feat: validate typed MCP tool arguments

mcp/server20260728/server.go

@@ -8,6 +8,7 @@ package mcpserver
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -50,9 +51,9 @@ type ResourceHandlerFunc func(context.Context, ReadResourceRequest) (mcptypes.Re
 // RegisterTypedTool decodes a tool's arguments into T before calling handler.
 // T is the application Go type for the tool input; no JSON Schema validator is
 // involved.
-func RegisterTypedTool[T any](s *MCPServer, definition mcptypes.Tool, handler func(context.Context, Headers, T) (mcptypes.CallToolResult, error)) {
+func RegisterTypedTool[T mcptypes.Validatable](s *MCPServer, definition mcptypes.Tool, handler func(context.Context, Headers, T) (mcptypes.CallToolResult, error)) {
 	s.RegisterTool(definition, func(ctx context.Context, request CallToolRequest) (mcptypes.CallToolResult, error) {
-		args, err := mcptypes.Decode[T](request.Params.Arguments)
+		args, err := mcptypes.DecodeValidated[T](request.Params.Arguments)
 		if err != nil {
 			return mcptypes.CallToolResult{}, fmt.Errorf("decode %s arguments: %w", definition.Name, err)
 		}
@@ -69,6 +70,7 @@ type MCPServer struct {
 	AllowedOrigins []string
 	mu             sync.RWMutex
 	tools          []registeredTool
+	toolByName     map[string]registeredTool
 	resources      []registeredResource
 }
 type registeredTool struct {
@@ -88,7 +90,12 @@ func (s *MCPServer) RegisterTool(definition mcptypes.Tool, handler ToolHandlerFu
 			panic("mcp: duplicate tool: " + definition.Name)
 		}
 	}
-	s.tools = append(s.tools, registeredTool{definition, handler})
+	if s.toolByName == nil {
+		s.toolByName = make(map[string]registeredTool)
+	}
+	registered := registeredTool{definition, handler}
+	s.tools = append(s.tools, registered)
+	s.toolByName[definition.Name] = registered
 }
 func (s *MCPServer) RegisterResource(definition mcptypes.Resource, handler ResourceHandlerFunc) {
 	s.mu.Lock()
@@ -145,6 +152,9 @@ func (s *MCPServer) handle(ctx context.Context, headers Headers, body []byte) an
 	if headers.Method != envelope.Method {
 		return s.error(envelope.ID, mcptypes.ErrHeaderMismatch, "Mcp-Method does not match request method")
 	}
+	if envelope.Method == "tools/call" && headers.Name == "" {
+		return s.error(envelope.ID, mcptypes.ErrHeaderMismatch, "missing Mcp-Name header")
+	}
 	var request mcptypes.Request
 	if err := json.Unmarshal(body, &request); err != nil || request.JSONRPC != mcptypes.JSONRPCVersion || request.Method == "" || (request.IDPresent && (request.ID == nil || request.ID.IsNull())) {
 		return s.error(envelope.ID, mcptypes.ErrInvalidRequest, "Invalid JSON-RPC request")
@@ -202,24 +212,25 @@ func (s *MCPServer) toolsList(id *mcptypes.RequestID) any {
 	return s.result(id, raw)
 }
 func (s *MCPServer) toolsCall(ctx context.Context, headers Headers, id *mcptypes.RequestID, raw json.RawMessage) any {
+	// Select the handler from Mcp-Name before decoding tool parameters.
+	s.mu.RLock()
+	registered, found := s.toolByName[headers.Name]
+	s.mu.RUnlock()
+	if !found {
+		return s.error(id, mcptypes.ErrInvalidParams, "Unknown tool")
+	}
 	params, err := mcptypes.Decode[mcptypes.CallToolParams](raw)
 	if err != nil || params.Name == "" {
 		return s.error(id, mcptypes.ErrInvalidParams, "Invalid params for tools/call")
 	}
-	s.mu.RLock()
+	if params.Name != headers.Name {
-	var handler ToolHandlerFunc
+		return s.error(id, mcptypes.ErrHeaderMismatch, "Mcp-Name does not match params.name")
-	for _, tool := range s.tools {
-		if tool.definition.Name == params.Name {
-			handler = tool.handler
-			break
-		}
 	}
-	s.mu.RUnlock()
+	value, err := registered.handler(ctx, CallToolRequest{headers, params})
-	if handler == nil {
-		return s.error(id, mcptypes.ErrInvalidParams, "Unknown tool")
-	}
-	value, err := handler(ctx, CallToolRequest{headers, params})
 	if err != nil {
+		if errors.Is(err, mcptypes.ErrInvalidArguments) {
+			return s.error(id, mcptypes.ErrInvalidParams, err.Error())
+		}
 		return s.error(id, mcptypes.ErrInternal, "Tool execution failed")
 	}
 	encoded, encodeErr := json.Marshal(value)

