feat: implement expand-task MCP command

- Create direct function wrapper in expand-task.js with error handling

- Add MCP tool integration for breaking down tasks into subtasks

- Update task-master-core.js to expose expandTaskDirect function

- Update changeset to document the new command

- Parameter support for subtask generation options (num, research, prompt, force)

mcp-server/src/core/direct-functions/expand-task.js

@@ -0,0 +1,152 @@
+/**
+ * expand-task.js
+ * Direct function implementation for expanding a task into subtasks
+ */
+
+import { expandTask } from '../../../../scripts/modules/task-manager.js';
+import { readJSON, writeJSON } from '../../../../scripts/modules/utils.js';
+import { findTasksJsonPath } from '../utils/path-utils.js';
+import path from 'path';
+import fs from 'fs';
+
+/**
+ * Direct function wrapper for expanding a task into subtasks with error handling.
+ *
+ * @param {Object} args - Command arguments
+ * @param {Object} log - Logger object
+ * @returns {Promise<Object>} - Task expansion result { success: boolean, data?: any, error?: { code: string, message: string }, fromCache: boolean }
+ */
+export async function expandTaskDirect(args, log) {
+  let tasksPath;
+  try {
+    // Find the tasks path first
+    tasksPath = findTasksJsonPath(args, log);
+  } catch (error) {
+    log.error(`Tasks file not found: ${error.message}`);
+    return { 
+      success: false, 
+      error: { 
+        code: 'FILE_NOT_FOUND_ERROR', 
+        message: error.message 
+      }, 
+      fromCache: false 
+    };
+  }
+
+  // Validate task ID
+  const taskId = args.id ? parseInt(args.id, 10) : null;
+  if (!taskId) {
+    log.error('Task ID is required');
+    return { 
+      success: false, 
+      error: { 
+        code: 'INPUT_VALIDATION_ERROR', 
+        message: 'Task ID is required' 
+      }, 
+      fromCache: false 
+    };
+  }
+
+  // Process other parameters
+  const numSubtasks = args.num ? parseInt(args.num, 10) : undefined;
+  const useResearch = args.research === true;
+  const additionalContext = args.prompt || '';
+  const force = args.force === true;
+
+  try {
+    log.info(`Expanding task ${taskId} into ${numSubtasks || 'default'} subtasks. Research: ${useResearch}, Force: ${force}`);
+    
+    // Read tasks data
+    const data = readJSON(tasksPath);
+    if (!data || !data.tasks) {
+      return { 
+        success: false, 
+        error: { 
+          code: 'INVALID_TASKS_FILE', 
+          message: `No valid tasks found in ${tasksPath}` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Find the specific task
+    const task = data.tasks.find(t => t.id === taskId);
+    
+    if (!task) {
+      return { 
+        success: false, 
+        error: { 
+          code: 'TASK_NOT_FOUND', 
+          message: `Task with ID ${taskId} not found` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Check if task is completed
+    if (task.status === 'done' || task.status === 'completed') {
+      return { 
+        success: false, 
+        error: { 
+          code: 'TASK_COMPLETED', 
+          message: `Task ${taskId} is already marked as ${task.status} and cannot be expanded` 
+        }, 
+        fromCache: false
+      };
+    }
+    
+    // Check for existing subtasks
+    const hasExistingSubtasks = task.subtasks && task.subtasks.length > 0;
+    
+    // Keep a copy of the task before modification
+    const originalTask = JSON.parse(JSON.stringify(task));
+    
+    // Tracking subtasks count before expansion
+    const subtasksCountBefore = task.subtasks ? task.subtasks.length : 0;
+    
+    // Create a backup of the tasks.json file
+    const backupPath = path.join(path.dirname(tasksPath), 'tasks.json.bak');
+    fs.copyFileSync(tasksPath, backupPath);
+    
+    // Directly modify the data instead of calling the CLI function
+    if (!task.subtasks) {
+      task.subtasks = [];
+    }
+    
+    // Save tasks.json with potentially empty subtasks array
+    writeJSON(tasksPath, data);
+    
+    // Call the core expandTask function
+    await expandTask(taskId, numSubtasks, useResearch, additionalContext);
+    
+    // Read the updated data
+    const updatedData = readJSON(tasksPath);
+    const updatedTask = updatedData.tasks.find(t => t.id === taskId);
+    
+    // Calculate how many subtasks were added
+    const subtasksAdded = updatedTask.subtasks ? 
+      updatedTask.subtasks.length - subtasksCountBefore : 0;
+    
+    // Return the result
+    log.info(`Successfully expanded task ${taskId} with ${subtasksAdded} new subtasks`);
+    return { 
+      success: true, 
+      data: { 
+        task: updatedTask,
+        subtasksAdded,
+        hasExistingSubtasks
+      }, 
+      fromCache: false 
+    };
+  } catch (error) {
+    log.error(`Error expanding task: ${error.message}`);
+    return { 
+      success: false, 
+      error: { 
+        code: 'CORE_FUNCTION_ERROR', 
+        message: error.message || 'Failed to expand task' 
+      }, 
+      fromCache: false 
+    };
+  }
+} 

