diff --git a/src/server/mcp.ts b/src/server/mcp.ts
index 765ba864f..855ddfc98 100644
--- a/src/server/mcp.ts
+++ b/src/server/mcp.ts
@@ -788,7 +788,7 @@ export class McpServer {
     /**
      * Registers a tool with a config object and callback.
      */
-    registerTool<InputArgs extends ZodRawShape, OutputArgs extends ZodRawShape>(
+    registerTool<OutputArgs extends ZodRawShape, InputArgs extends undefined | ZodRawShape = undefined>(
         name: string,
         config: {
             title?: string;