refactor: make CallToolRequest.Arguments more flexible (Breaking Change)

README.md

@@ -149,9 +149,10 @@ func main() {
 
     // Add the calculator handler
     s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-        op := request.Params.Arguments["operation"].(string)
-        x := request.Params.Arguments["x"].(float64)
-        y := request.Params.Arguments["y"].(float64)
+        args := request.GetArguments()
+        op := args["operation"].(string)
+        x := args["x"].(float64)
+        y := args["y"].(float64)
 
         var result float64
         switch op {
@@ -312,9 +313,10 @@ calculatorTool := mcp.NewTool("calculate",
 )
 
 s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-    op := request.Params.Arguments["operation"].(string)
-    x := request.Params.Arguments["x"].(float64)
-    y := request.Params.Arguments["y"].(float64)
+    args := request.GetArguments()
+    op := args["operation"].(string)
+    x := args["x"].(float64)
+    y := args["y"].(float64)
 
     var result float64
     switch op {
@@ -355,10 +357,11 @@ httpTool := mcp.NewTool("http_request",
 )
 
 s.AddTool(httpTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-    method := request.Params.Arguments["method"].(string)
-    url := request.Params.Arguments["url"].(string)
+    args := request.GetArguments()
+    method := args["method"].(string)
+    url := args["url"].(string)
     body := ""
-    if b, ok := request.Params.Arguments["body"].(string); ok {
+    if b, ok := args["body"].(string); ok {
         body = b
     }
 

client/inprocess_test.go

@@ -32,7 +32,7 @@ func TestInProcessMCPClient(t *testing.T) {
 			Content: []mcp.Content{
 				mcp.TextContent{
 					Type: "text",
-					Text: "Input parameter: " + request.Params.Arguments["parameter-1"].(string),
+					Text: "Input parameter: " + request.GetArguments()["parameter-1"].(string),
 				},
 				mcp.AudioContent{
 					Type:     "audio",

client/sse_test.go

@@ -36,7 +36,7 @@ func TestSSEMCPClient(t *testing.T) {
 			Content: []mcp.Content{
 				mcp.TextContent{
 					Type: "text",
-					Text: "Input parameter: " + request.Params.Arguments["parameter-1"].(string),
+					Text: "Input parameter: " + request.GetArguments()["parameter-1"].(string),
 				},
 			},
 		}, nil

examples/custom_context/main.go

@@ -81,7 +81,7 @@ func handleMakeAuthenticatedRequestTool(
 	ctx context.Context,
 	request mcp.CallToolRequest,
 ) (*mcp.CallToolResult, error) {
-	message, ok := request.Params.Arguments["message"].(string)
+	message, ok := request.GetArguments()["message"].(string)
 	if !ok {
 		return nil, fmt.Errorf("missing message")
 	}

examples/dynamic_path/main.go

@@ -20,7 +20,7 @@ func main() {
 
 	// Add a trivial tool for demonstration
 	mcpServer.AddTool(mcp.NewTool("echo"), func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-		return mcp.NewToolResultText(fmt.Sprintf("Echo: %v", req.Params.Arguments["message"])), nil
+		return mcp.NewToolResultText(fmt.Sprintf("Echo: %v", req.GetArguments()["message"])), nil
 	})
 
 	// Use a dynamic base path based on a path parameter (Go 1.22+)

examples/everything/main.go

@@ -312,7 +312,7 @@ func handleEchoTool(
 	ctx context.Context,
 	request mcp.CallToolRequest,
 ) (*mcp.CallToolResult, error) {
-	arguments := request.Params.Arguments
+	arguments := request.GetArguments()
 	message, ok := arguments["message"].(string)
 	if !ok {
 		return nil, fmt.Errorf("invalid message argument")
@@ -331,7 +331,7 @@ func handleAddTool(
 	ctx context.Context,
 	request mcp.CallToolRequest,
 ) (*mcp.CallToolResult, error) {
-	arguments := request.Params.Arguments
+	arguments := request.GetArguments()
 	a, ok1 := arguments["a"].(float64)
 	b, ok2 := arguments["b"].(float64)
 	if !ok1 || !ok2 {
@@ -382,7 +382,7 @@ func handleLongRunningOperationTool(
 	ctx context.Context,
 	request mcp.CallToolRequest,
 ) (*mcp.CallToolResult, error) {
-	arguments := request.Params.Arguments
+	arguments := request.GetArguments()
 	progressToken := request.Params.Meta.ProgressToken
 	duration, _ := arguments["duration"].(float64)
 	steps, _ := arguments["steps"].(float64)

mcp/tools.go

@@ -44,8 +44,8 @@ type CallToolResult struct {
 type CallToolRequest struct {
 	Request
 	Params struct {
-		Name      string         `json:"name"`
-		Arguments map[string]any `json:"arguments,omitempty"`
+		Name      string      `json:"name"`
+		Arguments any `json:"arguments,omitempty"` // Can be map[string]any or any other type
 		Meta      *struct {
 			// If specified, the caller is requesting out-of-band progress
 			// notifications for this request (as represented by
@@ -58,6 +58,21 @@ type CallToolRequest struct {
 	} `json:"params"`
 }
 
