feat: US-006 - Add AsyncCallback type and AsyncTool interface

- Define AsyncCallback function type for async tool completion notification
- Define AsyncTool interface with SetCallback method
- Add comprehensive godoc comments with usage examples
- This enables tools like SpawnTool to notify completion asynchronously

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

.ralph/prd.json

@@ -91,7 +91,7 @@
         "Typecheck passes"
       ],
       "priority": 6,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {

.ralph/progress.txt

@@ -6,12 +6,13 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
 
 ## Progress
 
-### Completed (4/21)
+### Completed (5/21)
 
 - US-001: Add ToolResult struct and helper functions
 - US-002: Modify Tool interface to return *ToolResult
 - US-004: Delete isToolConfirmationMessage function (already removed in commit 488e7a9)
 - US-005: Update AgentLoop tool result processing logic
+- US-006: Add AsyncCallback type and AsyncTool interface
 
 ### In Progress
 
@@ -24,7 +25,7 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
 | US-003 | Modify ToolRegistry to process ToolResult | Pending | registry.go already updated |
 | US-004 | Delete isToolConfirmationMessage function | Completed | Already removed in commit 488e7a9 |
 | US-005 | Update AgentLoop tool result processing logic | Completed | No test files in pkg/agent yet |
-| US-006 | Add AsyncCallback type and AsyncTool interface | Pending | |
+| US-006 | Add AsyncCallback type and AsyncTool interface | Completed | |
 | US-007 | Heartbeat async task execution support | Pending | |
 | US-008 | Inject callback into async tools in AgentLoop | Pending | |
 | US-009 | State save atomicity - SetLastChannel | Pending | |
@@ -85,3 +86,19 @@ Tool 返回值结构化重构 - 将 Tool 接口返回值从 (string, error) 改
   - **Useful context:** `opts.SendResponse` 参数控制是否发送响应给用户。当工具设置了 `ForUser` 时，即使 Silent=false，也只有在 `SendResponse=true` 时才会发送。
 
 ---
+
+## [2026-02-12] - US-006
+- What was implemented:
+  - 在 `pkg/tools/base.go` 中定义 `AsyncCallback` 函数类型
+  - 定义 `AsyncTool` 接口，包含 `SetCallback(cb AsyncCallback)` 方法
+  - 添加完整的 godoc 注释，包含使用示例
+
+- Files changed:
+  - `pkg/tools/base.go`
+
+- **Learnings for future iterations:**
+  - **Patterns discovered:** Go 接口的设计应该是可选的组合模式。`AsyncTool` 是一个可选接口，工具可以选择实现以支持异步操作。
+  - **Gotchas encountered:** 无
+  - **Useful context:** 这个模式将在 US-008 中用于 `SpawnTool`，让子代理完成时能够通知主循环。
+
+---

pkg/tools/base.go

@@ -2,6 +2,7 @@ package tools
 
 import "context"
 
+// Tool is the interface that all tools must implement.
 type Tool interface {
 	Name() string
 	Description() string
@@ -16,6 +17,58 @@ type ContextualTool interface {
 	SetContext(channel, chatID string)
 }
 
+// AsyncCallback is a function type that async tools use to notify completion.
+// When an async tool finishes its work, it calls this callback with the result.
+//
+// The ctx parameter allows the callback to be canceled if the agent is shutting down.
+// The result parameter contains the tool's execution result.
+//
+// Example usage in an async tool:
+//
+//	func (t *MyAsyncTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
+//	    // Start async work in background
+//	    go func() {
+//	        result := doAsyncWork()
+//	        if t.callback != nil {
+//	            t.callback(ctx, result)
+//	        }
+//	    }()
+//	    return AsyncResult("Async task started")
+//	}
+type AsyncCallback func(ctx context.Context, result *ToolResult)
+
+// AsyncTool is an optional interface that tools can implement to support
+// asynchronous execution with completion callbacks.
+//
+// Async tools return immediately with an AsyncResult, then notify completion
+// via the callback set by SetCallback.
+//
+// This is useful for:
+// - Long-running operations that shouldn't block the agent loop
+// - Subagent spawns that complete independently
+// - Background tasks that need to report results later
+//
+// Example:
+//
+//	type SpawnTool struct {
+//	    callback AsyncCallback
+//	}
+//
+//	func (t *SpawnTool) SetCallback(cb AsyncCallback) {
+//	    t.callback = cb
+//	}
+//
+//	func (t *SpawnTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
+//	    go t.runSubagent(ctx, args)
+//	    return AsyncResult("Subagent spawned, will report back")
+//	}
+type AsyncTool interface {
+	Tool
+	// SetCallback registers a callback function to be invoked when the async operation completes.
+	// The callback will be called from a goroutine and should handle thread-safety if needed.
+	SetCallback(cb AsyncCallback)
+}
+
 func ToolToSchema(tool Tool) map[string]interface{} {
 	return map[string]interface{}{
 		"type": "function",