# BaseExceptionFilter

**Kind:** Class

**Source:** [`packages/core/exceptions/base-exception-filter.ts`](https://github.com/nestjs/nest/blob/master/packages/core/exceptions/base-exception-filter.ts#L17)

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

`BaseExceptionFilter` is NestJS’s default HTTP exception handling foundation. It converts `HttpException` instances and compatible HTTP error objects into framework responses, while safely returning a generic 500 response and logging unexpected errors.

**Implements:** `ExceptionFilter`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `catch` | `catch(exception: T, host: ArgumentsHost)` | `void` |
| `handleUnknownError` | `handleUnknownError(exception: T, host: ArgumentsHost, applicationRef: AbstractHttpAdapter | HttpServer)` | `void` |
| `isExceptionObject` | `isExceptionObject(err: any)` | `err is Error` |
| `isHttpError` | `isHttpError(err: any)` | `err is { statusCode: number; message: string }` |

## Properties

| Property | Type |
|---|---|
| `httpAdapterHost` | `HttpAdapterHost` |

## Where it refuses work

- `BaseExceptionFilter` stops the work with an early return when `!(exception instanceof HttpException)`.

## Diagram

```mermaid
graph LR
  A[Thrown exception] --> B[BaseExceptionFilter.catch]
  B --> C{HttpException?}
  C -->|Yes| D[Extract status and response body]
  D --> E[Send response through HTTP adapter]
  C -->|No| F[handleUnknownError]
  F --> G{HTTP-style error object?}
  G -->|Yes| H[Use statusCode and message]
  G -->|No| I[Return generic 500 response]
  H --> E
  I --> E
```

## Usage

```ts
import { ArgumentsHost, Catch } from '@nestjs/common';
import { BaseExceptionFilter } from '@nestjs/core';

@Catch()
export class AllExceptionsFilter extends BaseExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    // Add custom logging, metrics, or tracing here.
    console.error('Unhandled exception:', exception);

    // Preserve Nest's standard HTTP exception behavior.
    super.catch(exception, host);
  }
}

// Register the filter globally:
// app.useGlobalFilters(new AllExceptionsFilter());
```

## AI Coding Instructions

- Extend `BaseExceptionFilter` when adding cross-cutting exception behavior while retaining NestJS’s default HTTP response handling.
- Always call `super.catch(exception, host)` unless the custom filter intentionally replaces the complete response flow.
- Use `handleUnknownError()` for non-`HttpException` errors; it handles safe 500 responses and error logging.
- Avoid exposing raw unknown error messages or stack traces in HTTP responses; rely on the base filter’s generic internal-error behavior.
- Ensure custom filters are registered through `app.useGlobalFilters()` or the `APP_FILTER` provider token.

## Relationships

- IMPORTS → `ArgumentsHost`
- IMPORTS → `ExceptionFilter`
- IMPORTS → `HttpException`
- IMPORTS → `HttpServer`
- IMPORTS → `HttpStatus`
- IMPORTS → `Inject`
- IMPORTS → `IntrinsicException`
- IMPORTS → `Logger`
- IMPORTS → `Optional`
- IMPORTS → `isObject`