# Express

URL: https://tailrace.dev/docs/reference/express

> Reference for @tailrace/express - tailraceExpress middleware and TailraceExpressOptions.



`@tailrace/express` provides OpenAI-compatible passthrough middleware for Express. It scans chat request bodies and JSON / SSE responses at the model boundary.

## Install [#install]

```bash
pnpm add @tailrace/core @tailrace/express express
```

Peer dependency: `express` `>=4`. Depends on `@tailrace/core` + [`@tailrace/http`](/docs/reference/http).

## Exports [#exports]

| Export                   | Description                        |
| ------------------------ | ---------------------------------- |
| `tailraceExpress`        | Returns Express request middleware |
| `TailraceExpressOptions` | Options type                       |

## Options [#options]

| Option       | Default               | Notes                         |
| ------------ | --------------------- | ----------------------------- |
| `mode`       | `"openai-compatible"` | Only mode in v0.1             |
| `agent`      | `"default"`           | `(req) => string`             |
| `workflowId` | `"default"`           | `string` or `(req) => string` |
| `onDecision` | -                     | Audit callback                |

Expects a JSON body (`express.json()` or equivalent) for chat requests. Boundary provider is the request body's `model` string as-is (no `${providerId}/${modelId}` rewrite).

## Block shape [#block-shape]

**422** JSON `{ error: { type: "policy_violation", entity, rule } }`. SSE: one `data:` error event then close (abort-equivalent only). Maps from [`PolicyViolationError`](/docs/reference/errors/POLICY_VIOLATION).

## See also [#see-also]

* [Block secrets in an Express app](/docs/guides/block-secrets-in-express)
* [Express integration](/docs/integrations/express)
* [HTTP shared pipeline](/docs/reference/http)