---
title: Fallback canary
description: How this renderer falls back for a block type it doesn't recognize, and the synthetic block used to test that path.
---

Slack Block Kit has a fallback convention: when a client doesn't recognize a block's `type`, it can render
that block's producer-supplied `fallback` array in its place instead of dropping the content. `fallback_canary`
is a synthetic block type this library ships to exercise that path in tests and demos. It isn't a block Slack
itself sends. Point any real, unrecognized block at a `fallback` array and this renderer handles it the same way.

<Preview
  payload={{
    blocks: [
      {
        type: "fallback_canary",
        fallback: [
          {
            type: "header",
            text: { type: "plain_text", text: "Fallback demo" },
          },
          {
            type: "section",
            text: {
              type: "mrkdwn",
              text: "Your client does not recognize this block, so it is showing the producer-supplied *fallback* instead of dropping the content.",
            },
          },
          {
            type: "context",
            elements: [{ type: "mrkdwn", text: "Rendered via the fallback convention" }],
          },
        ],
      },
    ],
  }}
/>

## Fields

| Prop | Type | Default | Description |
| - | - | - | - |
| `type` | `string` | - | Always `fallback_canary`. |
| `fallback` | `Block[]` | - | Blocks to render in place of this one. Slack renders the whole list inside the single wrapper it gave the unknown block, which is a flex row, so the fallback blocks sit side by side rather than stacking vertically. |

This isn't a standard Block Kit block type. It's a test fixture for the fallback mechanism itself. When this
library encounters a block `type` it doesn't have a component for, it looks for a `fallback` array on that
block and renders those blocks instead of showing nothing.

## Examples

### Unknown block with a fallback

The example above simulates what happens when your app sends a block type this renderer (or a Slack client)
doesn't know about yet, but included a `fallback` for older clients.

## Related

**[Section](/blocks/section)**

The most common block inside a fallback list.

**[Container](/blocks/container)**

Groups blocks visually, useful context for how fallback's flex-row wrapper differs.