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# PrefillErrorHandler

The `PrefillErrorHandler` is an **error processor** that handles assistant-response prefill errors. This error occurs when a conversation ends with an assistant message and the model rejects the request because it interprets it as prefilling the assistant response.

When the error is detected, the processor sends a hidden `system-reminder` signal with `continue` as its contents and signals a retry. The reminder is persisted as signal metadata, which keeps it available for retry reconstruction and raw history while standard UI-facing message conversions hide it.

Mastra adds this processor to every agent's `errorProcessors` by default, after `ProviderHistoryCompat` and before `StreamErrorRetryProcessor`. It only acts on the first retry attempt, so it needs no configuration. Supply your own instance to tune it. Anything already in your list is kept and the default is only added when its `id` is absent.

## How it works

1. The LLM API call fails with a known assistant-prefill rejection message
2. `PrefillErrorHandler` checks that this is the first retry attempt
3. It sends a hidden `system-reminder` signal with `continue` as its contents
4. It returns `{ retry: true }` to signal the LLM call should be retried with the modified messages

The processor now reacts to the API rejection itself instead of re-checking whether the conversation currently ends with an assistant message. This makes it resilient to cases where the provider rejects the request for prefill semantics even if the trailing message shape has already been transformed upstream.

## Usage example

To tune or explicitly scope prefill recovery, supply your own instance. The default `PrefillErrorHandler` isn't added when you do:

```typescript
import { Agent } from '@mastra/core/agent'
import { PrefillErrorHandler } from '@mastra/core/processors'

export const agent = new Agent({
  id: 'my-agent',
  name: 'my-agent',
  instructions: 'You are a helpful assistant.',
  model: 'anthropic/claude-opus-4-7',
  errorProcessors: [new PrefillErrorHandler()],
})
```

If you want custom recovery behavior, provide your own error processor with a `processAPIError` method:

```typescript
import { Agent } from '@mastra/core/agent'
import type { Processor } from '@mastra/core/processors'

const customErrorHandler: Processor = {
  id: 'custom-prefill-error-handler',
  processAPIError({ error, messageList, retryCount }) {
    // Your custom logic here
  },
}

export const agent = new Agent({
  id: 'my-agent',
  name: 'my-agent',
  instructions: 'You are a helpful assistant.',
  model: 'anthropic/claude-opus-4-7',
  errorProcessors: [customErrorHandler],
})
```

## Constructor parameters

The `PrefillErrorHandler` takes no constructor parameters.

## Properties

**id** (`'prefill-error-handler'`): Processor identifier.

**name** (`'Prefill Error Handler'`): Processor display name.

**processAPIError** (`(args: ProcessAPIErrorArgs) => ProcessAPIErrorResult | void`): Handles known assistant-prefill errors by sending a hidden system reminder signal and signaling retry. Only triggers on the first retry attempt.

## Related

- [Processor interface](https://mastra.ai/reference/processors/processor-interface)
- [Guardrails](https://mastra.ai/docs/agents/guardrails)