Documentation Index
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Observe or transform agent, model, and tool execution in a portable way.
| Phase | Use it for |
|---|---|
| Before agent | Request policy, context setup, or early rejection |
| After agent | Result checks or accounting |
| Before model | Input policy or routing metadata |
| After model | Result checks |
| Before tool | Tool policy and guardrails |
| After tool | Result checks and audit enrichment |
from harnest.lifecycle import lifecycle
@lifecycle.before_tool
async def check_tool(event):
if event.tool_name == "delete_account":
raise PermissionError("Tool is disabled")
| Property | Behavior |
|---|---|
| Ordering | Explicit order, then source path |
| Transformer | Can replace the current value |
| Observer | Reads an event without replacing it |
| Rejection | Stops work before the protected phase |
| Error observer | Cannot hide the original failure |
| Requirement | Choose |
|---|---|
| Same policy on ADK and LangGraph | Portable hook |
| ADK runner or plugin API | ADK native integration |
| LangGraph middleware or graph runtime | LangGraph native integration |