From c6c328e5174d7d91ac05b62126b5408f82a93a0f Mon Sep 17 00:00:00 2001
From: Mathias Bergqvist <mthbqv@gmail.com>
Date: Mon, 4 May 2026 20:59:15 +0200
Subject: [PATCH] fix(mcp): map tool-not-found to CodeNotFound via registry
 sentinel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 internal/mcp/server.go        |  4 +---
 internal/mcp/server_test.go   | 28 ++++++++++++++++++++++++++++
 internal/registry/registry.go |  5 ++++-
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/internal/mcp/server.go b/internal/mcp/server.go
index 929ae3d..613d7c6 100644
--- a/internal/mcp/server.go
+++ b/internal/mcp/server.go
@@ -90,7 +90,7 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 		out, err := s.opts.Registry.Dispatch(r.Context(), p.Name, p.Arguments)
 		if err != nil {
 			code := -32000
-			if errors.Is(err, ErrToolNotFound) {
+			if errors.Is(err, registry.ErrToolNotFound) {
 				code = CodeNotFound
 			}
 			writeJSON(w, http.StatusOK,
@@ -125,8 +125,6 @@ func (s *Server) handleGET(w http.ResponseWriter, r *http.Request) {
 	<-r.Context().Done()
 }
 
-var ErrToolNotFound = errors.New("tool not found")
-
 func writeJSON(w http.ResponseWriter, status int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
diff --git a/internal/mcp/server_test.go b/internal/mcp/server_test.go
index ac1c3c2..441cd06 100644
--- a/internal/mcp/server_test.go
+++ b/internal/mcp/server_test.go
@@ -117,3 +117,31 @@ func TestPostBodyTooLarge(t *testing.T) {
 	assert.NotEqual(t, http.StatusOK, rr.Code, "oversized body must not return 200")
 	assert.Equal(t, http.StatusBadRequest, rr.Code)
 }
+
+func TestToolsCallToolNotFound(t *testing.T) {
+	srv := newServer(t)
+	// Initialize to get a session ID.
+	init := postJSON(t, srv, map[string]any{
+		"jsonrpc": "2.0",
+		"id":      1,
+		"method":  "initialize",
+		"params":  map[string]any{"protocolVersion": "2025-06-18"},
+	}, "")
+	sid := init.Header().Get("Mcp-Session-Id")
+
+	rr := postJSON(t, srv, map[string]any{
+		"jsonrpc": "2.0",
+		"id":      2,
+		"method":  "tools/call",
+		"params":  map[string]any{"name": "nonexistent", "arguments": map[string]any{}},
+	}, sid)
+	require.Equal(t, http.StatusOK, rr.Code)
+
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+	rpcErr, ok := resp["error"].(map[string]any)
+	require.True(t, ok, "expected error field in response")
+	code := int(rpcErr["code"].(float64))
+	assert.Equal(t, -32002, code, "expected CodeNotFound (-32002) for missing tool")
+	assert.NotEmpty(t, rpcErr["message"])
+}
diff --git a/internal/registry/registry.go b/internal/registry/registry.go
index 01c870e..949ca64 100644
--- a/internal/registry/registry.go
+++ b/internal/registry/registry.go
@@ -4,8 +4,11 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 )
 
+var ErrToolNotFound = errors.New("tool not found")
+
 type ToolDescriptor struct {
 	Name        string          `json:"name"`
 	Description string          `json:"description"`
@@ -36,7 +39,7 @@ func (r *Registry) Tools() []ToolDescriptor {
 func (r *Registry) Dispatch(ctx context.Context, name string, args json.RawMessage) (json.RawMessage, error) {
 	t, ok := r.tools[name]
 	if !ok {
-		return nil, errors.New("tool not found: " + name)
+		return nil, fmt.Errorf("tool %q: %w", name, ErrToolNotFound)
 	}
 	return t.Call(ctx, args)
 }