@mastra/core/dist/docs/references/reference-file-based-agents-processors.md

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# Processors

A file-based agent discovers [processors](https://mastra.ai/docs/agents/processors) from its `processors/` directory. Use this page for the file-based convention; use the processors guide for built-in processors, custom processors, streaming behavior, and advanced patterns.

Files under `processors/input/` run before messages reach the model. Files under `processors/output/` run after the model responds. Each file default-exports one [`Processor`](https://mastra.ai/reference/processors/processor-interface), and the filename becomes its discovery key.

## Quickstart

Add an input processor by placing a processor file under `processors/input/`:

```typescript
import { PIIDetector } from '@mastra/core/processors'

export default new PIIDetector({
  strategy: 'block',
})
```

This processor runs before user messages reach the model.

## Add an output processor

Output processors are good for filtering or transforming model responses before they're returned. This example redacts PII from the final response.

```typescript
import { PIIDetector } from '@mastra/core/processors'

export default new PIIDetector({
  strategy: 'redact',
})
```

## Ordering

Discovered processors are added after any processors defined in [`config.ts`](https://mastra.ai/reference/file-based-agents/config). That means config processors run first.

```text
config.inputProcessors → processors/input/* → model
model → config.outputProcessors → processors/output/* → response
```

If `config.inputProcessors` or `config.outputProcessors` is a function, discovered processors of that type are ignored with a warning because function-valued processors can't be statically merged.

## Precedence with config

Use `config.ts` for processors that need to be created at runtime or shared across agents. Use file-based processors for static, per-agent processors that should be reviewed alongside the agent.

Precedence is:

1. Function-valued `config.inputProcessors` or `config.outputProcessors`
2. Array-valued `config.inputProcessors` or `config.outputProcessors`
3. Discovered files under `processors/input/` or `processors/output/`

Visit the [`Processor` reference](https://mastra.ai/reference/processors/processor-interface) for the full interface.