> ## Documentation Index
> Fetch the complete documentation index at: https://docs.usefused.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Runtime extension reference

> Find each lifecycle decorator, its allowed count, and its detailed guide.

Root files under `extensions/**/*.py` add behavior around the agent. Harnest discovers decorated functions and ignores ordinary helpers.

## Decorator index

| Decorator                                 |                 Allowed | Purpose                      | Guide                                                                                    |
| ----------------------------------------- | ----------------------: | ---------------------------- | ---------------------------------------------------------------------------------------- |
| `@lifecycle.resource`                     |                    Many | Application dependency       | [Application resources](/harnest/runtime/lifecycle/application-resources)                |
| `@context("name")`                        |                    Many | Publish a resource           | [Application resources](/harnest/runtime/lifecycle/application-resources)                |
| `@lifecycle.before_agent` / `after_agent` |                    Many | Agent policy and observation | [Invocation hooks](/harnest/runtime/lifecycle/invocation-hooks)                          |
| `@lifecycle.before_model` / `after_model` |                    Many | Model policy and observation | [Invocation hooks](/harnest/runtime/lifecycle/invocation-hooks)                          |
| `@lifecycle.before_tool` / `after_tool`   |                    Many | Tool policy and observation  | [Invocation hooks](/harnest/runtime/lifecycle/invocation-hooks)                          |
| `@lifecycle.output_policy`                |             Zero or one | Public SubAgent messages     | [Output policy](/harnest/runtime/lifecycle/output-policy)                                |
| `@lifecycle.adk_plugin`                   |                    Many | ADK-native behavior          | [Native integrations](/harnest/runtime/lifecycle/native-integrations)                    |
| `@lifecycle.langgraph_middleware`         |                    Many | LangGraph-native behavior    | [Native integrations](/harnest/runtime/lifecycle/native-integrations)                    |
| `@lifecycle.http_routes`                  |                    Many | Application HTTP routes      | [Custom HTTP endpoints](/harnest/runtime/custom-http-endpoints)                          |
| `@lifecycle.asset_store(name="...")`      | Many, with unique names | Multimodal asset storage     | [Store and retrieve media](/harnest/build/models-and-libraries/store-and-retrieve-media) |
| `@lifecycle.session_store`                |             Exactly one | Sessions and transcripts     | [Storage](/harnest/runtime/checkpoints-and-storage)                                      |
| `@lifecycle.checkpointer`                 |             Exactly one | In-progress execution        | [Storage](/harnest/runtime/checkpoints-and-storage)                                      |
| `@lifecycle.authenticate`                 |            Zero or more | Request identity             | [Authentication](/harnest/runtime/authentication-and-credentials)                        |
| `@lifecycle.credential_provider`          |             Zero or one | Downstream credentials       | [Authentication](/harnest/runtime/authentication-and-credentials)                        |
| `@lifecycle.telemetry_exporter`           |                    Many | Trace and log destination    | [Telemetry](/harnest/runtime/telemetry)                                                  |

## Factory rules

| Rule            | Behavior                                                                                         |
| --------------- | ------------------------------------------------------------------------------------------------ |
| Signature       | Synchronous and zero-argument unless it is a callback; `http_routes` receives one `AgentInvoker` |
| Return type     | Must match the decorator's resource or policy type                                               |
| Duplicates      | Single-owner factories reject duplicates                                                         |
| Ordering        | Explicit order, then source path                                                                 |
| Ownership       | Root agent only                                                                                  |
| Startup failure | Application does not serve                                                                       |
| Shutdown        | Harnest closes resources it owns                                                                 |

<Card title="Lifecycle overview" icon="arrows-rotate" href="/harnest/runtime/lifecycle">
  Choose the extension type before opening its focused guide.
</Card>