chore: Add instructions for callback signatures

PiperOrigin-RevId: 815549924

contributing/samples/adk_agent_builder_assistant/instruction_embedded.template

@@ -275,6 +275,76 @@ Use other tools only when the knowledge agent doesn't have enough information.
 7. **Keep TODO for complex**: For complex business logic, leave TODO comments
 8. **Follow current ADK patterns**: Always search for and reference the latest examples from contributing/samples
 
+### 🚨 CRITICAL: Callback Correct Signatures
+ADK supports different callback types with DIFFERENT signatures. Use FUNCTION-based callbacks (never classes):
+
+## 1. Agent Callbacks (before_agent_callbacks / after_agent_callbacks)
+
+**✅ CORRECT Agent Callback:**
+```python
+from typing import Optional
+from google.genai import types
+from google.adk.agents.callback_context import CallbackContext
+
+def content_filter_callback(context: CallbackContext) -> Optional[types.Content]:
+    """After agent callback to filter sensitive content."""
+    # Access the response content through context
+    if hasattr(context, 'response') and context.response:
+        response_text = str(context.response)
+        if "confidential" in response_text.lower():
+            filtered_text = response_text.replace("confidential", "[FILTERED]")
+            return types.Content(parts=[types.Part(text=filtered_text)])
+    return None  # Return None to keep original response
+```
+
+## 2. Model Callbacks (before_model_callbacks / after_model_callbacks)
+
+**✅ CORRECT Model Callback:**
+```python
+from typing import Optional
+from google.adk.models.llm_request import LlmRequest
+from google.adk.models.llm_response import LlmResponse
+from google.adk.agents.callback_context import CallbackContext
+
+def log_model_request(context: CallbackContext, request: LlmRequest) -> Optional[LlmResponse]:
+    """Before model callback to log requests."""
+    print(f"Model request: {{request.contents}}")
+    return None  # Return None to proceed with original request
+
+def modify_model_response(context: CallbackContext, response: LlmResponse) -> Optional[LlmResponse]:
+    """After model callback to modify response."""
+    # Modify response if needed
+    return response  # Return modified response or None for original
+```
+
+## 3. Tool Callbacks (before_tool_callbacks / after_tool_callbacks)
+
+**✅ CORRECT Tool Callback:**
+```python
+from typing import Any, Dict, Optional
+from google.adk.tools.base_tool import BaseTool
+from google.adk.tools.tool_context import ToolContext
+
+def validate_tool_input(tool: BaseTool, args: Dict[str, Any], context: ToolContext) -> Optional[Dict]:
+    """Before tool callback to validate input."""
+    # Validate or modify tool arguments
+    if "unsafe_param" in args:
+        del args["unsafe_param"]
+    return args  # Return modified args or None for original
+
+def log_tool_result(tool: BaseTool, args: Dict[str, Any], context: ToolContext, result: Dict) -> Optional[Dict]:
+    """After tool callback to log results."""
+    print(f"Tool {{tool.name}} executed with result: {{result}}")
+    return None  # Return None to keep original result
+```
+
+## Callback Signature Summary:
+- **Agent Callbacks**: `(CallbackContext) -> Optional[types.Content]`
+- **Before Model**: `(CallbackContext, LlmRequest) -> Optional[LlmResponse]`
+- **After Model**: `(CallbackContext, LlmResponse) -> Optional[LlmResponse]`
+- **Before Tool**: `(BaseTool, Dict[str, Any], ToolContext) -> Optional[Dict]`
+- **After Tool**: `(BaseTool, Dict[str, Any], ToolContext, Dict) -> Optional[Dict]`
+
 ## Important ADK Requirements
 
 **File Naming & Structure:**