mcp-server/src/core/task-master-core.js

@@ -15,6 +15,7 @@ import { generateTaskFilesDirect } from './direct-functions/generate-task-files.
 import { setTaskStatusDirect } from './direct-functions/set-task-status.js';
 import { showTaskDirect } from './direct-functions/show-task.js';
 import { nextTaskDirect } from './direct-functions/next-task.js';
+import { expandTaskDirect } from './direct-functions/expand-task.js';
 
 // Re-export utility functions
 export { findTasksJsonPath } from './utils/path-utils.js';
@@ -31,6 +32,7 @@ export {
   setTaskStatusDirect,
   showTaskDirect,
   nextTaskDirect,
+  expandTaskDirect,
 };
 
 /**
@@ -48,5 +50,6 @@ export const directFunctions = {
   setStatus: setTaskStatusDirect,
   showTask: showTaskDirect,
   nextTask: nextTaskDirect,
+  expandTask: expandTaskDirect,
   // Add more functions as we implement them
 }; 

mcp-server/src/tools/expand-task.js

@@ -0,0 +1,57 @@
+/**
+ * tools/expand-task.js
+ * Tool to expand a task into subtasks
+ */
+
+import { z } from "zod";
+import {
+  handleApiResult,
+  createErrorResponse
+} from "./utils.js";
+import { expandTaskDirect } from "../core/task-master-core.js";
+
+/**
+ * Register the expand-task tool with the MCP server
+ * @param {Object} server - FastMCP server instance
+ */
+export function registerExpandTaskTool(server) {
+  server.addTool({
+    name: "expand_task",
+    description: "Expand a task into subtasks for detailed implementation",
+    parameters: z.object({
+      id: z.string().describe("ID of task to expand"),
+      num: z.union([z.number(), z.string()]).optional().describe("Number of subtasks to generate"),
+      research: z.boolean().optional().describe("Use Perplexity AI for research-backed generation"),
+      prompt: z.string().optional().describe("Additional context for subtask generation"),
+      force: z.boolean().optional().describe("Force regeneration even for tasks that already have subtasks"),
+      file: z.string().optional().describe("Path to the tasks file"),
+      projectRoot: z
+        .string()
+        .optional()
+        .describe(
+          "Root directory of the project (default: current working directory)"
+        ),
+    }),
+    execute: async (args, { log }) => {
+      try {
+        log.info(`Expanding task with args: ${JSON.stringify(args)}`);
+        
+        // Call the direct function wrapper
+        const result = await expandTaskDirect(args, log);
+        
+        // Log result
+        if (result.success) {
+          log.info(`Successfully expanded task ID: ${args.id} with ${result.data.subtasksAdded} new subtasks${result.data.hasExistingSubtasks ? ' (appended to existing subtasks)' : ''}`);
+        } else {
+          log.error(`Failed to expand task: ${result.error.message}`);
+        }
+        
+        // Use handleApiResult to format the response
+        return handleApiResult(result, log, 'Error expanding task');
+      } catch (error) {
+        log.error(`Error in expand-task tool: ${error.message}`);
+        return createErrorResponse(`Failed to expand task: ${error.message}`);
+      }
+    },
+  });
+} 

mcp-server/src/tools/index.js

@@ -13,6 +13,7 @@ import { registerUpdateSubtaskTool } from "./update-subtask.js";
 import { registerGenerateTool } from "./generate.js";
 import { registerShowTaskTool } from "./show-task.js";
 import { registerNextTaskTool } from "./next-task.js";
+import { registerExpandTaskTool } from "./expand-task.js";
 
 /**
  * Register all Task Master tools with the MCP server
@@ -28,6 +29,7 @@ export function registerTaskMasterTools(server) {
   registerGenerateTool(server);
   registerShowTaskTool(server);
   registerNextTaskTool(server);
+  registerExpandTaskTool(server);
 }
 
 export default {