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George Weale c620a7347c fix(samples): repair the core, tools, plugins and skills samples
Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 962244200
2026-08-10 10:48:35 -07:00

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# ADK Agent with Plugin
### What is ADK Plugin?
At its core, ADK extensibility is built on
[**callbacks**](https://google.github.io/adk-docs/callbacks/): functions you
write that ADK automatically executes at key stages of an agent's lifecycle.
**A Plugin is simply a class that packages these individual callback functions
together for a broader purpose.**
While a standard Agent Callback is configured on a *single agent, a single tool*
for a *specific task*, a Plugin is registered *once* on the `Runner` and its
callbacks apply *globally* to every agent, tool, and LLM call managed by that
runner. This makes Plugins the ideal solution for implementing horizontal
features that cut across your entire application.
### What can plugins do?
Plugins are incredibly versatile. By implementing different callback methods, you
can achieve a wide range of functionalities.
- **Logging & Tracing**: Create detailed logs of agent, tool, and LLM activity
for debugging and performance analysis.
- **Policy Enforcement**: Implement security guardrails. For example, a
before_tool_callback can check if a user is authorized to use a specific
tool and prevent its execution by returning a value.
- **Monitoring & Metrics**: Collect and export metrics on token usage,
execution times, and invocation counts to monitoring systems like Prometheus
or Stackdriver.
- **Caching**: In before_model_callback or before_tool_callback, you can
check if a request has been made before. If so, you can return a cached
response, skipping the expensive LLM or tool call entirely.
- **Request/Response Modification**: Dynamically add information to LLM prompts
(e.g., in before_model_callback) or standardize tool outputs (e.g., in
after_tool_callback).
### Run the agent
Use following command to run the main.py
```bash
python3 -m contributing.samples.plugins.plugin_basic.main
```
It should output the following content. Note that the outputs from plugin are
printed.
```bash
[Plugin] Agent run count: 1
[Plugin] LLM request count: 1
** Got event from hello_world
Hello world: query is [hello world]
** Got event from hello_world
[Plugin] LLM request count: 2
** Got event from hello_world
```