fix: pass empty args to registerToolTask createTask handler when no inputSchema

When registerToolTask is used without inputSchema, the createTask handler
should receive an empty object as its first argument when the handler
expects two arguments (args, ctx). This fixes the case where users define
a two-argument handler but omit inputSchema - previously the second
argument (ctx) would be passed as the first argument instead.

Uses Function.length to detect handler arity: single-arg handlers
continue to receive just ctx, while two-arg handlers get ({}, ctx).

Closes #1471

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: corvid-agent

.changeset/fix-register-tool-task-args.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Fix `registerToolTask` to pass empty args object to `createTask` handler when no `inputSchema` is provided, allowing two-argument `(args, ctx)` handler signatures to work correctly.

packages/server/src/server/mcp.ts

@@ -1042,10 +1042,10 @@ function createToolExecutor(inputSchema: AnySchema | undefined, handler: AnyTool
                 throw new Error('No task store provided.');
             }
             const taskCtx: CreateTaskServerContext = { ...ctx, task: { store: ctx.task.store, requestedTtl: ctx.task?.requestedTtl } };
-            if (inputSchema) {
-                return taskHandler.createTask(args, taskCtx);
+            if (inputSchema || taskHandler.createTask.length > 1) {
+                return taskHandler.createTask(inputSchema ? args : {}, taskCtx);
             }
-            // When no inputSchema, call with just ctx (the handler expects (ctx) signature)
+            // When no inputSchema and handler expects single arg, call with just ctx
             return (taskHandler.createTask as (ctx: CreateTaskServerContext) => CreateTaskResult | Promise<CreateTaskResult>)(taskCtx);
         };
     }