---
title: Execution Boundary Hooks
description: Intercept the start, inputs, output, and end of crew and flow executions with the @on decorator
mode: "wide"
---

Execution boundary hooks intercept the outermost edges of a run — before any
work starts, when inputs are resolved, when the final result is ready, and when
the execution finishes. They fire for both crews and flows and are the right
place for run-level policy checks, input rewriting, and output sanitization.

## Overview

Four interception points cover the boundaries:

| Point | When | `ctx.payload` |
|-------|------|---------------|
| `EXECUTION_START` | A crew or flow is about to begin | inputs `dict` |
| `INPUT` | Resolved inputs for the execution | inputs `dict` |
| `OUTPUT` | The final result is ready | the output object |
| `EXECUTION_END` | The execution has finished (success or failure) | the output object, or `None` on failure |

For a crew, the output payload is a `CrewOutput`. For a flow, it is the final
flow-method result.

## Hook Signature

```python
from crewai.hooks import on, HookAborted, InterceptionPoint

@on(InterceptionPoint.EXECUTION_START)
def boundary_hook(ctx) -> Any | None:
    # Mutate ctx.payload in place, or
    # return a non-None value to replace it, or
    # raise HookAborted(reason, source) to stop the run
    return None
```

Boundary hooks follow the standard contract: proceed (`return None`), mutate in
place, replace by returning, or abort by raising
[`HookAborted`](/edge/en/learn/execution-hooks#aborting-an-operation). An abort at any
boundary propagates out of `kickoff()` with its reason.

## Context Schema

Each point receives a typed context. All contexts share the base fields:

```python
class InterceptionContext:
    payload: Any            # The interceptable value (see table above)
    agent: Any = None       # Not populated at execution boundaries
    agent_role: str | None  # Not populated at execution boundaries
    task: Any = None        # Not populated at execution boundaries
    crew: Any = None        # The Crew instance (crew runs only)
    flow: Any = None        # The Flow instance (flow runs only)
```

The per-point contexts add a named alias for the payload:

```python
class ExecutionStartContext(InterceptionContext):
    inputs: dict            # Same dict as payload

class InputContext(InterceptionContext):
    inputs: dict            # Same dict as payload

class OutputContext(InterceptionContext):
    output: Any             # The output object

class ExecutionEndContext(InterceptionContext):
    output: Any                    # The output object (None when status == "failed")
    status: str                    # "completed" or "failed"
    error: BaseException | None    # The exception when status == "failed"
```

<Note>
`ctx.inputs` aliases the **original** inputs dict, so in-place edits through
either name are equivalent. If an earlier hook *replaced* the payload by
returning a new dict, only `ctx.payload` is rebound — always read and write
`ctx.payload` when hooks might chain.
</Note>

## Crew Runs vs. Flow Runs

Boundary hooks fire on both runtimes, and crew execution internally rides on a
flow runtime. During a `crew.kickoff()`, a global boundary hook therefore fires
for the crew boundary (`ctx.crew` set, `ctx.flow` `None`) **and** for the
internal flow (`ctx.flow` set, `ctx.crew` `None`). Discriminate by runtime:

```python
@on(InterceptionPoint.OUTPUT)
def crew_output_only(ctx):
    if ctx.crew is None:
        return None  # Skip the internal flow (or a bare flow)
    ctx.payload.raw = ctx.payload.raw.strip()
```

## Common Use Cases

### Policy Check at Start

```python
@on(InterceptionPoint.EXECUTION_START)
def enforce_policy(ctx):
    if ctx.crew is not None and not ctx.payload.get("authorized"):
        raise HookAborted(reason="unauthorized execution", source="access-control")
```

### Input Rewriting

```python
@on(InterceptionPoint.INPUT)
def add_defaults(ctx):
    if ctx.crew is None:
        return None
    ctx.payload.setdefault("locale", "en-US")
    ctx.payload["topic"] = ctx.payload["topic"].strip().lower()
```

Rewritten inputs flow into task interpolation, so the run behaves as if it was
kicked off with the modified dict.

### Output Sanitization

```python
import re

@on(InterceptionPoint.OUTPUT)
def redact_emails(ctx):
    if ctx.crew is None:
        return None
    ctx.payload.raw = re.sub(
        r"\b[\w.+-]+@[\w-]+\.[\w.]+\b", "[EMAIL-REDACTED]", ctx.payload.raw
    )
```

`OUTPUT` runs before `EXECUTION_END`, and both see the (possibly replaced)
payload from earlier hooks; the final rewritten value is what `kickoff()`
returns.

### Observing Failures

`EXECUTION_END` fires exactly once per execution, on success and on failure
alike. When the run raises — a task error, a flow-method exception, or a
`HookAborted` from an earlier point — the hook receives `status="failed"` with
the exception in `ctx.error`, and the original exception still propagates out
of `kickoff()` unchanged:

```python
@on(InterceptionPoint.EXECUTION_END)
def report_outcome(ctx):
    if ctx.status == "failed":
        notify_policy_engine(status="failed", error=repr(ctx.error))
    else:
        notify_policy_engine(status="completed")
```

Two caveats: `EXECUTION_END` does not fire when `EXECUTION_START` never
dispatched (an abort at start counts as the execution never beginning), and
raising `HookAborted` from a failure-path `EXECUTION_END` dispatch is ignored —
there is nothing left to abort, and the original error wins.

## Ordering

For a crew run the boundary order is:

```
EXECUTION_START → before_kickoff callbacks → INPUT → tasks execute → OUTPUT → EXECUTION_END
```

Hooks at the same point run in registration order, global hooks first, then
crew-scoped hooks. Telemetry (`HookDispatchedEvent`) is emitted per dispatch.

## Managing Hooks in Tests

```python
from crewai.hooks import clear_all_hooks

clear_all_hooks()  # Clears every point, including boundaries
```

## Related Documentation

- [Execution Hooks Overview →](/edge/en/learn/execution-hooks)
- [Step Hooks →](/edge/en/learn/step-hooks)
- [LLM Call Hooks →](/edge/en/learn/llm-hooks)
- [Tool Call Hooks →](/edge/en/learn/tool-hooks)