mcp/server20260728/server_test.go

@@ -3,6 +3,7 @@ package mcpserver
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -15,6 +16,13 @@ type echoArgs struct {
 	Message string `json:"message"`
 }
 
+func (a echoArgs) Validate() error {
+	if a.Message == "" {
+		return errors.New("message is required")
+	}
+	return nil
+}
+
 func TestServeHTTPUsesHeadersBeforeTypedDecode(t *testing.T) {
 	var got Headers
 	var gotArgs echoArgs
@@ -41,6 +49,44 @@ func TestServeHTTPUsesHeadersBeforeTypedDecode(t *testing.T) {
 	}
 }
 
+func TestServeHTTPRejectsInvalidTypedArguments(t *testing.T) {
+	srv := &MCPServer{}
+	RegisterTypedTool(srv, mcptypes.Tool{Name: "echo"}, func(context.Context, Headers, echoArgs) (mcptypes.CallToolResult, error) {
+		return mcptypes.NewToolResultText("ok"), nil
+	})
+	req := httptest.NewRequest(http.MethodPost, "/mcp", strings.NewReader(`{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"echo","arguments":{}}}`))
+	req.Header.Set("Mcp-Method", "tools/call")
+	req.Header.Set("Mcp-Name", "echo")
+	res := httptest.NewRecorder()
+	srv.ServeHTTP(res, req)
+	var response mcptypes.ErrorResponse
+	if err := json.NewDecoder(res.Body).Decode(&response); err != nil {
+		t.Fatal(err)
+	}
+	if response.Error.Code != mcptypes.ErrInvalidParams {
+		t.Fatalf("code = %d, want %d", response.Error.Code, mcptypes.ErrInvalidParams)
+	}
+}
+
+func TestServeHTTPRejectsHeaderNameMismatch(t *testing.T) {
+	srv := &MCPServer{}
+	srv.RegisterTool(mcptypes.Tool{Name: "echo"}, func(context.Context, CallToolRequest) (mcptypes.CallToolResult, error) {
+		return mcptypes.NewToolResultText("ok"), nil
+	})
+	req := httptest.NewRequest(http.MethodPost, "/mcp", strings.NewReader(`{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"other"}}`))
+	req.Header.Set("Mcp-Method", "tools/call")
+	req.Header.Set("Mcp-Name", "echo")
+	res := httptest.NewRecorder()
+	srv.ServeHTTP(res, req)
+	var response mcptypes.ErrorResponse
+	if err := json.NewDecoder(res.Body).Decode(&response); err != nil {
+		t.Fatal(err)
+	}
+	if response.Error.Code != mcptypes.ErrHeaderMismatch {
+		t.Fatalf("code = %d, want %d", response.Error.Code, mcptypes.ErrHeaderMismatch)
+	}
+}
+
 func TestServeHTTPRejectsHeaderMethodMismatch(t *testing.T) {
 	srv := &MCPServer{}
 	req := httptest.NewRequest(http.MethodPost, "/mcp", strings.NewReader(`{"jsonrpc":"2.0","id":1,"method":"tools/list"}`))

mcp/types20260728/types.go

@@ -8,6 +8,7 @@ package mcptypes
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 )
 
