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# jev-ultrafast-mcp
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**English** · [简体中文](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/README.zh-CN.md)
**Hand the browser work off — an MCP server that drives the page for your agent.**
Your agent should not be opening the browser at all. A browser task goes over whole — the URL, the
goal, and the check that proves it — and the loop runs on the server. One tool call instead of
twenty. Three seconds instead of a minute. A cent instead of a frontier model's context. And it
never invents a target: it picks from what the page actually has, and the server refuses rather than
guesses.
**What it cost.** A cent for the whole day, and a cent is all of it:
## Quick start
Three commands, then restart your client.
```bash
git clone https://github.com/jiawei686/jev-ultrafast-mcp.git
cd jev-ultrafast-mcp
python3 -m venv .venv
.venv/bin/pip install -e . # Windows: .venv\Scripts\pip install -e .
python scripts/install.py # finds your MCP clients and writes their config
```
`install.py` looks for WorkBuddy, Claude Code, Claude Desktop, Codex CLI, Cursor, VS Code, Cline,
Windsurf and Gemini CLI, and writes the format each one expects — **merging** into your existing
config and saving a `.bak` first. Needs Python ≥ 3.10 and any Chromium-family browser.
Restart the client, and then just say what you want:
> **You:** Set this form to 3 adults, tick *Nonstop only*, then submit it.
> **Your agent:** `browser_goal(goal=…, url=…, verify=[…])` — **one** call; the page is opened and the
> loop runs server-side, and the result is checked by code afterwards.
> ([what that costs](#cheap-and-fast-and-here-is-the-bill))
> **You:** Open example.com and tell me what the page says.
> **Your agent:** `browser_open` → reads the element table → answers — a look is not a task, so it
> does not need the model.
> ([verbatim run](#what-a-session-actually-looks-like))
**Restarted and the tools are not there?** Some clients make you approve the server once. In
WorkBuddy that is *Connectors → Custom connectors → **Trust***. The approval is remembered against
the config itself, so if you later edit the config it asks once more.
## Installation
No checkout needed if you would rather install it as a package. It is on PyPI, so the name is
enough:
```bash
uvx jev-ultrafast-mcp # run it straight from PyPI, nothing installed
pip install jev-ultrafast-mcp # or install it yourself
```
A client config wants a stable interpreter path rather than `uvx`'s cache, so:
```bash
python3 -m venv ~/.jev-ultrafast-mcp/venv
~/.jev-ultrafast-mcp/venv/bin/pip install jev-ultrafast-mcp
```
That gives you a `jev-ultrafast-mcp` console script and a stable interpreter path to put in a
client config — verified against the latest `mcp` SDK on Python 3.13, every one of the ten tools
listed.
`scripts/install.py` is the other half: it finds your MCP clients and writes the config each one
expects, merging into the existing file and saving a `.bak` first.
```bash
python scripts/install.py --list # what is installed, and the file each one reads
python scripts/install.py --print # show the config it would write, change nothing
python scripts/install.py -c cursor,codex # only these two
python scripts/install.py --headed # keep a visible browser window
python scripts/install.py --allow-domains example.com,*.example.org
python scripts/install.py --uninstall # take the entry back out
```
Runtime dependencies: `mcp`, `websockets`, `httpx`. No Playwright, no Selenium, no
`browser-harness`.
---
## Cheap and fast, and here is the bill
A three-step goal on a real page, driven by `browser_goal`. This is everything your agent sent and
everything it got back — **one** turn, and the page never entered its context:
```
browser_goal(
goal="On this flight search form: set Passengers to 3 adults, tick the 'Nonstop only' "
"checkbox, then submit the search. Do not type into any city field.",
verify=[{"type": "text_contains", "text": "3 adults · nonstop"}],
)
goal: On this flight search form: set Passengers to 3 adults, …
status: done
steps: 3
turbo: 4 decisions · 14,626 tokens · 1.8s model + 1.1s page · 3.3s wall
trace:
1. SELECT e6 Passengers → ok (759ms model / 30ms browser)
2. TOGGLE e7 Nonstop only → ok (336ms model / 692ms browser)
3. CLICK e8 Search → ok (370ms model / 410ms browser)
4. DONE (conf 0.93)
verified: PASS
ok text_contains: '3 adults · nonstop' found in page text
```
**What the second time costs.** Nothing. The second time is a recorded macro, and a macro makes no
model calls at all — it does not even need a key.
**How long it took.** 3.3 s wall for the whole goal: 1.8 s of model, 1.1 s of page. Every run prints
that line itself, so the numbers are checkable rather than persuasive.
That run is not a mock-up. `scripts/turbo_check.py` reproduces it against a real Chrome and the real
model, and then **checks the page with code** rather than trusting the model's account of its own
work. The three actions — plus the reading and re-reading between them — all happened on the server.
Your agent spent one turn and never saw an element table.
The division of labour is the whole design decision, so it is yours to make per task:
| | agent drives | `browser_goal` drives |
|---|---|---|
| Tool calls for a 3-step flow | 6+ (observe, act, observe, act…) | **1** |
| Who holds the page in context | your agent | **the decision model, server-side** |
| Per-step cost | one agent turn | one typed request, no screenshot |
| Who names the target | the model writes a selector | **the model picks a `ref` from the page's own table** |
| If it goes wrong | a wrong click, usually silent | **the server refuses, with the reason** |
| Knowing it worked | the model's summary | **code-checked assertion, which wins the disagreement** |
| Second time around | run the model again | **macro replay, zero model calls** |
---
## What it is
Browser automation usually makes the *agent* do the driving: read the page, pick one element, act,
read again to see whether that worked. Ten clicks is ten turns, the page passes through the agent's
context every time, and a mis-click rarely announces itself.
This server can take that job instead. `browser_goal` is **one** tool call from your agent; the loop
runs here, server-side, with Jev — TypeSafe's decision model — choosing each step. The model never
writes a selector: it picks among the elements the page actually has, and the server refuses
anything that is not on the page rather than guessing. When it stops, `browser_assert` checks the
page it left behind in code, and a passing assertion outranks the model's own account of what it did.
