ b/internal/tools/mcp/doc.go
@@ -0,0 +1,4 @@
+// Package mcp implements MCP tool transport (MVP: stdio JSON-RPC) per design doc §7.3.
+//
+// [CallStdio] runs a subprocess, performs initialize / notifications/initialized, then tools/call.
+package mcp
diff --git a/internal/tools/mcp/errors.go b/internal/tools/mcp/errors.go
new file mode 100644
index 0000000..d242a75
--- /dev/null
+++ b/internal/tools/mcp/errors.go
@@ -0,0 +1,16 @@
+package mcp
+
+import "fmt"
+
+// rpcError is a JSON-RPC error returned by the MCP server (do not retry as transport).
+type rpcError struct {
+	detail string
+}
+
+func (e *rpcError) Error() string {
+	return "mcp: " + e.detail
+}
+
+func rpcErrorf(format string, args ...any) error {
+	return &rpcError{detail: fmt.Sprintf(format, args...)}
+}
diff --git a/internal/tools/mcp/mcp_test.go b/internal/tools/mcp/mcp_test.go
new file mode 100644
index 0000000..7e213af
--- /dev/null
+++ b/internal/tools/mcp/mcp_test.go
@@ -0,0 +1,50 @@
+package mcp
+
+import (
+	"context"
+	"os/exec"
+	"path/filepath"
+	"testing"
+
+	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
+)
+
+func mockMCPExecutable(t *testing.T) string {
+	t.Helper()
+	out := filepath.Join(t.TempDir(), "mockmcp")
+	cmd := exec.Command("go", "build", "-o", out, "./testdata/mockmcp")
+	cmd.Dir = "."
+	if b, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("build mockmcp: %v\n%s", err, b)
+	}
+	return out
+}
+
+func TestCallStdio_mockSubprocess(t *testing.T) {
+	bin := mockMCPExecutable(t)
+	ctx := context.Background()
+	out, meta, err := CallStdio(ctx, &spec.ToolMCP{
+		Transport: "stdio",
+		Command:   bin,
+	}, nil, "any", map[string]any{"repo": "acme/api", "n": float64(3)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if meta.DurationMs < 0 {
+		t.Fatalf("meta %+v", meta)
+	}
+	if out["repo"] != "acme/api" || out["n"] != float64(3) {
+		t.Fatalf("output %+v", out)
+	}
+}
+
+func TestCallStdio_retryOnTransportFailure(t *testing.T) {
+	ctx := context.Background()
+	_, _, err := CallStdio(ctx, &spec.ToolMCP{
+		Transport: "stdio",
+		Command:   "/nonexistent/binary/mcp-missing-xyz",
+	}, &spec.ToolRetry{MaxAttempts: 2, Backoff: "fixed"}, "x", nil)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}
diff --git a/internal/tools/mcp/testdata/mockmcp/main.go b/internal/tools/mcp/testdata/mockmcp/main.go
new file mode 100644
index 0000000..f07b5f6
--- /dev/null
+++ b/internal/tools/mcp/testdata/mockmcp/main.go
@@ -0,0 +1,62 @@
+// Mock MCP stdio server for tests: handles initialize and tools/call by echoing arguments as JSON text.
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"os"
+)
+
+func main() {
+	sc := bufio.NewScanner(os.Stdin)
+	enc := json.NewEncoder(os.Stdout)
+	for sc.Scan() {
+		line := sc.Bytes()
+		var msg map[string]any
+		if err := json.Unmarshal(line, &msg); err != nil {
+			continue
+		}
+		method, _ := msg["method"].(string)
+		if msg["id"] == nil {
+			continue
+		}
+		id := msg["id"]
+		switch method {
+		case "initialize":
+			_ = enc.Encode(map[string]any{
+				"jsonrpc": "2.0",
+				"id":      id,
+				"result": map[string]any{
+					"protocolVersion": "2024-11-05",
+					"capabilities":    map[string]any{"tools": map[string]any{}},
+					"serverInfo":      map[string]any{"name": "mockmcp", "version": "1"},
+				},
+			})
+		case "tools/call":
+			params, _ := msg["params"].(map[string]any)
+			args := map[string]any{}
+			if params != nil {
+				if a, ok := params["arguments"].(map[string]any); ok {
+					args = a
+				}
+			}
+			b, _ := json.Marshal(args)
+			_ = enc.Encode(map[string]any{
+				"jsonrpc": "2.0",
+				"id":      id,
+				"result": map[string]any{
+					"content": []any{map[string]any{"type": "text", "text": string(b)}},
+				},
+			})
+		default:
+			_ = enc.Encode(map[string]any{
+				"jsonrpc": "2.0",
+				"id":      id,
+				"error": map[string]any{
+					"code":    -32601,
