fix(server): handle undefined arguments for tools with all optional params (#1404)

Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>
Co-authored-by: Felix Weinberger <fweinberger@anthropic.com>

packages/server/src/server/mcp.ts

@@ -271,7 +271,7 @@ export class McpServer {
         // If that fails, use the schema directly (for union/intersection/etc)
         const inputObj = normalizeObjectSchema(tool.inputSchema);
         const schemaToParse = inputObj ?? (tool.inputSchema as AnySchema);
-        const parseResult = await safeParseAsync(schemaToParse, args);
+        const parseResult = await safeParseAsync(schemaToParse, args ?? {});
         if (!parseResult.success) {
             const error = 'error' in parseResult ? parseResult.error : 'Unknown error';
             const errorMessage = getParseErrorMessage(error);

test/integration/test/issues/test400.optional-tool-params.test.ts

@@ -0,0 +1,70 @@
+/**
+ * Regression test for https://github.com/modelcontextprotocol/typescript-sdk/issues/400
+ *
+ * When a tool has all optional parameters, some LLM models call the tool without
+ * providing an `arguments` field. This test verifies that undefined arguments are
+ * handled correctly by defaulting to an empty object.
+ */
+
+import { Client } from '@modelcontextprotocol/client';
+import { CallToolResultSchema, InMemoryTransport } from '@modelcontextprotocol/core';
+import { McpServer } from '@modelcontextprotocol/server';
+import type { ZodMatrixEntry } from '@modelcontextprotocol/test-helpers';
+import { zodTestMatrix } from '@modelcontextprotocol/test-helpers';
+
+describe.each(zodTestMatrix)('Issue #400: $zodVersionLabel', (entry: ZodMatrixEntry) => {
+    const { z } = entry;
+
+    test('should accept undefined arguments when all tool params are optional', async () => {
+        const mcpServer = new McpServer({
+            name: 'test server',
+            version: '1.0'
+        });
+        const client = new Client({
+            name: 'test client',
+            version: '1.0'
+        });
+
+        mcpServer.registerTool(
+            'optional-params-tool',
+            {
+                inputSchema: {
+                    limit: z.number().optional(),
+                    offset: z.number().optional()
+                }
+            },
+            async ({ limit, offset }) => ({
+                content: [
+                    {
+                        type: 'text',
+                        text: `limit: ${limit ?? 'default'}, offset: ${offset ?? 'default'}`
+                    }
+                ]
+            })
+        );
+
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+        await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
+
+        // Call tool without arguments (arguments is undefined)
+        const result = await client.request(
+            {
+                method: 'tools/call',
+                params: {
+                    name: 'optional-params-tool'
+                    // arguments is intentionally omitted (undefined)
+                }
+            },
+            CallToolResultSchema
+        );
+
+        expect(result.isError).toBeUndefined();
+        expect(result.content).toEqual([
+            {
+                type: 'text',
+                text: 'limit: default, offset: default'
+            }
+        ]);
+    });
+});