Four things follow from that:
- **One call, not one per click.** The run above took a 3-step goal on a real page through
**4 decisions, 14,626 tokens, 1.8 s model + 1.1 s page, 3.3 s wall** — for one turn of your
agent's context.
- **Accurate by construction.** A target is a `ref` from a numbered table of what is on the page,
not a selector or a coordinate the model invented, and the action is re-checked against the page
before it runs.
- **Free after the first run.** Record the path once; replay costs zero model calls, works with no
key at all, and refuses to proceed when the page no longer matches.
- **Text, not pixels.** No screenshots, no HTML dumps. It speaks CDP straight to a Chrome you
already have — no Playwright, no Selenium, no screenshot pipeline.
Everything *except* `browser_goal` — `browser_open`, `browser_observe`, `browser_act`,
`browser_assert`, `browser_macro` — needs no key, no account, and no network beyond the page itself,
from any MCP client: WorkBuddy, Claude Code, Codex, Cursor or VS Code. If you would rather keep your
hands on the wheel, that whole surface is still here.
```
browser_open → element table → browser_act [refs] → browser_assert
```
Inspired by [`browser-use/jev-ultrafast`](https://github.com/browser-use/jev-ultrafast) and
TypeSafe's typed-question API. Independent project, not affiliated with either — see
[`docs/DESIGN.md`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/docs/DESIGN.md) for what is different and why.
**Contents** ·
[Quick start](#quick-start) ·
[Cheap and fast](#cheap-and-fast-and-here-is-the-bill) ·
[What it is](#what-it-is) ·
[What a session looks like](#what-a-session-actually-looks-like) ·
[Connecting an agent](#connecting-an-agent) ·
[What you can ask it to do](#what-you-can-ask-it-to-do) ·
[What the agent reads](#what-the-agent-actually-reads) ·
[Why another browser MCP?](#why-another-browser-mcp) ·
[Tools](#tools) ·
[Configuration](#configuration) ·
[FAQ](#faq) ·
[Try it without an agent](#try-it-without-an-agent) ·
[See also](#see-also)
---
## What a session actually looks like
You say:
> Open example.com and tell me what the page says.
Your agent does this, and this is everything it sees:
```
browser_open("https://example.com")
[obs#1] https://example.com/ "Example Domain" scroll=0/216 reachable=1/1
e1 lnk More information...
browser_observe()
[delta#2] … 1 element
= no change (1 element)
```
Then it answers. No screenshot was taken, no HTML was dumped, and the page never entered a model's
context: your agent read the table and answered.
A more realistic one — searching a real site, with your agent doing the driving:
```
browser_open("https://duckduckgo.com")
e4 cmb* Search with DuckDuckGo ▸ ""
browser_act([{type, ref: "e4", text: "python asyncio tutorial"}, {keys, key: "Enter"}])
→ 2/2 ops ok, one round trip, page navigated
browser_observe()
[delta#3] https://duckduckgo.com/?…&q=python+asyncio+tutorial reachable=9/59
+ e5 lnk Python Asyncio Tutorial
+ e6 lnk Async IO in Python: A Complete Walkthrough
…
43 new, 0 changed, 0 gone
browser_assert([{url_contains, text: "q="}, {count_at_least, role: "link", min: 5}])
PASS
```
That is a verbatim run against the live web — `scripts/live_check.py` reproduces it end to end.
---
## Connecting an agent
| Client | Config file `install.py` writes | After installing |
|---|---|---|
| **WorkBuddy** | `~/.workbuddy-ai/mcp.json` (older installs: `~/.workbuddy/mcp.json`) | restart, then Connectors → Custom connectors → **Trust** |
| **Claude Code** | `~/.claude.json` (user scope) | or `claude mcp add --scope user …` |
| **Claude Desktop** | `~/Library/Application Support/Claude/claude_desktop_config.json` | quit the app fully and reopen |
| **Codex CLI** | `~/.codex/config.toml` | `codex mcp list` to confirm |
| **Cursor** | `~/.cursor/mcp.json` | reload the window |
| **VS Code (Copilot)** | `…/Code/User/mcp.json` | Agent mode only — not Ask/Edit |
| **Cline** | `…/Code/User/globalStorage/saoudrizwan.claude-dev/settings/cline_mcp_settings.json` | reload the window |
| **Windsurf** | `~/.codeium/windsurf/mcp_config.json` | reload the window |
| **Gemini CLI** | `~/.gemini/settings.json` | `gemini mcp list` to confirm |
<details>
<summary><b>Manual setup</b> — if you would rather not run the installer</summary>
Every client below needs the same three facts: an absolute interpreter path, the module, and one
environment variable. Substitute your own path for `/ABS/PATH`.
**WorkBuddy** — `~/.workbuddy-ai/mcp.json`
WorkBuddy reads its config directory from `WORKBUDDY_CONFIG_DIR` and otherwise falls back to
`~/.workbuddy`. A machine can carry both — an older app alongside the current one — and writing
the one the app is not reading registers nothing and logs nothing. `install.py` resolves this
the same way the app does and tells you when it had to choose.
Then **restart the app before looking for the tools.** The config file is only watched if it
already existed when the app launched, so a freshly created one is invisible until the next
start. After the restart the server appears as a first connection, and you approve it once.
```json
{
"mcpServers": {
"jev-ultrafast-mcp": {
"command": "/ABS/PATH/jev-ultrafast-mcp/.venv/bin/python",
"args": ["-m", "jev_ultrafast_mcp"],
"env": { "JEVMCP_HEADLESS": "1" }
}
}
}
```
**Claude Code**
```bash
claude mcp add --scope user jev-ultrafast-mcp \
--env JEVMCP_HEADLESS=1 \
-- /ABS/PATH/jev-ultrafast-mcp/.venv/bin/python -m jev_ultrafast_mcp
```
Or write the same `mcpServers` object by hand: `~/.claude.json` for user scope, `.mcp.json` in a
project for team scope (committed to git).
