fix: accept omitted tool arguments for zero-param tools like get_me

Treat nil or empty Arguments JSON as {} before unmarshaling so clients
that omit arguments match tools with empty input schemas.

Fixes #2587

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: root

pkg/inventory/server_tool.go

@@ -133,7 +133,11 @@ func NewServerToolWithContextHandler[In any, Out any](tool mcp.Tool, toolset Too
 		HandlerFunc: func(_ any) mcp.ToolHandler {
 			return func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 				var arguments In
-				if err := json.Unmarshal(req.Params.Arguments, &arguments); err != nil {
+				args := req.Params.Arguments
+				if len(args) == 0 {
+					args = json.RawMessage(`{}`)
+				}
+				if err := json.Unmarshal(args, &arguments); err != nil {
 					return &mcp.CallToolResult{
 						Content: []mcp.Content{
 							&mcp.TextContent{Text: fmt.Sprintf("invalid arguments: %s", err)},

pkg/inventory/server_tool_test.go

@@ -78,3 +78,29 @@ func TestNewServerToolWithContextHandler_ValidArguments_Succeeds(t *testing.T) {
 	require.True(t, ok)
 	assert.Equal(t, "success: octocat/hello-world", textContent.Text)
 }
+
+func TestNewServerToolWithContextHandler_NilArguments_TreatedAsEmptyObject(t *testing.T) {
+	tool := NewServerToolWithContextHandler(
+		mcp.Tool{Name: "zero_arg_tool"},
+		testToolsetMetadata("test"),
+		func(_ context.Context, _ *mcp.CallToolRequest, args map[string]any) (*mcp.CallToolResult, any, error) {
+			return &mcp.CallToolResult{
+				Content: []mcp.Content{
+					&mcp.TextContent{Text: "ok"},
+				},
+			}, args, nil
+		},
+	)
+
+	handler := tool.HandlerFunc(nil)
+	result, err := handler(context.Background(), &mcp.CallToolRequest{
+		Params: &mcp.CallToolParamsRaw{
+			Name:      "zero_arg_tool",
+			Arguments: nil,
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	assert.False(t, result.IsError)
+}