---
title: Open File Picker
description: Open the browser file picker from any clickable element.
icon: file-arrow-up
---

import { ComponentPreview } from '/snippets/component-preview.mdx'

`OpenFilePicker` is an action, not a component. It opens the browser's native file picker when triggered — attach it to any clickable element and it handles the rest: reading selected files to base64 and passing them to `onSuccess` as `$event`.

This is the counterpart to [DropZone](/components/drop-zone). Where DropZone gives you a dedicated upload area, OpenFilePicker turns any existing element into a file input. A button, a badge, an icon — anything with an `on_click`.

<Info>
  All file reading happens in the browser. Files are never sent to a server unless you explicitly wire up an `on_success` action (like `CallTool`) to do so.
</Info>

<ComponentPreview json={{"view":{"type":"Button","label":"Upload File","variant":"default","size":"default","disabled":false,"onClick":{"action":"openFilePicker","multiple":false}}}} playground="ZnJvbSBwcmVmYWJfdWkuY29tcG9uZW50cyBpbXBvcnQgQnV0dG9uCmZyb20gcHJlZmFiX3VpLmFjdGlvbnMgaW1wb3J0IE9wZW5GaWxlUGlja2VyCgpCdXR0b24oIlVwbG9hZCBGaWxlIiwgb25fY2xpY2s9T3BlbkZpbGVQaWNrZXIoKSkK">
<CodeGroup>
```python Python icon="python"
from prefab_ui.components import Button
from prefab_ui.actions import OpenFilePicker

Button("Upload File", on_click=OpenFilePicker())
```
```json Protocol icon="brackets-curly"
{
  "view": {
    "type": "Button",
    "label": "Upload File",
    "variant": "default",
    "size": "default",
    "disabled": false,
    "onClick": {"action": "openFilePicker", "multiple": false}
  }
}
```
</CodeGroup>
</ComponentPreview>

## Processing Files with a Tool

Files are read client-side, but they don't go anywhere until you tell them to. The typical pattern is to nest a `CallTool` inside `on_success` — pick the file, then send its base64 data to your MCP server for processing.

<ComponentPreview json={{"view":{"type":"Button","label":"Upload CSV","icon":"upload","variant":"outline","size":"default","disabled":false,"onClick":{"onSuccess":{"action":"toolCall","tool":"process_csv","arguments":{"file":"{{ $event }}"}},"action":"openFilePicker","accept":".csv","multiple":false}}}} playground="ZnJvbSBwcmVmYWJfdWkuY29tcG9uZW50cyBpbXBvcnQgQnV0dG9uCmZyb20gcHJlZmFiX3VpLmFjdGlvbnMgaW1wb3J0IE9wZW5GaWxlUGlja2VyCmZyb20gcHJlZmFiX3VpLmFjdGlvbnMubWNwIGltcG9ydCBDYWxsVG9vbAoKQnV0dG9uKAogICAgIlVwbG9hZCBDU1YiLAogICAgaWNvbj0idXBsb2FkIiwKICAgIHZhcmlhbnQ9Im91dGxpbmUiLAogICAgb25fY2xpY2s9T3BlbkZpbGVQaWNrZXIoCiAgICAgICAgYWNjZXB0PSIuY3N2IiwKICAgICAgICBvbl9zdWNjZXNzPUNhbGxUb29sKAogICAgICAgICAgICAicHJvY2Vzc19jc3YiLAogICAgICAgICAgICBhcmd1bWVudHM9eyJmaWxlIjogInt7ICRldmVudCB9fSJ9LAogICAgICAgICksCiAgICApLAopCg">
<CodeGroup>
```python Python icon="python"
from prefab_ui.components import Button
from prefab_ui.actions import OpenFilePicker
from prefab_ui.actions.mcp import CallTool

Button(
    "Upload CSV",
    icon="upload",
    variant="outline",
    on_click=OpenFilePicker(
        accept=".csv",
        on_success=CallTool(
            "process_csv",
            arguments={"file": "{{ $event }}"},
        ),
    ),
)
```
```json Protocol icon="brackets-curly"
{
  "view": {
    "type": "Button",
    "label": "Upload CSV",
    "icon": "upload",
    "variant": "outline",
    "size": "default",
    "disabled": false,
    "onClick": {
      "onSuccess": {
        "action": "toolCall",
        "tool": "process_csv",
        "arguments": {"file": "{{ $event }}"}
      },
      "action": "openFilePicker",
      "accept": ".csv",
      "multiple": false
    }
  }
}
```
</CodeGroup>
</ComponentPreview>

