Modify registerPrompt to accept undefined Args type

.changeset/moody-humans-grow.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server": patch
+---
+
+Fix `registerPrompt` generic to allow the no-args callback overload. The `Args` type parameter now defaults to `undefined`, matching `registerTool`.

packages/server/src/server/mcp.ts

@@ -915,7 +915,7 @@ export class McpServer {
      * );
      * ```
      */
-    registerPrompt<Args extends StandardSchemaWithJSON>(
+    registerPrompt<Args extends StandardSchemaWithJSON | undefined = undefined>(
         name: string,
         config: {
             title?: string;

packages/server/test/server/mcp.types.test.ts

@@ -0,0 +1,34 @@
+/**
+ * Compile-time type checks for McpServer registration methods.
+ *
+ * These verify that generic type parameters resolve correctly for the
+ * no-argsSchema and with-argsSchema overloads of registerPrompt.
+ */
+import type { GetPromptResult, ServerContext } from '@modelcontextprotocol/core';
+import * as z from 'zod/v4';
+
+import { McpServer } from '../../src/server/mcp.js';
+
+/* eslint-disable @typescript-eslint/no-unused-vars */
+
+declare const server: McpServer;
+declare const result: GetPromptResult;
+
+// Without argsSchema, the callback must accept (ctx) only.
+// Before the fix, Args had no default and could not be undefined, so the
+// PromptCallback conditional never resolved to the no-args branch.
+function registerPrompt_noArgs() {
+    server.registerPrompt('no-args', {}, (ctx: ServerContext) => result);
+
+    // @ts-expect-error -- callback cannot take an args parameter when argsSchema is omitted
+    server.registerPrompt('no-args', {}, (args: { code: string }, ctx: ServerContext) => result);
+}
+
+// With argsSchema, the callback must accept (args, ctx).
+function registerPrompt_withArgs() {
+    server.registerPrompt(
+        'with-args',
+        { argsSchema: z.object({ code: z.string() }) },
+        (args: { code: string }, ctx: ServerContext) => result
+    );
+}