+// GetArguments returns the Arguments as map[string]any for backward compatibility
+// If Arguments is not a map, it returns an empty map
+func (r CallToolRequest) GetArguments() map[string]any {
+	if args, ok := r.Params.Arguments.(map[string]any); ok {
+		return args
+	}
+	return map[string]any{}
+}
+
+// GetRawArguments returns the Arguments as-is without type conversion
+// This allows users to access the raw arguments in any format
+func (r CallToolRequest) GetRawArguments() any {
+	return r.Params.Arguments
+}
+
 // ToolListChangedNotification is an optional notification from the server to
 // the client, informing it that the list of tools it offers has changed. This may
 // be issued by servers without any previous subscription from the client.

mcp/tools_arguments_test.go

@@ -0,0 +1,98 @@
+package mcp
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCallToolRequestWithMapArguments(t *testing.T) {
+	// Create a request with map arguments
+	req := CallToolRequest{}
+	req.Params.Name = "test-tool"
+	req.Params.Arguments = map[string]any{
+		"key1": "value1",
+		"key2": 123,
+	}
+
+	// Test GetArguments
+	args := req.GetArguments()
+	assert.Equal(t, "value1", args["key1"])
+	assert.Equal(t, 123, args["key2"])
+
+	// Test GetRawArguments
+	rawArgs := req.GetRawArguments()
+	mapArgs, ok := rawArgs.(map[string]any)
+	assert.True(t, ok)
+	assert.Equal(t, "value1", mapArgs["key1"])
+	assert.Equal(t, 123, mapArgs["key2"])
+}
+
+func TestCallToolRequestWithNonMapArguments(t *testing.T) {
+	// Create a request with non-map arguments
+	req := CallToolRequest{}
+	req.Params.Name = "test-tool"
+	req.Params.Arguments = "string-argument"
+
+	// Test GetArguments (should return empty map)
+	args := req.GetArguments()
+	assert.Empty(t, args)
+
+	// Test GetRawArguments
+	rawArgs := req.GetRawArguments()
+	strArg, ok := rawArgs.(string)
+	assert.True(t, ok)
+	assert.Equal(t, "string-argument", strArg)
+}
+
+func TestCallToolRequestWithStructArguments(t *testing.T) {
+	// Create a custom struct
+	type CustomArgs struct {
+		Field1 string `json:"field1"`
+		Field2 int    `json:"field2"`
+	}
+
+	// Create a request with struct arguments
+	req := CallToolRequest{}
+	req.Params.Name = "test-tool"
+	req.Params.Arguments = CustomArgs{
+		Field1: "test",
+		Field2: 42,
+	}
+
+	// Test GetArguments (should return empty map)
+	args := req.GetArguments()
+	assert.Empty(t, args)
+
+	// Test GetRawArguments
+	rawArgs := req.GetRawArguments()
+	structArg, ok := rawArgs.(CustomArgs)
+	assert.True(t, ok)
+	assert.Equal(t, "test", structArg.Field1)
+	assert.Equal(t, 42, structArg.Field2)
+}
+
+func TestCallToolRequestJSONMarshalUnmarshal(t *testing.T) {
+	// Create a request with map arguments
+	req := CallToolRequest{}
+	req.Params.Name = "test-tool"
+	req.Params.Arguments = map[string]any{
+		"key1": "value1",
+		"key2": 123,
+	}
+
+	// Marshal to JSON
+	data, err := json.Marshal(req)
+	assert.NoError(t, err)
+
+	// Unmarshal from JSON
+	var unmarshaledReq CallToolRequest
+	err = json.Unmarshal(data, &unmarshaledReq)
+	assert.NoError(t, err)
+
+	// Check if arguments are correctly unmarshaled
+	args := unmarshaledReq.GetArguments()
+	assert.Equal(t, "value1", args["key1"])
+	assert.Equal(t, float64(123), args["key2"]) // JSON numbers are unmarshaled as float64
+}

mcp/utils.go

@@ -675,10 +675,11 @@ func ParseReadResourceResult(rawMessage *json.RawMessage) (*ReadResourceResult,
 }
 
 func ParseArgument(request CallToolRequest, key string, defaultVal any) any {
-	if _, ok := request.Params.Arguments[key]; !ok {
+	args := request.GetArguments()
+	if _, ok := args[key]; !ok {
 		return defaultVal
 	} else {
-		return request.Params.Arguments[key]
+		return args[key]
 	}
 }
 

mcptest/mcptest_test.go

@@ -52,7 +52,7 @@ func TestServer(t *testing.T) {
 
 func helloWorldHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 	// Extract name from request arguments
-	name, ok := request.Params.Arguments["name"].(string)
+	name, ok := request.GetArguments()["name"].(string)
 	if !ok {
 		name = "World"
 	}