+					"message": "method not found",
+				},
+			})
+		}
+	}
+}
diff --git a/internal/tools/mcp/transport_stdio.go b/internal/tools/mcp/transport_stdio.go
new file mode 100644
index 0000000..14119ca
--- /dev/null
+++ b/internal/tools/mcp/transport_stdio.go
@@ -0,0 +1,160 @@
+package mcp
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os/exec"
+	"sync"
+	"sync/atomic"
+)
+
+// StdioTransport runs an MCP server over newline-delimited JSON-RPC on stdin/stdout.
+type StdioTransport struct {
+	command string
+	args    []string
+
+	cmd   *exec.Cmd
+	stdin io.WriteCloser
+	br    *bufio.Reader
+
+	nextID int64
+	mu     sync.Mutex
+}
+
+// NewStdioTransport configures a stdio MCP session (Start not yet called).
+func NewStdioTransport(command string, args []string) *StdioTransport {
+	cp := append([]string(nil), args...)
+	return &StdioTransport{command: command, args: cp}
+}
+
+// Start launches the subprocess and wires stdio pipes.
+func (t *StdioTransport) Start(ctx context.Context) error {
+	if t == nil {
+		return fmt.Errorf("mcp: nil transport")
+	}
+	t.cmd = exec.CommandContext(ctx, t.command, t.args...)
+	in, err := t.cmd.StdinPipe()
+	if err != nil {
+		return err
+	}
+	out, err := t.cmd.StdoutPipe()
+	if err != nil {
+		return err
+	}
+	t.stdin = in
+	t.br = bufio.NewReader(out)
+	t.cmd.Stderr = io.Discard
+	return t.cmd.Start()
+}
+
+// Close ends the session (best-effort kill).
+func (t *StdioTransport) Close() error {
+	if t == nil {
+		return nil
+	}
+	if t.stdin != nil {
+		_ = t.stdin.Close()
+		t.stdin = nil
+	}
+	if t.cmd != nil && t.cmd.Process != nil {
+		_ = t.cmd.Process.Kill()
+		_ = t.cmd.Wait()
+		t.cmd = nil
+	}
+	return nil
+}
+
+func (t *StdioTransport) writeMessage(v any) error {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return err
+	}
+	if _, err := t.stdin.Write(append(b, '\n')); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (t *StdioTransport) readLineCtx(ctx context.Context) ([]byte, error) {
+	type res struct {
+		line []byte
+		err  error
+	}
+	ch := make(chan res, 1)
+	go func() {
+		line, err := t.br.ReadBytes('\n')
+		ch <- res{line, err}
+	}()
+	select {
+	case <-ctx.Done():
+		if t.cmd != nil && t.cmd.Process != nil {
+			_ = t.cmd.Process.Kill()
+		}
+		return nil, ctx.Err()
+	case r := <-ch:
+		return r.line, r.err
+	}
+}
+
+// RoundTrip sends a JSON-RPC request and returns the result field for the matching id.
+func (t *StdioTransport) RoundTrip(ctx context.Context, method string, params any) (json.RawMessage, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	id := atomic.AddInt64(&t.nextID, 1)
+	req := map[string]any{
+		"jsonrpc": "2.0",
+		"id":      id,
+		"method":  method,
+		"params":  params,
+	}
+	if err := t.writeMessage(req); err != nil {
+		return nil, err
+	}
+
+	for {
+		line, err := t.readLineCtx(ctx)
+		if err != nil {
+			return nil, err
+		}
+		var msg map[string]any
+		if err := json.Unmarshal(line, &msg); err != nil {
+			continue
+		}
+		if _, hasMethod := msg["method"].(string); hasMethod && msg["id"] == nil {
+			continue
+		}
+		rid, ok := msg["id"]
+		if !ok {
+			continue
+		}
+		if !jsonRPCIDMatches(rid, id) {
+			continue
+		}
+		if errObj, ok := msg["error"]; ok && errObj != nil {
+			return nil, rpcErrorf("rpc error: %v", errObj)
+		}
+		raw, err := json.Marshal(msg["result"])
+		if err != nil {
+			return nil, err
+		}
+		return json.RawMessage(raw), nil
+	}
+}
+
+func jsonRPCIDMatches(rid any, want int64) bool {
+	switch x := rid.(type) {
+	case float64:
+		return int64(x) == want
+	case json.Number:
+		n, err := x.Int64()
+		return err == nil && n == want
+	case int64:
+		return x == want
+	default:
+		return false
+	}
+}
diff --git a/internal/tools/registry.go b/internal/tools/registry.go
index cfbc027..062c719 100644
--- a/internal/tools/registry.go
+++ b/internal/tools/registry.go
@@ -8,10 +8,11 @@ import (
 	"time"
 
