harness-browser 0.1.2

AI-friendly browser automation via CDP with profile-based login persistence

harness-browser

AI-friendly browser automation via Chrome DevTools Protocol (CDP).

English · 中文

An agent-first browser runtime built on pure CDP. Predictable DOM snapshots, persistent profile sessions, and a typed Python API designed for LLM tool-calling — no Playwright, no driver layer in between.

Why harness-browser

Concern	How we address it
Token cost	4-level DOM with interactive mode (~200–500 tokens) returns only clickable/typeable elements with stable refs — not raw HTML
Stable element targeting	Refs (btn_2, inp_search) survive layout reflows and are auto-invalidated on navigation, so the agent never points at a stale node
Login persistence	One Chrome user-data-dir per profile under ~/.harness-browser/profiles/<name>/ — log in once, every subsequent run reuses cookies and storage
No Playwright tax	Pure CDP over WebSocket (websockets>=12.0); no browser binaries shipped, no patched Chromium, no driver layer
Observability	Every action emits ActionMetrics (duration_ms, dom_nodes_scanned, estimated_tokens, screenshot_size_kb) plus before_action / after_action / action_error / page_navigated hooks
Configuration	Seven BROWSER_USE_* env vars cover paths, ports, timeouts, and remote/Docker Chrome via BROWSER_USE_CDP_WS_URL — no code changes between dev, CI, and prod
Agent integrations	Stateless browser_tool(action=..., profile=...) for any framework, MCP server (python -m harness_browser.mcp_server), and a ready-to-copy Claude Code skill in skills/

Features

• Pure CDP — direct WebSocket connection, no Playwright dependency
• Profile-based login persistence — Chrome user-data-dir per profile, cookies/sessions reused across runs
• 4-level DOM output — minimal (~50 tokens), interactive (~200–500 tokens), full (~1000–3000 tokens), structured (JSON)
• Ref system — stable element references across actions, invalidated on navigation
• Hook system — before_action, after_action, action_error, page_navigated
• Per-action metrics — duration_ms, estimated_tokens, screenshot_size_kb
• Environment-variable configuration — all paths, ports, and timeouts configurable without code changes
• Remote/Docker Chrome support — bypass launcher via BROWSER_USE_CDP_WS_URL
• MCP Server — expose actions as MCP tools for Claude Code and other MCP clients
• Drop-in Claude Code skill — copy skills/harness-browser/ into any agent project
• Strict typing — mypy strict, ruff clean, 34 unit tests covering DOM, refs, hooks, settings, and CDP framing

Requirements

• Python 3.11+
• Chrome or Chromium

# Ubuntu/Debian
sudo apt install chromium-browser

# macOS
brew install --cask google-chrome

Installation

pip install harness-browser

Quick Start

Python API

import asyncio
from harness_browser import BrowserSession

async def main():
    async with await BrowserSession.create(profile="default") as sess:
        await sess.navigate("https://example.com")
        result = await sess.dom_tree(level="interactive")
        print(result.content)
        # → [ref=inp_1] input[text] placeholder="Search"
        # → [ref=btn_2] button "Go"
        await sess.click(ref="btn_2")

asyncio.run(main())

AI Framework Usage (stateless)

from harness_browser import browser_tool

# All calls route to the same session by profile name
result = await browser_tool(action="navigate", url="https://github.com", profile="work")
result = await browser_tool(action="dom_tree", level="interactive", profile="work")
result = await browser_tool(action="click", ref="btn_search", profile="work")
result = await browser_tool(action="type", text="harness", profile="work")

DOM Levels

Level	Tokens	Use case
minimal	~50	Confirm page loaded, check title/URL
interactive	~200–500	Find clickable/typeable elements (default)
full	~1000–3000	Read page content
structured	varies	JSON for programmatic processing

Login State Reuse

Profiles persist Chrome sessions in ~/.harness-browser/profiles/<name>/:

# First run: navigate to login page, log in manually
await browser_tool(action="navigate", url="https://github.com/login", profile="github")

# All future runs: login state reused automatically
await browser_tool(action="navigate", url="https://github.com/settings", profile="github")

Hook System

async with await BrowserSession.create(profile="work") as sess:
    @sess.on("before_action")
    async def log_action(event):
        print(f"[{event['action']}] starting")

    @sess.on("after_action")
    async def log_metrics(metrics):
        print(f"  done in {metrics.duration_ms}ms (~{metrics.estimated_tokens} tokens)")

    await sess.navigate("https://example.com")

MCP Server

python -m harness_browser.mcp_server

Add to Claude Code settings.json:

{
  "mcpServers": {
    "harness-browser": {
      "command": "python",
      "args": ["-m", "harness_browser.mcp_server"],
      "env": {
        "BROWSER_USE_MODE": "auto",
        "BROWSER_USE_PROFILES_DIR": "/data/browser-profiles"
      }
    }
  }
}

All BROWSER_USE_* environment variables can be passed through the MCP env block — this is the recommended way to configure mode, profile location, and remote CDP endpoints for an MCP-hosted browser.