**Codex CLI** — `~/.codex/config.toml`. Codex uses TOML, and the table is `mcp_servers`, not
`mcpServers`:
```toml
[mcp_servers.jev-ultrafast-mcp]
command = "/ABS/PATH/jev-ultrafast-mcp/.venv/bin/python"
args = ["-m", "jev_ultrafast_mcp"]
startup_timeout_sec = 20
[mcp_servers.jev-ultrafast-mcp.env]
JEVMCP_HEADLESS = "1"
```
The same entry works as `codex mcp add jev-ultrafast-mcp --env JEVMCP_HEADLESS=1 -- /ABS/PATH/…/python -m jev_ultrafast_mcp`.
**Cursor** — `~/.cursor/mcp.json` for every project, `.cursor/mcp.json` for one. Same `mcpServers`
object as WorkBuddy.
**VS Code (Copilot)** — `.vscode/mcp.json`, or Command Palette → *MCP: Open User Configuration* for
all workspaces. VS Code is the odd one out twice over: the key is `servers`, and every entry must
declare `"type": "stdio"` or it is silently skipped.
```json
{
"servers": {
"jev-ultrafast-mcp": {
"type": "stdio",
"command": "/ABS/PATH/jev-ultrafast-mcp/.venv/bin/python",
"args": ["-m", "jev_ultrafast_mcp"],
"env": { "JEVMCP_HEADLESS": "1" }
}
}
}
```
**Claude Desktop** — `claude_desktop_config.json` (`%APPDATA%\Claude\` on Windows), same
`mcpServers` object. Restart the app from the tray, not just the window.
</details>
The browser does not start until the first `browser_open`, and the tab it drives is a **background
tab it owns** — focus emulation keeps animations and menus running without stealing your window.
### Making sure your agent actually hands it over
Pointing the client at the server is half of it. The other half is that the agent has to *know* to
hand over — and that part is not automatic everywhere.
A server sends a short `instructions` block when a client connects, and this one's first rule is
that a browser task goes to `browser_goal`, in one call, with the URL. Clients that read it behave.
Not all of them do: **WorkBuddy delivers a server's tools to the model and drops its
`instructions`** — measured, not assumed: the tool schemas are in the recorded request payload, the
instructions text is not. A host in that position does the obvious thing and drives the page itself,
one call per click, which is exactly the work this server exists to take away.
Two ways to close that gap — either is enough:
1. **Install the skill.** [`skills/jev-ultrafast-mcp/SKILL.md`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/skills/jev-ultrafast-mcp/SKILL.md)
carries the same rule in the form a client reads as a skill, plus the traps that waste a run.
Copy it into your client's skills folder (`~/.workbuddy/skills/` for WorkBuddy):
```bash
mkdir -p ~/.workbuddy/skills/jev-ultrafast-mcp
cp /path/to/jev-ultrafast-mcp/skills/jev-ultrafast-mcp/SKILL.md ~/.workbuddy/skills/jev-ultrafast-mcp/
```
2. **Or say it once.** "Browser tasks go to `browser_goal`" is enough for most sessions — an agent
told that keeps doing it.
**How to tell it took.** Ask for something that needs a click. If `browser_goal` comes back with a
`url` inside the call, the handoff is live. If the agent opens the page and starts walking the
element table for you instead, the rule did not arrive — install the skill, or say it once.
---
## What you can ask it to do
| Say this | What happens |
|---|---|
| "Fill in this form and submit it" | **handed over whole** — one `browser_goal` carrying the URL, the goal and a check; the loop runs server-side |
| "Walk this flow in staging and tell me if it worked" | the same one call — `verify` decides PASS/FAIL by code, and it outranks the model's own account |
| "Do this same thing again tomorrow" | record a **macro**; replay costs zero model calls and needs no key at all |
| "Open this page and tell me what it says" | a look, not a task: reads the visible text and the controls — no model, no key |
| "Did the deploy actually ship?" | `browser_assert` returns PASS/FAIL, not an opinion |
| "Log in and download last month's invoice" | you log in by hand once; the profile persists |
| "Click through checkout in staging" | payment-like buttons come back as `needs_confirmation` |
## What it is **not**
Being clear about this saves everyone time:
- **It never looks at pixels.** A captcha, a chart, a canvas-only app — anything that needs real
visual judgement — is out of scope. Use a screenshot-and-vision agent for those, or use the
`screenshot` op here to capture evidence for a *human*.
- **It is not a scraper framework.** One browser, one session at a time. No proxy rotation, no
concurrency, no crawling at scale.
- **It is not a recorder for humans.** There is no click-to-record UI; macros are recorded by the
agent driving the task normally.
---
## What the agent actually reads
Not a DOM dump, not a screenshot — a table of the controls it can act on. Each row is a `ref`
(element number), a role code, flags, and the accessible name; editable things carry their current
value, and selectable things carry their options:
```
[obs#1] http://127.0.0.1:54409/fixture.html "Ultrafast Fixture" scroll=0/860 reachable=16/16
e1 lnk Home
e2 lnk About
e3 lnk Open popup
e4 inp* Where from? ▸ ""
e5 cmb* Where to? ▸ ""
e6 cmb Passengers ▸ 1 adult opts{1 adult=1 | 2 adults=2 | 3 adults=3 | 4 adults=4}
e7 chk· Nonstop only
e8 btn Search
e10 inp* Password ▸ ""
e11 file CV accept=.pdf,.txt
e12 btn Delete account
```
Flags: `*` editable · `»` off-screen (the server scrolls it into view) · `⊘` covered by something
else · `⊗` present but disabled · `⋮` opens a menu, hover it first · `▾` expanded · `✓`/`·` checked
state. `reachable=16/19` means three controls exist but are covered, off-screen, or disabled right
now — a disabled one is still listed so you can see what the form is waiting for, and is never put to
you as something to click.
