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A blocks mini app’s UI is built from a component tree, rendered natively by the site. You can’t write HTML, so XSS is impossible.

Component list

Value ranges

Unknown component types and unknown props are rejected, not ignored — if you think you rendered a button but nothing shows up, that’s a bug you should be able to see, and the server rejects the invocation with E_INVALID_BLOCKS.
Component tree limits: 500 nodes / 32 levels of nesting / 256 KB.

Example

A button’s action is passed to onAction as ctx.action_id; the values of input and select are collected into ctx.inputs.