@@ -108,6 +109,8 @@ type ErrorObject struct {
 	Data    json.RawMessage `json:"data,omitempty"`
 }
 
+var ErrInvalidArguments = errors.New("invalid tool arguments")
+
 const (
 	ErrParse                           = -32700
 	ErrInvalidRequest                  = -32600
@@ -252,3 +255,52 @@ func Decode[T any](data json.RawMessage) (T, error) {
 	err := json.Unmarshal(data, &value)
 	return value, err
 }
+
+// Validatable is implemented by typed tool argument structs that enforce
+// required fields and application-level constraints.
+type Validatable interface {
+	Validate() error
+}
+
+// DecodeValidated decodes typed tool arguments and runs their validation.
+func DecodeValidated[T Validatable](data json.RawMessage) (T, error) {
+	value, err := Decode[T](data)
+	if err != nil {
+		return value, fmt.Errorf("%w: %v", ErrInvalidArguments, err)
+	}
+	if err := value.Validate(); err != nil {
+		return value, fmt.Errorf("%w: %v", ErrInvalidArguments, err)
+	}
+	return value, nil
+}
+
+// NewToolResultText returns a successful text-only tool result.
+func NewToolResultText(text string) CallToolResult {
+	return CallToolResult{ResultType: "complete", Content: []Content{{Type: "text", Text: text}}}
+}
+
+// NewToolResultError returns an in-band tool execution error.
+func NewToolResultError(message string) CallToolResult {
+	result := NewToolResultText(message)
+	result.IsError = true
+	return result
+}
+
+// NewToolResultStructured encodes a typed Go value as structuredContent.
+func NewToolResultStructured[T any](value T, text string) (CallToolResult, error) {
+	raw, err := json.Marshal(value)
+	if err != nil {
+		return CallToolResult{}, fmt.Errorf("encode structured tool result: %w", err)
+	}
+	return CallToolResult{ResultType: "complete", Content: []Content{{Type: "text", Text: text}}, StructuredContent: raw}, nil
+}
+
+// NewToolResultStructuredOnly encodes a typed Go value and uses its JSON form
+// as the text fallback for clients that do not support structuredContent.
+func NewToolResultStructuredOnly[T any](value T) (CallToolResult, error) {
+	raw, err := json.Marshal(value)
+	if err != nil {
+		return CallToolResult{}, fmt.Errorf("encode structured tool result: %w", err)
+	}
+	return CallToolResult{ResultType: "complete", Content: []Content{{Type: "text", Text: string(raw)}}, StructuredContent: raw}, nil
+}

mcp/types20260728/types_test.go

@@ -0,0 +1,41 @@
+package mcptypes
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+type typedResult struct {
+	Step  string `json:"step"`
+	Count int    `json:"count"`
+}
+
+func TestNewToolResultStructuredUsesTypedValue(t *testing.T) {
+	result, err := NewToolResultStructured(typedResult{Step: "done", Count: 2}, "finished")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var got typedResult
+	if err := json.Unmarshal(result.StructuredContent, &got); err != nil {
+		t.Fatal(err)
+	}
+	if got.Step != "done" || got.Count != 2 {
+		t.Fatalf("structured result = %#v", got)
+	}
+	if result.ResultType != "complete" || result.Content[0].Text != "finished" {
+		t.Fatalf("result = %#v", result)
+	}
+}
+
+func TestNewToolResultStructuredOnlyUsesJSONFallback(t *testing.T) {
+	result, err := NewToolResultStructuredOnly(typedResult{Step: "done", Count: 2})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Content[0].Text != `{"step":"done","count":2}` {
+		t.Fatalf("text fallback = %q", result.Content[0].Text)
+	}
+	if result.IsError {
+		t.Fatal("structured result marked as error")
+	}
+}