# Event Handlers

Event handlers are the code that reacts to events in your Bkper Books. When a transaction is checked, an account is created, or any other event occurs, your handler receives it and can take action — calculate taxes, sync data between books, post to external services, and more.

![Bkper Event Handler](https://bkper.com/images/bots/bkper-tax-bot/bkper-tax-bot.gif)

## How it works

1. You declare which events your app handles in [`bkper.yaml`](https://bkper.com/docs/build/apps/configuration.md)
2. Bkper sends an HTTP POST to your webhook URL when those events fire
3. Your handler processes the event and returns a response

On the [Bkper Platform](https://bkper.com/docs/build/apps/overview.md), events are routed to `/events` on your app's single Worker — including local development via tunnels. For [self-hosted](https://bkper.com/docs/build/apps/self-hosted.md) setups, you configure the webhook URL directly.

## Agent identity

Event handlers **run on behalf of the user who installed the app**. Their transactions and activities are identified in the UI by the app's logo and name:

![Event handler agents identified in the activity stream](https://bkper.com/docs/_astro/bkper-bot-agents.CtsWIZEd.png)

## Responses

Handler responses are recorded in the activity that triggered the event. You can view and replay them by clicking the response at the bottom of the activity:

![Event handler responses in the activity stream](https://bkper.com/docs/_astro/bkper-bot-responses.UQXhqdai.png)

### Response format

Your handler must return a response in this format:

```ts
{ result?: string | string[] | boolean; error?: string; warning?: string }
```

- The `result` is recorded as the handler response in the book activity
- If you return `{ result: false }`, the response is suppressed and not recorded
- Errors like `{ error: "This is an error" }` show up as error responses

To show the full error stack trace for debugging:

```ts
try {
    // handler logic
} catch (err) {
    return { error: err instanceof Error ? err.message : String(err) };
}
```

### HTML in responses

If you return an **HTML snippet** (e.g., a link) in the result, it will be rendered in the response popup.

## Development mode

Event handlers run in _Development Mode_ when executed by the **developer or owner** of the App.

In development mode, both successful results and errors are shown as responses:

![Event handler error in development mode](https://bkper.com/docs/_astro/bkper-bot-error.4eq2AKEM.png)

You can click a response to **replay** failed executions — useful for debugging without recreating the triggering event.

To find transactions with bot errors in a book, run the query:

```
error:true
```

## Preventing loops

When your event handler creates or modifies transactions, those changes fire new events. To prevent infinite loops, check the `event.agent.id` field:

```ts
function handleEvent(event: bkper.Event) {
    // Skip events triggered by this app
    if (event.agent?.id === 'your-app-id') {
        return { result: false };
    }

    // Process the event
    // ...
}
```

This pattern is essential for any handler that writes back to the same book.

## Authentication

Platform-hosted event handlers use the same server-side Bkper API pattern as `/api/*` routes:

```ts
const bkper = new Bkper();
const book = new Book(event.book, bkper.getConfig());
```

Dispatch consumes the event delivery token, strips platform headers before your Worker runs, and platform outbound auth injects the OAuth token and app agent identity on Bkper API calls.

Do not read `bkper-oauth-token`, `bkper-agent-id`, or `Authorization` headers in platform app code.

> **Note**
> During local development, events are routed through the Cloudflare tunnel started by `bkper app dev`. Local outbound uses your CLI credentials when the handler calls Bkper.
For [self-hosted](https://bkper.com/docs/build/apps/self-hosted.md) setups, the event auth headers are sent to both `webhookUrl` and `webhookUrlDev` and must be handled directly by your infrastructure.

## Event routing pattern

On the Bkper Platform, your server Worker uses [Hono](https://hono.dev) to receive webhook calls at `/events`. A typical pattern routes events by type:

```ts
import { Bkper, Book } from 'bkper-js';

app.post('/events', async c => {
    const event: bkper.Event = await c.req.json();

    if (!event.book) {
        return c.json({ error: 'Missing book in event payload' }, 400);
    }

    const bkper = new Bkper();
    const book = new Book(event.book, bkper.getConfig());

    switch (event.type) {
        case 'TRANSACTION_CHECKED':
            return c.json(await handleTransactionChecked(book, event));
        default:
            return c.json({ result: false });
    }
});
```

