From 797078a7bb3e74224c828d85028ea296c622aa35 Mon Sep 17 00:00:00 2001
From: Felix Weinberger <fweinberger@anthropic.com>
Date: Mon, 30 Mar 2026 20:36:46 +0000
Subject: [PATCH] fix(server): allow registerPrompt callback without args
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The Args generic now extends StandardSchemaWithJSON | undefined with a
default of undefined, matching registerTool. This lets PromptCallback
resolve to the (ctx) => result overload when argsSchema is omitted.

Co-authored-by: Anders Søgaard <andershagbard@gmail.com>
---
 .changeset/moody-humans-grow.md               |  5 +++
 packages/server/src/server/mcp.ts             |  2 +-
 packages/server/test/server/mcp.types.test.ts | 34 +++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/moody-humans-grow.md
 create mode 100644 packages/server/test/server/mcp.types.test.ts

diff --git a/.changeset/moody-humans-grow.md b/.changeset/moody-humans-grow.md
new file mode 100644
index 000000000..fbd1992d2
--- /dev/null
+++ b/.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`.
diff --git a/packages/server/src/server/mcp.ts b/packages/server/src/server/mcp.ts
index 4d9f81c50..c85dcde93 100644
--- a/packages/server/src/server/mcp.ts
+++ b/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;
diff --git a/packages/server/test/server/mcp.types.test.ts b/packages/server/test/server/mcp.types.test.ts
new file mode 100644
index 000000000..c435c5f4d
--- /dev/null
+++ b/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
+    );
+}