Return protocol errors for invalid tool arguments

Author: FU-max-boop

packages/server/src/server/mcp.ts

@@ -208,8 +208,8 @@ export class McpServer {
                 await this.validateToolOutput(tool, result, request.params.name);
                 return result;
             } catch (error) {
-                if (error instanceof ProtocolError && error.code === ProtocolErrorCode.UrlElicitationRequired) {
-                    throw error; // Return the error to the caller without wrapping in CallToolResult
+                if (error instanceof ProtocolError) {
+                    throw error; // Return protocol-level errors to the caller without wrapping in CallToolResult
                 }
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }

packages/server/test/server/mcp.compat.test.ts

@@ -2,9 +2,10 @@ import type { JSONRPCMessage } from '@modelcontextprotocol/core';
 import { InMemoryTransport, isStandardSchema, LATEST_PROTOCOL_VERSION } from '@modelcontextprotocol/core';
 import { describe, expect, expectTypeOf, it, vi } from 'vitest';
 import * as z from 'zod/v4';
+
 import { McpServer } from '../../src/index.js';
-import type { InferRawShape } from '../../src/server/mcp.js';
 import { completable } from '../../src/server/completable.js';
+import type { InferRawShape } from '../../src/server/mcp.js';
 
 describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () => {
     it('registerTool accepts a raw shape for inputSchema and auto-wraps it', () => {
@@ -119,6 +120,55 @@ describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () =
 
         await server.close();
     });
+
+    it('returns a protocol error for invalid tool arguments', async () => {
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+
+        const handler = vi.fn(async () => ({
+            content: [{ type: 'text' as const, text: 'should not run' }]
+        }));
+        server.registerTool('echo', { inputSchema: { x: z.number() } }, handler);
+
+        const [client, srv] = InMemoryTransport.createLinkedPair();
+        await server.connect(srv);
+        await client.start();
+
+        const responses: JSONRPCMessage[] = [];
+        client.onmessage = m => responses.push(m);
+
+        await client.send({
+            jsonrpc: '2.0',
+            id: 1,
+            method: 'initialize',
+            params: {
+                protocolVersion: LATEST_PROTOCOL_VERSION,
+                capabilities: {},
+                clientInfo: { name: 'c', version: '1.0.0' }
+            }
+        } as JSONRPCMessage);
+        await client.send({ jsonrpc: '2.0', method: 'notifications/initialized' } as JSONRPCMessage);
+        await client.send({
+            jsonrpc: '2.0',
+            id: 2,
+            method: 'tools/call',
+            params: { name: 'echo', arguments: { x: 'not-a-number' } }
+        } as JSONRPCMessage);
+
+        await vi.waitFor(() => expect(responses.some(r => 'id' in r && r.id === 2)).toBe(true));
+
+        expect(handler).not.toHaveBeenCalled();
+        const response = responses.find(r => 'id' in r && r.id === 2) as {
+            error?: { code: number; message: string };
+            result?: unknown;
+        };
+        expect(response.result).toBeUndefined();
+        expect(response.error).toMatchObject({
+            code: -32_602,
+            message: expect.stringContaining('Invalid arguments for tool echo')
+        });
+
+        await server.close();
+    });
 });
 
 describe('InferRawShape', () => {