After an action it reports **only what changed** — that is the single biggest saving in a long loop:
```
[delta#2] http://127.0.0.1:54409/fixture.html "Ultrafast Fixture" reachable=16/16
~ e4 inp* Where from? ▸ "Zurich" (was "")
~ e7 chk✓ Nonstop only
2 changed, 0 new, 0 gone
```
An action that accomplished nothing is the most expensive thing in an agent loop, because the model
retries it. So it is spelled out in one line:
```
[delta#3] … 16 elements
= no change (16 elements)
```
New rows appear with `+` and disappear with `-`. When two controls share a name, the row carries the
context that tells them apart:
```
+ e17 btn Select @Zurich → Anywhere Option 1 · 1 adult · nonstop Select
+ e18 btn Select @Zurich → Anywhere Option 2 · 1 adult · nonstop Select
```
And a ref that no longer points at anything is refused, with a reason instead of a wrong click:
```json
[{"op": "click", "ok": false, "ref": "e999", "error": "detached"}]
```
### See that table for your own page
The extension in
[`chrome-extension/`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/chrome-extension/README.md)
is a window on it. Load it unpacked, click it on any page, and you get the same rows the model gets —
drawn by the same observer and a port of the same renderer, so a `ref` in the popup means what it
means in a session. The second read of a page renders as a delta, which is how you watch a page
change.
It also **runs a macro with no model in the loop** — the resolver, the dispatcher and the report
writer are all ports of the server's own code, so a replay there is the replay here.
Both halves are why it asks for four permissions and no host access at all: `activeTab` (the tab you
clicked it on, and nothing else), `scripting`, `storage`, and `debugger`. The last one is the cost of
the replay, and it is not a shortcut: `element.click()` produces `isTrusted: false` events a site is
entitled to ignore, so a replayed click that is to be believed has to come through the DevTools
protocol. The extension's own
[README](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/chrome-extension/README.md) says
what that buys, what it costs, and which four ops it declines to do rather than reach outside the tab
you pointed it at.
---
## Why another browser MCP?
Two differences, and the first one is the reason this exists.
**The driving is not the agent's job.** A browser flow is a loop, and in most servers that loop
lives in the calling agent: read the page, name one element, wait, read again. Fine for two steps,
absurd for twenty — twenty turns of an expensive context to do what a smaller model could have done
in one call. Here the loop lives on the server: one `browser_goal` call carries the URL, the goal
and the check, and the agent never opens the browser itself. The manual tools stay for the two cases
that need them — reading a page, which is not a task and should not cost a model call, and the
fallback when no model key is configured.
**The model never invents a target.** Most browser MCP servers hand over CDP primitives —
`click_at_xy`, a CSS selector, `evaluate`. Maximum flexibility, minimum safety: a wrong selector
fails silently or, worse, succeeds on the wrong element. Here a target is a `ref` from a numbered
table of what is on the page, turning that ref into a real click is the server's problem, and the
server refuses rather than guesses. That is also what makes the handoff safe: whatever is driving
is choosing among options the page actually has, so accuracy does not rest on it being careful.
| | primitives-based browser MCP | **jev-ultrafast-mcp** |
|---|---|---|
| Who runs the loop | the calling agent, every step | **the server — one `browser_goal` call** |
| How a target is named | a selector / coordinate / JS the model writes | **a `ref` from an element table** |
| Extra model calls | none | **one small decision model per step, and only inside `browser_goal`** |
| API keys required | none | **none for the browser tools**; a decision-model key only for `browser_goal` |
| Ref lifetime | n/a (agent re-invents each step) | **stable across observations** |
| Re-reading the page | full dump every time | **delta** — `+` added, `~` changed, `-` removed, `= no change` |
| Round trips | one per action | **batched — many ops per call** |
| Ambiguous target | agent guesses | **server refuses with a reason** |
| Shadow DOM / iframes | usually unsupported | **traversed, with frame-offset-aware scrolling** |
| Pages that render late | depends on the agent sleeping | **waits for elements to appear, bounded** |
| Repeating a flow | re-runs the model | **macro replay at zero model cost** |
| Knowing it worked | the model eyeballs the page | **deterministic `browser_assert`, which overrules the model** |
| Destructive clicks | whatever the model decides | **`needs_confirmation`, domain envelope, secret redaction** |
Batching and deltas are not cosmetic. In the bundled end-to-end run, 29 ops and their follow-up
observations cost **15.4 KB** of context, of which **13.6 KB was deltas and 1.8 KB full tables** —
the model re-reads only the part of the page that moved.
---
## Tools
Ten tools. Most sessions need four of them.
| Tool | Description |
|---|---|
| `browser_open` | Open a URL in its own tab, and return the element table |
| `browser_observe` | Re-read the page: a delta, or the full table on demand |
| `browser_act` | Run a list of ops in one round trip, then return a delta |
| `browser_assert` | Deterministic checks on the page, with no model judgement |
| `browser_macro` | Record a flow once, then replay it with zero model calls |
| `browser_goal` | Hand the whole task over: the decision model drives the page |
| `browser_tabs` | List, open, switch and close tabs |
| `browser_sessions` | List the live browser sessions |
| `browser_close` | Tear a session down |
| `browser_doctor` | Self-check: which browser was found, and whether it answers |
### `browser_open(url, session="default", hint="")`
Opens a URL in its own tab and returns the full element table. `hint` restates your goal in one
line and is echoed back.
### `browser_observe(session="default", mode="auto", include_text=True, include_json=False)`
Re-reads the page. `auto` emits a delta; `full` forces the whole table, `delta` forces a diff.
`= no change` means the last action did nothing — **change strategy, do not retry**.
