Update ADK doc according to issue #1438 - 17

[adk-bot]

17. Document context injection and context_param_name for FunctionTool

Doc file: docs/tools-custom/function-tools.md

Current state:

Return Type

The preferred return type for a Function Tool is a dictionary in Python, a Map in Java, or an object in TypeScript. This allows you to structure the response with key-value pairs, providing context and clarity to the LLM. If your function returns a type other than a dictionary, the framework automatically wraps it into a dictionary with a single key named "result".

Proposed Change:

Return Type

The preferred return type for a Function Tool is a dictionary in Python, a Map in Java, or an object in TypeScript. This allows you to structure the response with key-value pairs, providing context and clarity to the LLM. If your function returns a type other than a dictionary, the framework automatically wraps it into a dictionary with a single key named "result".

Context Injection

If your function needs access to the agent's context (e.g., to read/write state, access authentication, or use artifacts), add a parameter named tool_context with type ToolContext to your function signature. The ADK will automatically inject the context when calling your function.

def my_tool(arg1: str, tool_context: ToolContext):
    # ... use tool_context ...

You can customize the parameter name used for injection by passing context_param_name to the FunctionTool constructor.

Reasoning:
The FunctionTool now supports customizing the context parameter name via context_param_name. Also, explicitly documenting context injection clarifies how to access ToolContext in custom functions.

Reference: src/google/adk/tools/function_tool.py

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