# PreconditionFailedException

**Kind:** Class

**Source:** [`packages/common/exceptions/precondition-failed.exception.ts`](https://github.com/nestjs/nest/blob/master/packages/common/exceptions/precondition-failed.exception.ts#L11)

**Part of:** [Common](subsystem-packages-common)

Defines an HTTP exception for *Precondition Failed* type errors.

`PreconditionFailedException` represents an HTTP 412 *Precondition Failed* error. Use it when a request cannot be completed because one or more client-supplied preconditions—such as an ETag, version, or conditional header—do not match the current resource state.

**Extends:** `HttpException`

## Diagram

```mermaid
graph LR
  Client[Client request with precondition] --> Handler[Application handler]
  Handler --> Check{Precondition valid?}
  Check -->|Yes| Update[Process request]
  Check -->|No| Exception[PreconditionFailedException]
  Exception --> Response[HTTP 412 Precondition Failed response]
```

## Usage

```ts
import { PreconditionFailedException } from '@nestjs/common';

async function updateDocument(
  id: string,
  expectedVersion: number,
  content: string,
) {
  const document = await documentsService.findById(id);

  if (document.version !== expectedVersion) {
    throw new PreconditionFailedException(
      'The document has changed since it was last retrieved.',
    );
  }

  return documentsService.update(id, content);
}
```

## AI Coding Instructions

- Throw this exception only when a request's explicit precondition fails, such as an ETag, `If-Match` header, or optimistic-lock version check.
- Prefer a clear, client-actionable message that explains the resource must be refreshed before retrying.
- Do not use this exception for validation errors; use the appropriate bad-request or validation exception instead.
- Allow the framework exception filter to serialize the exception into the standard HTTP 412 response shape.

## Used by

1 reference from 1 file. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (1)

- `DurableService` — `integration/scopes/src/durable/durable.service.ts`:10