fix(prompts): register arg-less prompts via the SDK no-argsSchema form

A user-created prompt with zero arguments was registered with an empty
argsSchema ({}); use the MCP SDK's documented two-arg (no-argsSchema) form
instead so arg-less prompts are unambiguously valid. Adds a no-arg mock test
plus a real-McpServer registration test that asserts no per-prompt errors are
swallowed (validates both the Zod-shape and no-arg forms against the live SDK).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: A.R.

packages/mcp-server/src/prompts.ts

@@ -48,15 +48,21 @@ export function registerPrompts(server: McpServer): void {
     });
 
     try {
-      server.registerPrompt(
-        prompt.name,
-        {
-          title: prompt.name,
-          description: prompt.description,
-          argsSchema: buildArgsSchema(prompt.arguments),
-        },
-        (args) => render(args as Record<string, unknown>),
-      );
+      // No-arg prompts use the SDK's two-arg form (no argsSchema); arg-bearing
+      // prompts pass a Zod shape built from their declared arguments.
+      if (prompt.arguments.length === 0) {
+        server.registerPrompt(prompt.name, { title: prompt.name, description: prompt.description }, () => render());
+      } else {
+        server.registerPrompt(
+          prompt.name,
+          {
+            title: prompt.name,
+            description: prompt.description,
+            argsSchema: buildArgsSchema(prompt.arguments),
+          },
+          (args) => render(args as Record<string, unknown>),
+        );
+      }
     } catch (err) {
       console.error(`[perplexity-mcp] failed to register prompt '${prompt.name}':`, err);
     }

packages/mcp-server/test/prompts.test.js

@@ -1,7 +1,8 @@
 import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { registerPrompts } from "../src/prompts.js";
 import { BUILTIN_PROMPTS, writePromptsConfig } from "../src/prompts-config.js";
 
@@ -86,4 +87,33 @@ describe("registerPrompts", () => {
     };
     expect(() => registerPrompts(server)).not.toThrow();
   });
+
+  it("registers a no-argument custom prompt via the no-argsSchema form", () => {
+    writePromptsConfig(
+      { overrides: {}, custom: [{ name: "no-args", description: "static", arguments: [], template: "fixed text" }] },
+      dir,
+    );
+    const server = mockServer();
+    registerPrompts(server);
+    const noArgs = server.calls.find((c) => c.name === "no-args");
+    expect(noArgs).toBeDefined();
+    expect(noArgs.config.argsSchema).toBeUndefined();
+    expect(noArgs.cb().messages[0].content.text).toBe("fixed text");
+  });
+
+  it("registers cleanly against a real McpServer (no swallowed per-prompt errors)", () => {
+    // Built-ins exercise the argsSchema (Zod-shape) form; the custom prompt
+    // exercises the no-argsSchema form. registerPrompts catches+logs per-prompt
+    // failures, so assert console.error was never called to prove the real SDK
+    // accepted every registration.
+    writePromptsConfig(
+      { overrides: {}, custom: [{ name: "no-args", description: "static", arguments: [], template: "fixed text" }] },
+      dir,
+    );
+    const server = new McpServer({ name: "test", version: "0.0.0" });
+    const errSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+    expect(() => registerPrompts(server)).not.toThrow();
+    expect(errSpy).not.toHaveBeenCalled();
+    errSpy.mockRestore();
+  });
 });