### `browser_act(ops, session="default", dry_run=False, stop_on_error=True, observe_after=True)`
Executes ops in order in **one round trip**, then returns a delta.
| op | fields |
|---|---|
| `click` | `ref` |
| `type` | `ref`, `text`, `clear`=true, `submit`=false, `slow` |
| `select` | `ref`, `value` (option value or label) |
| `toggle` | `ref`, `state` (omit to flip) |
| `hover` / `upload` | `ref` / `ref`, `path` |
| `keys` | `key` (`"Enter"`, `"Meta+A"`, `"ArrowDown"`), or `keys` (a list) — a **bare single character is text**, not a key press, so it is typed into whatever has focus and is refused on a field whose value must not leave the page |
| `scroll` | `dir`, `amount`, `ref` |
| `nav` / `back` / `forward` / `reload` | `url` (for `nav`) |
| `wait` / `wait_for_ref` / `wait_for_text` / `wait_for_load` | `ms` / `ref`,`timeout_ms` / `text` / `timeout_ms` |
| `screenshot` | `path` (a filename inside the shots directory), `full`, `format` (`jpeg` or `png`) |
| `tab` | `action`=`list\|new\|switch\|close`, `target_id`, `index`, `url` |
| `eval` | `js` — only when `JEVMCP_ALLOW_JS=1` |
```json
{"ops": [
{"op": "type", "ref": "e4", "text": "Zurich"},
{"op": "select", "ref": "e6", "value": "3 adults"},
{"op": "toggle", "ref": "e7"},
{"op": "click", "ref": "e8"}
]}
```
A failing op reports why: `occluded`, `detached`, `target_changed`, `page_changed`,
`needs_confirmation`, `blocked_by_policy`. Reach for `browser_observe`, not a retry.
A control whose accessible name matches a confirmation rule (`buy now`, `delete account`,
`unsubscribe`, …) comes back as `needs_confirmation` until the op carries `"confirm": true`. That
applies to **every op that clicks**, not to the op named `click` — `toggle` presses the control too.
The role that decides whether a name is an action name at all (a checkbox labelled "Delete account"
is not a click this stops for; a button is) is read from the element the server observed, never from
the op, so a `"role"` in your request cannot lift the rail. The same rule governs `type`: a field is
sensitive if the page flagged it **or** if its name and role say so — the union the observation masks
on, so the field whose value is hidden in the table is the field `type` asks about.
For tabs, prefer `target_id` over `index`. Indexes are positional and get renumbered whenever the
tab list changes, so an index read one call ago can address a different tab.
### `browser_assert(checks, session="default")`
Deterministic checks — no model judgement about whether it worked.
```json
{"checks": [
{"type": "url_matches", "pattern": "*/checkout*"},
{"type": "text_contains", "text": "Order confirmed"},
{"type": "element_exists", "role": "button", "name": "Continue"},
{"type": "value_equals", "ref": "e4", "value": "Zurich"},
{"type": "count_at_least", "role": "link", "min": 3}
]}
```
### `browser_macro(action, session="default", name="", params={}, ...)`
`record_start` → drive the task → `record_stop` → `run`. Replay costs **no model calls**: it
navigates back to where the task began and re-resolves every step by role + accessible name,
refusing weak or ambiguous matches rather than clicking the wrong thing. `params` fills
`{{placeholders}}` in typed text and URLs.
### `browser_goal(goal, url="", session="default", max_steps=20, verify=[...])`
Hands the whole task over. Pass `url` and the goal and the page is opened and driven to the end
server-side using TypeSafe speculative fan-out (one request per step) — one call, one turn. Leave
`url` out to carry on from the page the session is already showing. Needs a key for the decision
model: `TYPESAFE_API_KEY` for Jev's own API, or `OPENROUTER_API_KEY` with `JEV_PROVIDER=openrouter`
(equivalently, `TYPESAFE_BASE_URL` pointed at OpenRouter's decisions route). Returns
`verified: PASS/FAIL` when `verify` checks are supplied.
Reading a page is not a task: `browser_open`, `browser_observe` and `browser_assert` are direct,
free and keyless, so a look stays cheap. The handoff is for work that changes the page.
Every run also reports its own bill — `turbo: 4 decisions · 14,626 tokens · 1.8s model + 1.1s page ·
3.3s wall` — so what the handoff cost is visible in the answer, alongside how much of the wall time
was the model and how much was the page.
Every way the decision model can fail — no key, no credits, unreachable, a malformed answer, a body
that is not JSON — comes back as `turbo_unavailable:` with nothing executed. The trace of the steps
already taken is kept, so a run that dies on step five still reports what steps one to four did.
`status` is mostly the model's own summary, but the loop writes it too, and the values it writes are
worth recognising: `stopped: no progress` after three consecutive actions changed nothing — the page
is as finished as it is going to get, so the run stops there rather than spending the rest of
`max_steps` discovering it — and `stopped: hit max_steps` when the budget simply ran out. The two
read differently on purpose: the first says there is nothing left to do, the second says there might
be.
`verify` is checked by code, so when the summary and the page disagree the assertion decides: if the
page passes your checks the run reports `status: done` whatever the model said, and the trace
records that it overruled. This is the ordinary shape of a goal whose last action removes what it
acted on — click a check-in button and the button is gone, so the model, finding nothing left to do,
reports `BLOCKED` on a goal that in fact succeeded.
### `browser_tabs` · `browser_sessions` · `browser_close` · `browser_doctor`
Tab management (list / new / switch / close), session listing, teardown, and a self-check that
reports which browser was found and whether it is reachable.
