> ## Documentation Index > Fetch the complete documentation index at: https://docs.heylua.ai/llms.txt > Use this file to discover all available pages before exploring further. # Block syntax > The rules every Lua app and channel uses to find ::: blocks in a reply: openers, closers, the one-line form, unclosed blocks, code, and what happens to text around a block Every surface that shows an agent's reply reads its `:::` blocks with the same rules: the web widget, Lua Desktop, the mobile app, Frontline, the dashboard chat, the messaging channels, email, and voice. A reply that renders as cards in one place is read as the same blocks everywhere else; what each channel then does with a block is in the [channel matrix](/channels/formatting/overview#rendering-by-channel). A message you send from code with [`Channels.send`](/build/send-proactive-messages) to SMS, Front, or a generated inbox is not read for blocks: it arrives as written. The canonical form is the one the platform teaches the model: ```text theme={null} ::: actions - Book the Deluxe Ocean View - Show me cheaper rooms ::: ``` The rules below say which other spellings count, so a block the model writes slightly differently still renders instead of reaching the end user as raw text. ## Openers A block opens at the start of a line with `:::` followed by the block name. These all open the same `actions` block: | Written | Reads as | | - | - | | `::: actions` | The canonical form | | `:::actions` | No space after the colons | | `::: actions` or a tab | Extra spaces or a tab | | `::: Actions` | Names are not case-sensitive | | ` ::: actions` | An indented opener | | `Sure!::: actions` | An opener glued to the end of a line of text: the text stays, the block opens | Text after the name on the opener line is the block's first body line, so `::: actions - Yes` followed by `- No` on the next line gives two entries. An opener glued to text counts only when it is a known block name at the end of the line. `:::` anywhere else in a line is text, never a marker: `ratio 3:::2`, an ARN such as `arn:aws:s3:::bucket`, and `` `a:::b` `` in inline code all show as written. ## Closers A block closes at a line that holds only colons: | Written | Reads as | | - | - | | `:::` | The canonical closer | | ` :::` or `:::` with trailing spaces | Indented, or followed by spaces | | `::` or `::::` | Any line of two or more colons | | `- No:::` | A closer glued to the end of the last body line | | `::: Thanks for waiting!` | A closer followed by text that isn't a block name: the block closes and the text shows after it | A closer with no block open, such as a stray `:::` line, is removed. ## One-line blocks A whole block can sit on one line. It ends at the first run of two or more colons after a space, and the rest of the line is read again as text that can hold more blocks: ```text theme={null} ::: reaction emoji=👍 ::: ``` Text before a one-line block stays on its line, and text after it continues the same line. ## Missing closers The model sometimes forgets a closer. Two rules keep the reply readable: * **A new opener closes the open block.** In `::: images` … `::: actions` … `:::`, the images block ends where `::: actions` starts. * **A block still open when the reply ends renders** as if it had been closed. For `actions`, `images`, `links`, `payment`, `documents`, `reaction`, and `navigate`, the block keeps only its own lines, such as entries, images, or links, and any prose after them shows as normal text. A `list-item` or `horizontal-list-item` card keeps its free text as the description. While a reply streams, a block appears once its closer arrives, and a half-written marker line is held back, so the end user never sees `::: act` flash on screen. ## Code Nothing inside code is a block. A block written inside a fenced code block (` ``` ` or `~~~`) or inside inline code shows as code, so you can quote the syntax in a reply: ````text theme={null} To show buttons, write: ``` ::: actions - Yes ::: ``` ```` A line that opens and closes its own triple backticks, such as ` ```::: actions``` `, is inline code, not a fence. ## Text around and inside blocks * **Line breaks:** a literal `\n` (a backslash followed by `n`) in the reply becomes a line break, except inside inline code or a code block, where it stays as written. * **Several blocks of one kind** are all read. The apps show each one where it sits; the messaging channels and email merge them, so two `actions` blocks become one set of choices with repeats removed. See each block's page for the channels that send only the first payment link. * **Prose inside a block** whose body is a list, such as `Pick one:` inside `::: actions`, shows as text just before the block. * **Unknown names:** the marker lines of a block the surface doesn't know, such as `::: summary`, are removed and its body shows as normal text. * **Each part of a message is read on its own.** A reply made of several text parts never has a block that starts in one part and ends in the next. ## Body rules Each block's page lists its fields. A few rules apply across them: | Rule | Detail | | - | - | | Card headings | `# Title` and `#Title` are the same, as are `## Subheading` and `##Subheading`. The first `##` line is the subheading; later ones stay in the description. | | Entries | `actions` entries start with `-`, `*`, `•`, or `+`. Empty and repeated entries are removed. | | Links in a card | A `[label](url)` in a card's description stays a link. | | Image URLs | `images`, card images, `payment`, and `documents` accept `http` and `https` URLs only. | | Link URLs | `links` accepts `http`, `https`, `mailto:`, and `tel:`. | | `navigate` | A relative path on your site, such as `/pricing`. | | Other schemes | Anything else, including `javascript:` and `data:`, is removed. | | Broken URLs | A URL split across two lines inside `(…)` is joined back together. | ## See also * [Response formatting](/channels/formatting/overview) — which blocks exist and where each one renders * [list-item](/channels/formatting/list-item) — the card most replies start with * [form](/channels/formatting/form) — the one block whose body is YAML This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.