# BaseRpcExceptionFilter

**Kind:** Class

**Source:** [`packages/microservices/exceptions/base-rpc-exception-filter.ts`](https://github.com/nestjs/nest/blob/master/packages/microservices/exceptions/base-rpc-exception-filter.ts#L15)

**Part of:** [Microservices](subsystem-packages-microservices)

`BaseRpcExceptionFilter` is the default foundation for handling exceptions thrown by NestJS microservice RPC handlers. It converts known `RpcException` instances into error observables and safely maps unknown errors to a standardized internal-error response while logging unexpected failures.

**Implements:** `RpcExceptionFilter`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `catch` | `catch(exception: T, host: ArgumentsHost)` | `Observable<R>` |
| `handleUnknownError` | `handleUnknownError(exception: T, status: string)` | `void` |
| `isError` | `isError(exception: any)` | `exception is Error` |

## Where it refuses work

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

## Diagram

```mermaid
graph LR
  A[RPC Handler throws exception] --> B[BaseRpcExceptionFilter.catch]
  B --> C{Is RpcException?}
  C -- Yes --> D[Extract exception error payload]
  D --> E[Return error Observable]
  C -- No --> F[handleUnknownError]
  F --> G{Is Error instance?}
  G -- Yes --> H[Log unexpected error]
  G -- No --> I[Create unknown error response]
  H --> I
  I --> E
```

## Usage

```ts
import { Catch, ArgumentsHost } from '@nestjs/common';
import {
  BaseRpcExceptionFilter,
  RpcException,
} from '@nestjs/microservices';
import { Observable } from 'rxjs';

@Catch()
export class RpcExceptionFilter extends BaseRpcExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost): Observable<unknown> {
    // Add custom logging, metrics, or error normalization here.
    return super.catch(exception, host);
  }
}

// In an RPC controller:
throw new RpcException({
  status: 'VALIDATION_ERROR',
  message: 'The provided email address is invalid.',
});
```

## AI Coding Instructions

- Extend this class when custom RPC exception behavior is needed; delegate to `super.catch()` unless intentionally replacing the default mapping.
- Throw `RpcException` for expected, client-safe microservice errors so its payload is preserved in the RPC response.
- Do not expose raw unknown error messages or stack traces to RPC consumers; use `handleUnknownError()` for safe fallback responses.
- Use `isError()` before relying on `message` or `stack` properties when handling values that may not be native `Error` instances.
- Ensure custom filters are registered with the relevant microservice/controller context so they run for RPC transport requests.

## Relationships

- IMPORTS → `ArgumentsHost`
- IMPORTS → `IntrinsicException`
- IMPORTS → `Logger`
- IMPORTS → `RpcExceptionFilter`
- IMPORTS → `isObject`
- IMPORTS → `MESSAGES`