fastbrowse 0.2.0

A browser agent that picks instead of generating.

Jev chooses each action, an LLM plans and reads, and code owns verification, safety and secrets.

Why

Most browser agents generate each action from a screenshot. fastbrowse indexes the page into candidates and has Jev, a choice model, pick one, so it cannot click something that was never on the page. Every claim in an answer cites a quote stored verbatim from the page, and Jev checks each claim against its quote.

Six live tasks, two passes each, against hosted Browser Use on the same day: the latest httpx version on PyPI (as text and as structured output), the top Hacker News story, httpx's license on GitHub, the year of Gödel's incompleteness theorems via Wikipedia search, and a saucedemo login and add-to-cart. Each answer is graded against the live source (method):

Speed. fastbrowse beats hosted Browser Use on median and mean at about a fiftieth of the cost. Best successful run per task, fastbrowse against hosted: pypi-version 9.5s against 18.1s, pypi-structured 11.8s against 15.7s, saucedemo-cart 20.0s against 90.7s; hosted is still ahead by 1 to 2.6s on hn-top, github-license and wiki-godel. What did it: a direct address for the task proposed while the start page loads, a plan written from the task alone on a small model, settling on DOM quiet instead of every image and tracker, short facts picked by Jev, hedged requests against provider tails, and no low-confidence recovery for steps that do not act (details).

Twelve runs is a smoke test, not a benchmark: single runs swing by 4 to 5 seconds, and one wiki-godel run took 34s on a slow read. passed counts only complete; the hosted miss ended with "Task ended unexpectedly".

How it works

Jev never writes an action, it picks one of the candidates on the page, so it cannot click something that is not there. The LLM plans, reads and writes. Code owns the gates: irreversible actions stop without --authorize, secrets reach models by name only, and every claim in an answer cites a quote from the page. More in docs/design.md.

How it compares

jev-ultrafast is Browser Use's navigation agent, with a measured 7.1 second Google Flights run, and fastbrowse shares its core techniques (one browser call per page read, operation and target chosen in one Jev request). The column describes its main branch as of 2026-09-18; an experimental branch adds a planner with requirement checks.

Try it

Needs Python 3.14, uv, and Chrome (not needed with --cloud).

git clone https://github.com/agent-labs-dev/fastbrowse.git && cd fastbrowse
uv sync
cp .env.example .env            # add AI_GATEWAY_API_KEY or TYPESAFE_API_KEY, and OPENROUTER_API_KEY
uv run fastbrowse "What is the title of the top story right now?" --start https://news.ycombinator.com/

   0 read  -> executed
complete ($0.0071, 1 steps)
The top story on Hacker News is titled "...".

Steps go to stderr and the answer to stdout. --json prints every step, the quotes behind the answer, and cost by component.

export SAUCE_PASSWORD=secret_sauce
uv run fastbrowse "Log in as standard_user with the saved password and add the backpack to the cart." \
  --start https://www.saucedemo.com/ --secret password=SAUCE_PASSWORD --authorize

Signed-in sites

Sign in once by hand in a profile of its own, then point runs at it. The agent reuses the session and never sees a password.

google-chrome --user-data-dir="$HOME/.fastbrowse/amazon" https://www.amazon.com/   # sign in, then close Chrome
uv run fastbrowse "Add a UGREEN USB-A to USB-C cable, 2m, to my cart." \
  --start https://www.amazon.com/ --profile ~/.fastbrowse/amazon --headed

Or sign in from your vault: with the Bitwarden CLI signed in, name the item. The models see only the names username and password; the values stay in this process and are typed only on a site the item's saved URIs cover. With --profile, later runs stay signed in.

export BW_SESSION="$(bw unlock --raw)"
uv run fastbrowse "Add a UGREEN USB-A to USB-C cable, 2m, to my cart." \
  --start https://www.amazon.com/ --bitwarden Amazon --profile ~/.fastbrowse/amazon --headed

Models

The LLM defaults to google/gemini-3.8-flash at low reasoning effort, with google/gemini-3.5-flash-lite for planning, proposing a direct address and typing field text. Override with FASTBROWSE_LLM_MODEL (every purpose), FASTBROWSE_LLM_MODEL_<PURPOSE> (PLAN, READ, FIELD_TEXT, SHORTCUT, RECOVER, COMPOSE, VERIFY) and FASTBROWSE_LLM_REASONING (low, medium, high). Flash-lite for every purpose is faster but scored 8/12 live, so it is used only where its output is checked downstream.

Results

The exit code is 0 only for complete.

Embed it

import asyncio

from pydantic import BaseModel

from fastbrowse import run_task
from fastbrowse.models import Limits


class Release(BaseModel):
    package: str
    version: str


async def main() -> None:
    result = await run_task(
        "Find the httpx package and report its name and latest released version.",
        start="https://pypi.org/",
        output_schema=Release,
        limits=Limits(max_dollars=0.10),
    )
    print(result.status, result.data, f"${result.cost.known_dollars:.4f}")
    for evidence in result.evidence:
        print(f'  "{evidence.quote}" from {evidence.url}')


asyncio.run(main())

complete {'package': 'httpx', 'version': '0.28.1'} $0.0114
  "httpx 0.28.1" from https://pypi.org/project/httpx/
  "pip install httpx" from https://pypi.org/project/httpx/

run_task builds the browser and clients, runs the agent, and closes the browser on every path. The result has status, answer, data, evidence, final_url and an itemized cost.

Jev comes from Typesafe directly or through the Vercel AI Gateway, whichever key is set; with both, FASTBROWSE_JEV_SOURCE picks one, and FASTBROWSE_JEV_BASE_URL sends it through a proxy. Anything else, such as a cache or a recorded fixture, can be passed as run_task(jev=...): an object with one evaluate(state, questions) method (the JevClient protocol in jev.py). The LLM works the same way.

Safety model

• Irreversible actions. Before any button or submit, Jev is asked whether it commits something that cannot be undone. Without --authorize, a yes stops the run. This is a classifier, not a guarantee: a page can word a harmful control to look harmless.
• Secrets. Models see secret names only. A value is resolved at the moment of typing, only for its declared origin, and is redacted from everything the run returns, in raw, URL-encoded and JSON-escaped form. A password field is typed only from a stored secret, never generated.
• Page content is data. Every prompt says so, and completion is judged against quotes and page state rather than the model's say-so.

Evals and development

uv sync --all-extras                                         # the hosted-arm SDK too, which pyright checks
uv run python -m fastbrowse.evals.runner                     # local fixtures, under half a cent a task
uv run --extra browser-use python -m fastbrowse.evals.live   # live head-to-head; --arms fast skips hosted
uv run ruff format . && uv run ruff check . && uv run pyright && uv run pytest
uv run python scripts/no_slop.py && uv run vale sync && uv run vale README.md docs src scripts tests

Grades come only from things the agent cannot write: requests the fixture server recorded, truth from a site's own API, or the URL the browser ended on. See docs/evals.md, docs/design.md, and docs/jev.md for every Jev assumption checked against Typesafe's documentation.

License

MIT, copyright Agent Labs. Adapted third-party code is credited in NOTICE.