 	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
+	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/tools/mcp"
 	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/tools/native"
 )
 
-// Registry resolves workflow uses strings against declared tools and dispatches by transport (MVP: native, mock).
+// Registry resolves workflow uses strings against declared tools and dispatches by transport (MVP: native, mock, mcp stdio).
 type Registry struct {
 	graph  *spec.ProjectGraph
 	native *native.Registry
@@ -87,7 +88,16 @@ func (r *Registry) Call(ctx context.Context, req ToolCallRequest) (ToolCallRespo
 			ToolCallMeta{DurationMs: 1, CostUSD: 0},
 			start,
 		), nil
+	case "mcp":
+		if tr.Spec.MCP == nil {
+			return ToolCallResponse{}, fmt.Errorf("tools: mcp tool %q missing mcp configuration", toolName)
+		}
+		out, meta, err := mcp.CallStdio(ctx, tr.Spec.MCP, tr.Spec.Retry, operation, req.With)
+		if err != nil {
+			return ToolCallResponse{}, err
+		}
+		return normalizeResponse(out, ToolCallMeta{DurationMs: meta.DurationMs, CostUSD: meta.CostUSD}, start), nil
 	default:
-		return ToolCallResponse{}, fmt.Errorf("tools: tool %q type %q not supported by MVP registry (native|mock only)", toolName, tr.Spec.Type)
+		return ToolCallResponse{}, fmt.Errorf("tools: tool %q type %q not supported by MVP registry (native|mock|mcp)", toolName, tr.Spec.Type)
 	}
 }
diff --git a/internal/tools/tools_test.go b/internal/tools/tools_test.go
index 064434d..d44324e 100644
--- a/internal/tools/tools_test.go
+++ b/internal/tools/tools_test.go
@@ -3,6 +3,8 @@ package tools
 import (
 	"context"
 	"errors"
+	"os/exec"
+	"path/filepath"
 	"testing"
 
 	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
@@ -75,6 +77,48 @@ func TestParseUses_githubExample(t *testing.T) {
 	}
 }
 
+func mockMCPBinaryFromTools(t *testing.T) string {
+	t.Helper()
+	out := filepath.Join(t.TempDir(), "mockmcp")
+	cmd := exec.Command("go", "build", "-o", out, "./mcp/testdata/mockmcp")
+	cmd.Dir = "."
+	if b, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("build mockmcp: %v\n%s", err, b)
+	}
+	return out
+}
+
+func testGraphMCP(bin string) *spec.ProjectGraph {
+	return &spec.ProjectGraph{
+		Tools: map[string]*spec.ToolResource{
+			"mc": {
+				APIVersion: spec.APIVersionV0,
+				Kind:       spec.KindTool,
+				Metadata:   spec.Metadata{Name: "mc"},
+				Spec: spec.ToolSpec{
+					Type: "mcp",
+					MCP:  &spec.ToolMCP{Transport: "stdio", Command: bin},
+				},
+			},
+		},
+	}
+}
+
+func TestRegistry_MCP_stdio_mockServer(t *testing.T) {
+	bin := mockMCPBinaryFromTools(t)
+	reg := NewRegistry(testGraphMCP(bin))
+	resp, err := reg.Call(context.Background(), ToolCallRequest{
+		Uses: "tool.mc.echo",
+		With: map[string]any{"hello": "world"},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.Output["hello"] != "world" {
+		t.Fatalf("output %+v", resp.Output)
+	}
+}
+
 func TestMockExecutor_isolated(t *testing.T) {
 	ctx := context.Background()
 	m := &MockExecutor{

From 1fa00b40e9db7954b3c15e7cdd4a61ab36302731 Mon Sep 17 00:00:00 2001
From: Leonardo Araujo <leonardo.aa88@gmail.com>
Date: Sat, 11 Apr 2026 18:55:25 -0300
Subject: [PATCH 2/2] fix(test): use mockmcp.exe on Windows for go build -o
 output

exec.Command requires a .exe path on windows-latest CI; without it the
built binary path is not recognized as executable.

Made-with: Cursor
---
 internal/tools/mcp/mcp_test.go | 7 ++++++-
 internal/tools/tools_test.go   | 7 ++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/internal/tools/mcp/mcp_test.go b/internal/tools/mcp/mcp_test.go
index 7e213af..94fdbe0 100644
--- a/internal/tools/mcp/mcp_test.go
+++ b/internal/tools/mcp/mcp_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
@@ -11,7 +12,11 @@ import (
 
 func mockMCPExecutable(t *testing.T) string {
 	t.Helper()
-	out := filepath.Join(t.TempDir(), "mockmcp")
+	name := "mockmcp"
+	if runtime.GOOS == "windows" {
+		name += ".exe"
+	}
+	out := filepath.Join(t.TempDir(), name)
 	cmd := exec.Command("go", "build", "-o", out, "./testdata/mockmcp")
 	cmd.Dir = "."
 	if b, err := cmd.CombinedOutput(); err != nil {
diff --git a/internal/tools/tools_test.go b/internal/tools/tools_test.go
index d44324e..56a1d50 100644
--- a/internal/tools/tools_test.go
+++ b/internal/tools/tools_test.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"github.com/LAA-Software-Engineering/agentic-control-plane/internal/spec"
@@ -79,7 +80,11 @@ func TestParseUses_githubExample(t *testing.T) {
 
 func mockMCPBinaryFromTools(t *testing.T) string {
 	t.Helper()
-	out := filepath.Join(t.TempDir(), "mockmcp")
+	name := "mockmcp"
+	if runtime.GOOS == "windows" {
+		name += ".exe"
+	}
+	out := filepath.Join(t.TempDir(), name)
 	cmd := exec.Command("go", "build", "-o", out, "./mcp/testdata/mockmcp")
 	cmd.Dir = "."
 	if b, err := cmd.CombinedOutput(); err != nil {