---
## Configuration
All optional; the defaults are the point.
| Variable | Required | Default | Description |
|---|---|---|---|
| No | `JEVMCP_CHROME` | auto-detected | Chrome/Chromium/Edge/Brave executable |
| No | `JEVMCP_MODE` | `launch` | `launch` a browser, or `attach` to a running CDP endpoint |
| No | `JEVMCP_CDP_URL` | — | `http://127.0.0.1:9222` when `mode=attach`, or a `ws://` URL to skip discovery |
| No | `JEVMCP_ATTACH_PROFILE_DIR` | *(browser defaults)* | data directory of the browser being attached to, if `DevToolsActivePort` is not found automatically |
| No | `JEVMCP_HEADLESS` | `1` | `0` for a visible window |
| No | `JEVMCP_FOREGROUND` | `0` | `1` activates the owned tab |
| No | `JEVMCP_SANDBOX` | `auto` | `auto` retries with `--no-sandbox` if the browser aborts on startup |
| No | `JEVMCP_WINDOW` | `1280x860` | browser window size |
| No | `JEVMCP_PROFILE_DIR` | `~/.jev-ultrafast-mcp/chrome-profile` | persistent profile — log in once, stay logged in |
| No | `JEVMCP_ALLOW_DOMAINS` | *(all)* | comma-separated; navigation elsewhere is refused |
| No | `JEVMCP_DENY_DOMAINS` | *(none)* | comma-separated blocklist |
| No | `JEVMCP_CONFIRM_PATTERNS` | pay / delete / unsubscribe … | clicks matching these need `"confirm": true` |
| No | `JEVMCP_ALLOW_JS` | `0` | enables `eval` and `js` assertions |
| No | `JEVMCP_ALLOW_UPLOADS` | `1` | gates the `upload` op |
| No | `JEVMCP_MAX_ACTIONS` | `250` | element-table cap, applied by usefulness |
| No | `JEVMCP_MAX_TEXT` | `6000` | visible-text cap per observation |
| No | `JEVMCP_SETTLE_TIMEOUT` | `4.0` | after opening a URL, how long to wait for the page to stop fetching and its element table to stop changing |
| No | `JEVMCP_SETTLE_POLL_MS` | `120` | how often to re-read while waiting |
| No | `JEVMCP_STATE_DIR` | `~/.jev-ultrafast-mcp` | profile, macros and screenshots |
| No | `JEV_PROVIDER` | `typesafe` | which API pays for the decision model: `typesafe` (Jev's own) or `openrouter` |
| No | `TYPESAFE_API_KEY` | — | key for Jev's own API |
| No | `TYPESAFE_BASE_URL` | `https://api.typesafe.ai/v1/systemone` | where the decision model lives; a custom endpoint, and what the provider is inferred from when `JEV_PROVIDER` is unset |
| No | `OPENROUTER_API_KEY` | — | key for OpenRouter's decisions route, when `JEV_PROVIDER=openrouter` |
| No | `TYPESAFE_MODEL` | `jev-latest` | decision-model slug |
| No | `TEXT_MODEL_API_KEY` | — | optional; only for the small text helper `browser_goal` uses to type a value into a field. Unset, the helper inherits the decision model's provider |
| No | `TEXT_MODEL_BASE_URL` | `https://api.deepseek.com/v1` | endpoint for that helper; setting it *or* `TEXT_MODEL_API_KEY` is what opts out of inheriting the decision model's provider |
| No | `TEXT_MODEL` | `deepseek-chat` | model for that helper; required when the helper inherits a provider, since `deepseek-chat` is not an OpenRouter slug |
`JEVMCP_MODE=attach` is the "use the browser I already have open" route — the one to take when the
login you need already lives in your own profile. Chrome 144+ exposes that through
`chrome://inspect/#remote-debugging`, and its server answers 404 to `/json/version` by design; jev
falls back to `DevToolsActivePort` rather than treating that as "nothing is listening". Attach mode
only ever touches the tab it opens: `browser_close` detaches rather than quitting, and the same holds
when the server exits. Your other windows, and the session in them, are never closed.
Everything above the last eight rows is local: it configures a browser on your machine. Only the
decision-model group talks to the network, and only when `browser_goal` actually runs.
The decision model has two routes and `JEV_PROVIDER` names which one you are on. `typesafe` is Jev's
own API. `openrouter` is the same model through OpenRouter's decisions route, needs no TypeSafe
account, and is the route that predates the variable: `TYPESAFE_BASE_URL` still overrides the URL
either way, and is what the provider is inferred from when `JEV_PROVIDER` is unset, so a
configuration written before the name existed keeps working unchanged.
Only the OpenRouter route also serves a chat API, so only there can the text helper inherit. Set
`JEV_PROVIDER=openrouter` and name a chat model with `TEXT_MODEL`, and one `OPENROUTER_API_KEY`
covers both. Jev's own API answers typed questions and never writes prose, so `TYPE_TEXT` under it
needs a chat provider of its own through `TEXT_MODEL_API_KEY`. Either way the key and the URL are
always taken from the same provider — a key borrowed across providers buys a 401 that names the
company you did not call, and a run diagnosed from the wrong provider's error is a run nobody
diagnoses.
Two of these are worth setting before you point an agent at your own accounts:
`JEVMCP_ALLOW_DOMAINS` pins the browser to a set of hosts and refuses everything else, and a
persistent `JEVMCP_PROFILE_DIR` means you log in once by hand instead of teaching the model your
password.
---
## FAQ
**So this is just another model doing the work? Who is in charge?**
You are, and you choose per task. `browser_goal` puts Jev — a small decision model — in charge of
one goal: which of the page's elements to touch, one step at a time. It is not a general agent, it
has no memory between goals, and it never writes code or selectors, only picks from options the
server hands it. Your agent still decides *what* to ask for, and `verify` decides whether it
actually happened. If you would rather be in the loop for every step, do not call that one tool —
nothing else sends anything anywhere.
**Do I need an API key or an account?**
Not for the browser tools. `browser_open`, `browser_observe`, `browser_act`, `browser_assert`,
`browser_macro` and the tab/session tools never call out — no telemetry, no phone-home, nothing
leaves your machine. `browser_goal` is the exception, and it is opt-in: it sends your goal and the
current element table to a decision model, which is why it needs a key. Leave that one tool unused
and nothing about the page goes anywhere.
**Will a browser window pop up and take over my screen?**
No. It runs headless by default and drives a **background tab it owns** — animations and menus still
work, but nothing steals focus. `--headed` (or `JEVMCP_HEADLESS=0`) shows the window if you want to
watch it work.