## The Event object

The event payload has the following structure:

```ts
{
    /** The id of the Book associated to the Event */
    bookId?: string;

    /** The Book object associated with the Event */
    book?: {
        agentId?: string;
        collection?: Collection;
        createdAt?: string;
        datePattern?: string;
        decimalSeparator?: "DOT" | "COMMA";
        fractionDigits?: number;
        id?: string;
        lastUpdateMs?: string;
        lockDate?: string;
        name?: string;
        ownerName?: string;
        pageSize?: number;
        period?: "MONTH" | "QUARTER" | "YEAR";
        periodStartMonth?: "JANUARY" | "FEBRUARY" | "MARCH" | "APRIL"
            | "MAY" | "JUNE" | "JULY" | "AUGUST" | "SEPTEMBER"
            | "OCTOBER" | "NOVEMBER" | "DECEMBER";
        permission?: "OWNER" | "EDITOR" | "POSTER" | "RECORDER"
            | "VIEWER" | "NONE";
        properties?: { [name: string]: string };
        timeZone?: string;
        timeZoneOffset?: number;
    };

    /** The user in charge of the Event */
    user?: {
        avatarUrl?: string;
        name?: string;
        username?: string;
    };

    /** The Event agent, such as the App, Bot or Bank institution */
    agent?: {
        id?: string;
        logo?: string;
        name?: string;
    };

    /** The creation timestamp, in milliseconds */
    createdAt?: string;

    /** The event data */
    data?: {
        /** The object payload. Depends on the event type. */
        object?: any;
        /** The object previous attributes when updated */
        previousAttributes?: { [name: string]: string };
    };

    /** The unique id that identifies the Event */
    id?: string;

    /** The resource associated to the Event */
    resource?: string;

    /** The type of the Event */
    type?: EventType;
}
```

The event payload is the same structure exposed by the [REST API](https://bkper.com/docs/build/scripts/rest-api.md). If you use TypeScript, add the [`@bkper/bkper-api-types`](https://www.npmjs.com/package/@bkper/bkper-api-types) package to your project for full type definitions.

For update events, `data.previousAttributes` contains the fields that changed and their previous values — useful for computing diffs or reacting only to specific field changes.

## Event types

Declare which events your app handles in `bkper.yaml`:

```yaml
events:
    - TRANSACTION_CHECKED
    - TRANSACTION_POSTED
    - ACCOUNT_CREATED
```

The complete current set of event types:

| Event | Description |
| --- | --- |
| `FILE_CREATED` | A file was attached to the book. |
| `FILE_UPDATED` | An attached file was updated. |
| `TRANSACTION_CREATED` | A draft transaction was created. |
| `TRANSACTION_UPDATED` | A transaction was updated. |
| `TRANSACTION_DELETED` | A transaction was deleted. |
| `TRANSACTION_POSTED` | A draft transaction was posted and now affects balances. |
| `TRANSACTION_CHECKED` | A posted transaction was checked (reviewed and locked). |
| `TRANSACTION_UNCHECKED` | A checked transaction was unchecked and becomes editable again. |
| `TRANSACTION_RESTORED` | A deleted transaction was restored. |
| `ACCOUNT_CREATED` | An account was created. |
| `ACCOUNT_UPDATED` | An account was updated. |
| `ACCOUNT_DELETED` | An account was deleted. |
| `QUERY_CREATED` | A saved query was created. |
| `QUERY_UPDATED` | A saved query was updated. |
| `QUERY_DELETED` | A saved query was deleted. |
| `GROUP_CREATED` | A group was created. |
| `GROUP_UPDATED` | A group was updated. |
| `GROUP_DELETED` | A group was deleted. |
| `COMMENT_CREATED` | A comment was added. |
| `COMMENT_DELETED` | A comment was deleted. |
| `COLLABORATOR_ADDED` | A collaborator was added to the book. |
| `COLLABORATOR_UPDATED` | A collaborator's permissions were updated. |
| `COLLABORATOR_REMOVED` | A collaborator was removed from the book. |
| `INTEGRATION_CREATED` | An integration was created in the book. |
| `INTEGRATION_UPDATED` | An integration was updated. |
| `INTEGRATION_DELETED` | An integration was deleted. |
| `BOOK_CREATED` | A book was created. |
| `BOOK_AUDITED` | A balances audit completed for the book. |
| `BOOK_UPDATED` | Book settings were updated. |
| `BOOK_DELETED` | The book was deleted. |