---
title: "Native browser syntax tools in Bun MCP"
description: "Bounded inert HTML and shared CSS AST parsing inside the existing bun mcp server."
---

# Native browser syntax tools

The existing `bun mcp` registry exposes `html_parse` and `css_parse`. Their
callbacks invoke the same Rust libraries used by the native Bun browser APIs;
they do not launch another process or another MCP server.

`html_parse` calls `bun_browser_dom::parse_html`, applies an optional CSS selector
(default `*`) and returns element IDs, tag names and text previews. Set
`include_html: true` for HTML serialization previews. HTML5 recovery and entity
decoding come from the browser DOM. Script elements remain inert. IDs belong to
this result only; a subsequent call parses an independent document.

```json
{
  "name": "html_parse",
  "arguments": {
    "html": "<table><tr><td>A &amp; B</table>",
    "selector": "table > tbody > tr > td",
    "include_html": true,
    "limit": 20
  }
}
```

`css_parse` calls `bun_css::browser::parse_stylesheet`, the shared Bun
browser/bundler AST projection. It returns ordered rule preludes and optional
bodies, including statement at-rules. It enables author-style error recovery
without minification, CSS-module rewriting or URL resolution.

```json
{ "name": "css_parse", "arguments": { "css": "@layer reset, app; .a { height: 7px }", "limit": 20 } }
```

## Limits and errors

Inputs are limited to 2 MiB of UTF-8 per call. Selectors contain at most 512 UTF-8
bytes. The integer result limit is 1–100, default 20. Invalid input types,
selectors or limits return the existing native registry's MCP `isError` tool
result with an `Invalid arguments` diagnostic. Malformed protocol requests and
unknown tool names retain the server's JSON-RPC errors. A fatal CSS
parser/printer failure also returns an MCP `isError` result.

HTML text previews contain at most 512 bytes; optional HTML previews and CSS
rule bodies contain at most 1024 bytes. CSS preludes contain at most 512 bytes.
UTF-8 boundaries are preserved. A 48 KiB JSON budget may reduce the returned
count further. `matches`/`rules_total`, `returned`, `truncated` and per-field
truncation flags distinguish full counts from previews. Existing MCP token
pagination still applies to the text result; previews are not full-document
exports and field truncation does not have a resumable cursor.

Both tools are read-only, idempotent and closed-world. They do not fetch URLs,
execute JavaScript, create persistent sessions, navigate pages, compute the
cascade, render screenshots or expose host permissions to HTML. DOM trees and
CSS arenas remain local to the call. No V8 isolate, JS reference or browser
session is stored in MCP's shared state.

## Qualification

Run the existing protocol harness against the candidate on the declared SSH
VPS:

```sh
bun bd test test/cli/mcp/mcp.test.ts -t "native browser MCP"
```

The harness covers HTML5 recovery, inert scripts, isolation between calls,
shared CSS recovery/URLs/order, invalid-argument errors, UTF-8 previews, count
limits and output budgets. Source wiring alone does not qualify candidate
runtime behavior. Full page-navigation/browser-runtime MCP migration remains
separate unfinished work.