**How do I use it on a site I am logged into?**
Set `JEVMCP_PROFILE_DIR` to a persistent directory, open the browser once by hand, log in, and the
session is remembered. That is far better than teaching an agent your password — and password
fields are redacted in observations when you do type them.
**Can it just use the browser I already have open, with my logins in it?**
Yes. `JEVMCP_MODE=attach` plus `JEVMCP_CDP_URL=http://127.0.0.1:9222` drives your own Chrome. In
Chrome 144+ you switch debugging on from `chrome://inspect/#remote-debugging` — no restart, so your
tabs and logins survive — and Chrome asks you to approve the client. The first connection waits on
that click, so give it a moment before deciding it failed.
**Do I have to approve that click for every action?**
No. The approval is per browser session, not per connection and not per action. Once you have
approved it, every later action rides the same open WebSocket and never prompts again — not even
from a fresh process (measured: three new processes connecting twenty minutes after the click, all
accepted with no prompt). So the cost is one click per browser session, not one per operation. To
drop even that, use the default `JEVMCP_MODE=launch`: it starts its own browser on a real
debugging port with no approval dialog at all, at the price of logging in once in that profile.
**`curl http://127.0.0.1:9222/json/version` returns 404. Is debugging even on?**
Probably yes. The server behind `chrome://inspect/#remote-debugging` is **WebSocket-only** and
deliberately serves no HTTP discovery endpoints, so a 404 there is the documented behaviour rather
than a broken setup (and it is not the same thing as `--remote-debugging-port=9222`, even though both
print port 9222). jev does not rely on it: when `/json/version` does not answer, it reads the port and
the browser WebSocket path out of Chrome's `DevToolsActivePort` file. If your browser keeps its data
directory somewhere unusual, point `JEVMCP_ATTACH_PROFILE_DIR` at it.
**Nothing is happening and the page looks empty.**
A page that renders from JavaScript can briefly look empty. After it opens a URL the server waits for
the page to stop fetching *and* for its element table to stop changing (up to `JEVMCP_SETTLE_TIMEOUT`)
before it hands the page to the model — waiting on stillness alone is not enough, because an app that
has not fetched its bundle yet is a shell, and a shell is perfectly still. If a site is stuck behind a
cookie wall or a consent dialog, the element table will show it — look for the overlay warning in the
observation header.
**There is a captcha. Can it solve it?**
No, and that is deliberate — it never looks at pixels. Use a screenshot-and-vision agent for that.
**Is it on PyPI? Is it in the MCP registry?**
On PyPI, yes — `pip install jev-ultrafast-mcp`, or `uvx jev-ultrafast-mcp` to run it without
installing anything. In the registry, not yet, but nothing is left to do by hand:
[`server.json`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/server.json) validates
against the registry's schema and the release workflow submits it on every tag over OIDC, so it
lands with the next release. See
[Publishing](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/CONTRIBUTING.md#publishing) for
what that involves.
**How is this different from the Playwright MCP?**
Playwright's server exposes page primitives; the agent writes selectors and coordinates. This one
exposes a numbered table of controls and refuses ambiguous targets. If you need pixel-level control
or a mature recorded-testing ecosystem, use Playwright. If you want an agent that cannot silently
click the wrong button, use this.
**Is this an alternative to browser-use?**
They solve the same problem from opposite ends. [`browser-use`](https://github.com/browser-use/browser-use)
is a library that runs the agent loop in-process; this is an MCP server that gives *your* existing
agent the same kind of hands. The optional turbo path here is a port of
[`browser-use/jev-ultrafast`](https://github.com/browser-use/jev-ultrafast), which asks a decision
model one typed question per step instead of free-form text.
**Is it safe to let it loose on my accounts?**
It is built assuming it should not be trusted. Destructive-sounding clicks come back as
`needs_confirmation` instead of executing, `JEVMCP_ALLOW_DOMAINS` refuses navigation outside a
domain you list, sensitive fields are redacted, and `eval` is off unless you turn it on. Start with
a domain allowlist and an account you do not mind breaking.
A field whose value must not leave the page is redacted in the table, needs `"confirm": true` to
type into, and is written into a macro as `{{secret}}` rather than in the clear. There are two ops
that can put text into one, and the second is easy to miss: `keys` with a bare single character
(`{"op": "keys", "keys": "a"}`) sends `Input.insertText`, not a key press, so it types into whatever
has focus. That op is refused on a sensitive field rather than confirmed — it carries no ref and a
macro records a character sequence, so there is no placeholder a replay could put back. Use `type`
with the field's ref.
The one capability here that is **on by default and not bounded** is `upload`: it hands any existing
file or directory on this machine to the page, and a page on a host your envelope allows can then
receive it. Set `JEVMCP_ALLOW_UPLOADS=0` if the task does not need to attach anything. Screenshots
are the mirror image and *are* bounded — they can only be written inside the state directory.
---
## Try it without an agent
```bash
.venv/bin/python scripts/smoke.py # headless, 61 checks
.venv/bin/python scripts/smoke.py --headed # watch it drive
```
This launches Chrome, serves `tests/fixture.html`, and drives the real code paths: batch execution,
autocomplete, a covering modal, shadow DOM, a same-origin iframe, a file upload, a password field, a
destructive-click guard, stale refs, macro record/replay, tab handoff, screenshots, and a page that
renders after `readyState` already says "complete".
```
1. Observation — one atomic read, indexed refs
[ok ] element table is not empty — 16 elements
[ok ] shadow DOM element indexed — Shadow action -> e14
...
5. Occlusion — precomputed, not discovered by a failed click
[ok ] covered control flagged before any click — e8 occluded=True
[ok ] click on a covered control is refused with a reason — occluded
...
61/61 checks passed
```
To test against the real web rather than a fixture:
```bash
.venv/bin/python scripts/live_check.py # Bing + DuckDuckGo + tabs + screenshots
.venv/bin/python scripts/live_check.py --headed # watch it happen
```
That one needs the network and third-party sites, so it is deliberately not part of CI. Unreachable
sites are reported as *skipped*, and the summary says so plainly, so a fully-skipped run cannot be
mistaken for a passing one.