## Multiple Files

Set `multiple=True` to let users select several files at once. `$event` is always `list[FileUpload]` regardless of `multiple`, but the OS file picker restricts multi-select to when `multiple=True`.

```python Multiple file upload
from prefab_ui.components import Button
from prefab_ui.actions import OpenFilePicker
from prefab_ui.actions.mcp import CallTool

Button(
    "Upload Images",
    on_click=OpenFilePicker(
        accept="image/*",
        multiple=True,
        on_success=CallTool(
            "process_images",
            arguments={"images": "{{ $event }}"},
        ),
    ),
)
```

## File Size Limits

The `max_size` parameter rejects files that exceed a byte limit before they ever reach `onSuccess`. Oversized files are silently dropped with a toast notification — the user sees the error, your tool never has to deal with it.

```python With size limit
from prefab_ui.components import Button
from prefab_ui.actions import OpenFilePicker
from prefab_ui.actions.mcp import CallTool

Button(
    "Upload (max 5MB)",
    on_click=OpenFilePicker(
        max_size=5_000_000,
        on_success=CallTool("upload", arguments={"file": "{{ $event }}"}),
    ),
)
```

## Error Handling

The `on_error` callback on the nested `CallTool` fires if the server-side processing fails. The error message is available as `$error`.

```python With error handling
from prefab_ui.components import Button
from prefab_ui.actions import OpenFilePicker, ShowToast
from prefab_ui.actions.mcp import CallTool

Button(
    "Upload",
    on_click=OpenFilePicker(
        accept=".csv",
        on_success=CallTool(
            "process_csv",
            arguments={"file": "{{ $event }}"},
            on_success=ShowToast("File processed!", variant="success"),
            on_error=ShowToast(
                "Processing failed: {{ $error }}",
                variant="error",
            ),
        ),
    ),
)
```

## The `FileUpload` Type

Both OpenFilePicker and [DropZone](/components/drop-zone) produce the same `$event` shape. Prefab ships a `FileUpload` type you can use to annotate your tool parameters — see [The FileUpload Type](/components/drop-zone#the-fileupload-type) for details and usage.

## User Activation

One constraint worth knowing: `OpenFilePicker` must execute before any async server actions in the same chain. Browsers require a recent user gesture to open the file picker — if a `CallTool` or `SendMessage` fires first, the activation window expires and the picker silently won't open. This is the natural ordering anyway (pick the file, then send it), but it's worth understanding why.

## API Reference

<Card icon="code" title="OpenFilePicker Parameters">
<ParamField body="accept" type="str | None" default="None">
  File type filter. Accepts MIME types (`"image/*"`), extensions (`".csv"`), or a comma-separated mix.
</ParamField>

<ParamField body="multiple" type="bool" default="False">
  Allow selecting multiple files. `$event` is always `list[FileUpload]`; this controls whether the OS file picker allows multi-select.
</ParamField>

<ParamField body="max_size" type="int | None" default="None">
  Maximum file size in bytes. Oversized files are rejected with a toast notification.
</ParamField>

<ParamField body="on_success" type="Action | list[Action] | None" default="None">
  Action(s) to run after files are selected. `$event` is always `list[FileUpload]`.
</ParamField>

<ParamField body="on_error" type="Action | list[Action] | None" default="None">
  Action(s) to run if file reading fails.
</ParamField>
</Card>

## Protocol Reference

```json OpenFilePicker
{
  "action": "openFilePicker",
  "accept?": "string",
  "multiple?": false,
  "maxSize?": "number"
}
```

For the complete protocol schema, see [OpenFilePicker](/protocol/open-file-picker).
