# ExternalExceptionFilter

**Kind:** Class

**Source:** [`packages/core/exceptions/external-exception-filter.ts`](https://github.com/nestjs/nest/blob/master/packages/core/exceptions/external-exception-filter.ts#L3)

**Part of:** [Core](subsystem-packages-core)

`ExternalExceptionFilter` defines the exception-handling contract used to process errors raised outside the normal request flow. Implement its `catch()` method to inspect an exception, access the current execution context, and return either a response value or a promise.

## Methods

| Method | Signature | Returns |
|---|---|---|
| `catch` | `catch(exception: T, host: ArgumentsHost)` | `R | Promise<R>` |

## Diagram

```mermaid
graph LR
  A[Unhandled exception] --> B[ExternalExceptionFilter.catch]
  B --> C[ArgumentsHost]
  C --> D[Access transport context]
  B --> E[Create error response]
  E --> F[Return value or Promise]
```

## Usage

```ts
import type { ArgumentsHost } from '@nestjs/common';
import type { ExternalExceptionFilter } from '@nestjs/core';

class LoggingExceptionFilter implements ExternalExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    const response = host.switchToHttp().getResponse();

    console.error('Unhandled external exception:', exception);

    return response.status(500).json({
      statusCode: 500,
      message: 'Internal server error',
    });
  }
}

// Register the filter with the relevant application integration point.
const filter = new LoggingExceptionFilter();
```

## AI Coding Instructions

- Implement `catch()` with the expected exception and `ArgumentsHost` parameters; it may return a value directly or a `Promise`.
- Use `ArgumentsHost` to select the active transport context, such as HTTP, RPC, or WebSocket, before writing a response.
- Avoid assuming every exception is an `Error`; safely handle strings, objects, and unknown thrown values.
- Preserve framework-specific response conventions, including status codes and response serialization.
- Log unexpected exceptions with useful context, but avoid exposing internal error details to clients.

## How it works

## `ExternalExceptionFilter<T, R>`

`ExternalExceptionFilter` is a generic exception-filter base class. Its `catch` method accepts an exception of type `T` and an `ArgumentsHost`, is declared to return `R | Promise<R>`, but always throws the received exception instead of returning normally. [packages/core/exceptions/external-exception-filter.ts:3-6](packages/core/exceptions/external-exception-filter.ts#L3-L6) [packages/core/exceptions/external-exception-filter.ts:14](packages/core/exceptions/external-exception-filter.ts#L14)

## Relationships

- IMPORTS → `ArgumentsHost`
- IMPORTS → `IntrinsicException`
- IMPORTS → `Logger`