And to prove turbo mode itself — the one path that spends money, and therefore the one nothing else
exercises end to end:
```bash
.venv/bin/python scripts/turbo_check.py # the model drives: dropdown, checkbox, submit
.venv/bin/python scripts/turbo_check.py --headed # watch it decide
```
It serves the same fixture, points a real Chrome at it, and lets Jev drive the goal, then verifies
the page the model left behind with code rather than trusting its claim of success. Without a key it
prints `skipped` and exits 0, so the exit code and the word agree.
### A check-in that stops paying for itself
`examples/checkin.html` is a stand-in for the thing people actually automate: a daily button.
`scripts/checkin.py` drives it in three stages, cheapest first — and the point is that only the
first run ever costs anything.
```bash
.venv/bin/python scripts/checkin.py --port 8901 # learn once, then never again
.venv/bin/python scripts/checkin.py --port 8901 --record # re-learn, ignoring the saved macro
```
**1. Already done.** Read the page. If today's check-in is already on it, stop — no click, no model
call, nothing to undo.
**2. Replay.** Run the macro the first run recorded: zero model calls, a few hundred milliseconds,
and it refuses rather than guessing when the page has moved on. This is the stage that runs on every
ordinary day, and it needs no key at all.
**3. Explore.** Only when there is no macro, or the saved one no longer matches. The decision model
works the page out, and what it did is recorded as a macro so stage 2 takes over tomorrow. Its path
is only saved when the page proves it worked.
Pinning `--port` matters for the demo: a page's origin includes its port, so a second run on a
different port is a different site to the browser, with an empty `localStorage` and no memory of
having checked in.
For a real site:
```bash
.venv/bin/python scripts/checkin.py --url https://example.com/rewards \
--goal "Click the daily check-in button" --expect "已签到"
.venv/bin/python scripts/checkin.py --url https://example.com/rewards --replay-only
```
The goal and the proof are separate arguments on purpose. `--goal` is what the model is asked to do;
`--expect` is the text the page must show afterwards, checked by code — so a run is judged by the
page, never by the model's summary of its own work. Sign in once with `--headed --wait 120`; the
browser profile persists, so later runs reuse the session. `--replay-only` never calls the model,
which is the flag you want in a cron job.
---
## Layout
```
jev_ultrafast_mcp/
js/observer.js in-page observer: stable refs, shadow/frame traversal, verify/resolve
cdp.py synchronous CDP client + Chrome launch (no wrapper library)
browser.py sessions, guarded execution, op dispatch, macro recording
observe.py element model, compact renderer, delta computation
macros.py semantic descriptors, scored re-resolution, storage
assertions.py deterministic checks
policy.py optional TypeSafe turbo policy (speculative fan-out)
safety.py domain envelope, redaction, confirmation rules
config.py environment-driven configuration
server.py the MCP surface
scripts/
install.py writes the right config for each MCP client on this machine
smoke.py end-to-end proof against a real browser
mcp_check.py drives the server over real stdio MCP
live_check.py the same, against real websites (needs the network)
turbo_check.py lets the decision model drive a real browser (needs a key)
extension_check.py the extension in a real Chrome: its table, and a macro it replayed
checkin.py a real check-in: learn once with the model, then replay for free
chrome-extension/
lib/observer.js a byte-identical copy of jev_ultrafast_mcp/js/observer.js
lib/render.js a port of observe.py, held to the real renderer by generated fixtures
lib/macro.js a port of macros.py, held to the real resolver by generated fixtures
lib/session.js a port of browser.py's op dispatcher, held to the real one by generated fixtures
lib/report.js a port of server.py's report writers, so a report reads the same anywhere
lib/store.js macros in chrome.storage.local, under the server's own {{placeholder}} rules
background.js the service worker, and the only file that calls the debugger API
examples/
checkin.html the daily-button page checkin.py drives
assets/
social-preview.png the card GitHub shows when this repository is shared
make_social_preview.py renders it, so the words on it are placed rather than generated
llms.txt what this server is, for agents that read before recommending it
server.json the MCP registry entry, submitted by the release workflow
```
## Development
```bash
.venv/bin/pip install -e ".[dev]"
.venv/bin/ruff check .
.venv/bin/python -m pytest -q
.venv/bin/python scripts/smoke.py # 61 checks, real browser
.venv/bin/python scripts/mcp_check.py # 17 checks, real stdio MCP
```
All of it runs in CI on Python 3.10, 3.12 and 3.13 against headless Chrome. Read
[`CONTRIBUTING.md`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/CONTRIBUTING.md) before changing how targets are resolved — that logic is the
whole point of the project.
## See also
- [`browser-use/jev-ultrafast`](https://github.com/browser-use/jev-ultrafast) — the project the
optional turbo path is a port of: one typed question per step, answered by a decision model.
- [TypeSafe](https://typesafe.ai) — the typed-question decision API behind `browser_goal`.
- [Model Context Protocol](https://modelcontextprotocol.io) — the protocol this server speaks.
- [The official MCP registry](https://github.com/modelcontextprotocol/registry) — where clients go
looking for servers like this one.
- [Chrome DevTools Protocol](https://chromedevtools.github.io/devtools-protocol/) — what it drives
the browser with, through no wrapper library.
## License
MIT — see [`LICENSE`](https://github.com/jiawei686/jev-ultrafast-mcp/blob/main/LICENSE).
<!--
The MCP registry proves ownership of a PyPI package by looking for the line below in the README,
because this file is the description PyPI publishes. It has to match `name` in `server.json`
exactly; `tests/test_docs.py` fails if the two drift, and the registry rejects the entry with a
400 if the marker is missing -- which is how this was found, by running the submission.
Hidden in a comment on purpose: it is a token for a validator, not something to read.
https://modelcontextprotocol.io/registry/package-types
-->
<!-- mcp-name: io.github.jiawei686/jev-ultrafast-mcp -->
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