Available MCP tools: browser_navigate, browser_dom_tree, browser_screenshot, browser_click, browser_type, browser_eval_js.

Screenshots

screenshot writes a PNG to disk and returns its path — never raw base64. That keeps token usage flat regardless of image size and lets dashboards preview the file directly.

# default: timestamped file in BROWSER_USE_SCREENSHOTS_DIR
result = await sess.screenshot()
print(result.content)
# → /home/user/.harness-browser/screenshots/harness-1779462725763.png

# full scrollable page (uses Page.getLayoutMetrics + captureBeyondViewport)
await sess.screenshot(full_page=True)

# crop to a single element discovered via dom_tree
await sess.screenshot(element_ref="btn_2")

# pin the file path — every call overwrites the same file
await sess.screenshot(path="/tmp/latest.png")

result.metadata carries the page url / title / width / height / size_kb / full_page so callers can render context without an extra Runtime.evaluate.

Claude Code Skill

A ready-to-use skill ships under skills/:

# Copy into another agent project as a Claude Code skill
cp -r skills/harness-browser /path/to/other-project/.codebuddy/skills/
# or the Chinese variant
cp -r skills/harness-browser-zh /path/to/other-project/.codebuddy/skills/

The skill teaches the agent the standard navigate → dom_tree → click/type loop and the ref discipline (always re-fetch DOM after navigation).

Actions Reference

Action	Required	Optional
navigate	url
dom_tree		level (default: interactive)
screenshot		element_ref, full_page, path
click	one of: ref, selector, x+y
type	text	ref
scroll		direction, amount
hover	ref
eval_js	expression
go_back
go_forward
reload
list_tabs
new_tab		url
switch_tab	tab_id
close_tab		tab_id
close_session

Configuration

All settings can be configured via environment variables. No code changes required.

Environment Variable	Default	Description
BROWSER_USE_PROFILES_DIR	~/.harness-browser/profiles	Root directory for Chrome user-data-dirs
BROWSER_USE_SCREENSHOTS_DIR	~/.harness-browser/screenshots	Directory where the screenshot action writes PNG files
BROWSER_USE_CDP_HOST	localhost	Host or IP serving Chrome's CDP HTTP/WebSocket endpoint
BROWSER_USE_CDP_PORT_START	9222	First CDP debug port assigned to profiles
BROWSER_USE_MODE	auto	Launch mode: auto / headed / headless. auto picks headed when DISPLAY/WAYLAND_DISPLAY is set (or on macOS/Windows), else headless
BROWSER_USE_CHROME_BIN	auto-detect	Absolute path to Chrome/Chromium executable
BROWSER_USE_CDP_TIMEOUT	30.0	Seconds to wait for a CDP command response
BROWSER_USE_LAUNCH_RETRIES	20	Times to poll Chrome after launch
BROWSER_USE_LAUNCH_DELAY	0.25	Seconds between launch poll attempts
BROWSER_USE_CDP_WS_URL	—	Direct connect: bypass launcher, connect to this WebSocket URL

Common scenarios

Custom profile storage:

export BROWSER_USE_PROFILES_DIR=/data/browser-profiles

Force headed or headless mode (default is auto, which picks based on DISPLAY):

export BROWSER_USE_MODE=headless   # always headless (CI, containers)
export BROWSER_USE_MODE=headed     # always headed (force a window even without DISPLAY)
# unset / "auto" → headed when a desktop is detected, headless otherwise

Non-standard Chrome path:

export BROWSER_USE_CHROME_BIN=/opt/google/chrome/chrome

Connect to a remote or Docker Chrome (bypasses launcher entirely):

# Start Chrome with --remote-debugging-port=9222 --remote-debugging-address=0.0.0.0
export BROWSER_USE_CDP_WS_URL="ws://remote-host:9222/devtools/browser/xxxxxxxx"

Talk to Chrome on another host or container (keeps the attach/launcher logic, just changes the host):

# Chrome already running with --remote-debugging-port=9222 --remote-debugging-address=0.0.0.0
export BROWSER_USE_CDP_HOST=10.0.0.42
# harness will hit http://10.0.0.42:9222/json/version and use that page's WS URL

Override settings in code (useful for testing or multi-instance setups):

from harness_browser import BrowserSession, HarnessSettings

cfg = HarnessSettings(
    cdp_port_start=9300,
    cdp_timeout=60.0,
    profiles_dir="/data/profiles",
)
sess = await BrowserSession.create(profile="work", settings=cfg)

Development

# Clone
git clone https://git.woa.com/orcakit/browser-use.git
cd browser-use

# Install with dev extras
uv sync --extra dev

# Install pre-commit hooks
pre-commit install

# Run tests
make test

# Lint + type check
make lint

# Format
make format

# Build wheel
make build

Contributing

See CONTRIBUTING.md.

License

MIT