registerToolTask handlers receive wrong arguments when inputSchema is omitted

[mattzcarey]

Summary

When using registerToolTask without providing an inputSchema, handlers that expect two arguments (args, extra) receive extra as undefined because the executor only passes one argument.

Reproduction

server.experimental.tasks.registerToolTask(
    'test-tool',
    {
        description: 'A test tool'
        // Note: no inputSchema provided
    },
    {
        async createTask(_args, extra) {
            // extra is undefined here!
            const task = await extra.taskStore.createTask({ ttl: extra.taskRequestedTtl });
            // TypeError: Cannot read properties of undefined (reading 'taskStore')
            return { task };
        },
        // ...
    }
);

Root Cause

In createToolExecutor (packages/server/src/server/mcp.ts), when inputSchema is undefined, the executor calls the handler with only one argument:

if (!inputSchema) {
    return handler(extra);  // Only passes extra as first argument
}
// vs when inputSchema is defined:
return handler(parsedArgs, extra);  // Passes both args and extra

When a handler is defined as async createTask(_args, extra), JavaScript will:

• With handler(extra): _args = extra, extra = undefined
• With handler(parsedArgs, extra): _args = parsedArgs, extra = extra ✓

Workaround

Always provide inputSchema: z.object({}) even for tools that don't require arguments:

server.experimental.tasks.registerToolTask(
    'test-tool',
    {
        description: 'A test tool',
        inputSchema: z.object({})  // Required for two-argument handlers
    },
    // ...
);

Potential Solutions

1. Always pass two arguments: Change the executor to always call handler({}, extra) when inputSchema is undefined
2. TypeScript enforcement: Use function overloads or conditional types to enforce that handlers without inputSchema must use single-argument signature
3. Documentation: Document that inputSchema must be provided when using two-argument handler signatures

Environment

• SDK version: v2 (drop-zod-v3-support branch)
• Affects: registerToolTask in experimental tasks API