h5i-browser

A lightweight headless browser for coding agents, written in Rust. No Chromium, no V8.

It is headless but it still renders. A DOM-only parser never rasterises the page; this one does, so it can take screenshots and stream a live view. The engine is assembled rather than written. Blitz owns the DOM and drives Stylo for CSS, vello_cpu rasterises on the CPU, and Boa runs page script. Every request is policy-checked and receipted before it reaches the wire.

Build

cargo build --release

The binary is h5i-browser. Run h5i-browser --help, or h5i-browser <command> --help, for the full command line.

Usage

Open a page

Load a page and report what it says and what it tried to reach. Nothing remote is reachable until an origin is granted with --allow:

# Render a local file (loopback and local files need no grant)
h5i-browser open ./page.html

# Fetch a remote page, granting just its origin
h5i-browser open --allow https://example.com https://example.com

# The page as a model should read it, plus a screenshot
h5i-browser open --allow https://example.com --text --screenshot shot.png \
  https://example.com

# Run page script (opt-in), with a request log on disk
h5i-browser open --allow https://example.com --script \
  --receipts requests.jsonl https://example.com

Drive a live session

serve holds one page open and streams a live view over WebSocket; session verbs act on that held page (this is the agent-facing half):

# In one terminal: hold a page open and advertise the control port
h5i-browser serve --allow https://example.com \
  --stream-file /tmp/view.json https://example.com

# In another: read the page, act on it, read it again
h5i-browser session snapshot
h5i-browser session type @e3 "hello"
h5i-browser session submit @e3
h5i-browser session snapshot

# Save what the session did, then replay it against a fresh serve
h5i-browser session script --save run.txt
h5i-browser replay run.txt

Inspect the engine

# What this engine can and cannot do, as JSON
h5i-browser capabilities

# Fonts, proxy, and what the current policy would allow
h5i-browser doctor --allow https://example.com