Flintrail

The fastest browser for AI agents.

Drive a real Chrome live. Record a task once. Replay it in milliseconds, with no LLM in the loop.

Or skip the page entirely and replay the site's own API calls with your session.

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Why

Browser MCP servers today make the model do everything, every time: read the page, pick one element, click, read the page again. Each click is a tool call, and each tool call is a model turn that costs seconds. Do the same task tomorrow and the agent pays the full price again.

Flintrail treats the browser like a JIT compiler treats code:

1. Live, the agent drives the page, but batches a whole sequence of actions into one call and gets back a compact snapshot with clickable refs.
2. Record, the same calls are captured with multi-clue element fingerprints and {{variables}}.
3. Replay, the flow runs at browser speed with no model between steps. When a site changes, the run stops at the exact failing step and hands the page back so the agent repairs only that part.
4. API replay, Flintrail sees the site's own fetch/XHR calls and can replay them from inside the page, with the same cookies and session. No rendering, no clicking.

Benchmark

Same 5-step task (fill a field, tick a box, change page, click a client-rendered button that calls an API, wait for the result), median of 10 runs, headless Chromium. The numbers are tool-side wall time only. In a real agent every tool call also costs a model turn, so going from 7 calls to 2 saves several more seconds on top.

The hatched bar is the floor: the same clicks as a raw Playwright script, with no MCP and no agent. Any tool that clicks through the UI is bounded by it, which is why the per-call ratios stay modest. Flintrail adds about 50 ms per call on top of that floor, Playwright MCP about 260 ms. The only way under the floor is to stop clicking, which is what API replay does.

Reproduce it: npm run bench (installs nothing global; Playwright MCP is a dependency of bench/ only).

Architecture

Install

Claude Desktop: download flintrail.mcpb and double-click it.

Claude Desktop, Claude Code, Cursor (stdio)

{

"mcpServers": { "flintrail": { "command": "npx", "args": ["-y", "flintrail@latest"] } } }

Claude Code: claude mcp add flintrail -- npx -y flintrail@latest

From a local clone (git clone https://github.com/Soflutionltd/flintrail.git && cd flintrail && npm install && npm run build), start node /absolute/path/to/flintrail/bin/supervise.cjs.

Automatic updates

Every install below keeps itself up to date, without restarting Claude or Cursor. The MCP client talks to a small supervisor (bin/supervise.cjs) that stays connected while the server runs behind it. At start-up and every 6 hours it asks npm for the latest flintrail, installs it in the background in ~/.soflution/mcp/flintrail/, and switches over as soon as no request is in flight (the client is told the tool list may have changed). A version that fails to start is rolled back at once and never picked again on that machine. npm checks the integrity of every download, and releases are published from GitHub Actions with provenance.

The only network call is that version check to registry.npmjs.org; nothing about your machine or your use is sent. Turn it off with NO_UPDATE_NOTIFIER=1 (or SOFLUTION_AUTO_UPDATE=0). Started directly with node dist/..., flintrail does not update itself and only mentions a newer version in its first tool result.

Any agent, any machine (HTTP)

node dist/cli.js --http --host 127.0.0.1 --port 8931

token is generated once in ~/.flintrail/token (or pass --token / FLINTRAIL_TOKEN)

Clients connect to http://host:8931/mcp with Authorization: Bearer . Put it behind a tunnel or reverse proxy with TLS if it leaves the machine.

Browser modes

Flintrail refuses to attach to a personal browser (Brave, Edge, Opera, or on macOS anything that is not Chrome for Testing) unless FLINTRAIL_ALLOW_ANY_BROWSER=1. Your everyday browser stays yours.

Browser extension

The extension is the easiest way in: no command-line flags, no debugging port, works in Chrome, Edge, Brave, Arc or any Chromium 125+.

1. Install. Download flintrail-extension- .zip from the latest release, unzip it, open chrome://extensions, enable *Developer mode*, click *Load unpacked* and pick the folder. (Chrome Web Store listing in progress.)
2. Run Flintrail in extension mode, for example in Claude Desktop:

   { "mcpServers": { "flintrail": { "command": "npx", "args": ["-y", "flintrail@latest"], "env": { "FLINTRAIL_MODE": "extension" } } } }

1. Connect. Click the Flintrail icon, paste the token from ~/.flintrail/token, click *Connect*. The extension reconnects by itself afterwards.

How it works: Flintrail starts a relay on 127.0.0.1:8932. The extension connects to it with the token and executes CDP commands with chrome.debugger, one tab at a time. Playwright sees an ordinary CDP browser, so every tool, recording and replay works unchanged. The agent only sees tabs it opened itself or tabs you share with *Share with agent*. Chrome shows its "started debugging this browser" bar while a tab is driven, by design.

Teach by demonstration

You do not have to explain a task to the agent. Type a flow name in the popup, click *Start recording*, do the task yourself in the tab, click *Stop and save*. The extension records clicks, typing, selects, checkboxes and Enter with the same element fingerprints the engine uses, and never records password fields. Then:

fl_flow_set_var("paybyphone-renew", 0, "plate")   # turn the typed plate into {{plate}}

fl_run("paybyphone-renew", { "plate": "AB-123-CD" })

The flow now runs without you, without the LLM, in milliseconds per step.

Limits in extension mode: browser-level CDP (download folder, permissions, window size) is not available to extensions, so download saves through the browser's normal download flow. Chrome dismisses dialogs of background tabs driven by chrome.debugger before anyone can answer, so in this mode {do:"dialog"} answers the next alert / confirm / prompt from inside the page instead.

Tools

Example

// one call, five actions

{ "tool": "fl_act", "tab": "t1", "actions": [ { "do": "fill", "ref": "e1", "value": "AA-111-AA", "var": "plate" }, { "do": "check", "ref": "e3" }, { "do": "click", "role": "button", "name": "Continue" }, { "do": "click", "selector": "[data-testid=confirm]" }, { "do": "wait", "text": "active" } ]}

Then fl_run("permit", { "plate": "ZZ-999-ZZ" }) repeats it in under a second.

// files, native dialogs, coordinates, mobile width

{ "tool": "fl_act", "tab": "t1", "actions": [ { "do": "upload", "ref": "e4", "files": ["/Users/me/statement.csv"], "var": "statement" }, { "do": "dialog", "accept": true }, { "do": "click", "role": "button", "name": "Delete" }, { "do": "mouse", "x": 640, "y": 380 }, { "do": "resize", "width": 390, "height": 844 } ]}

upload works on a file input, even a hidden one, and on a button or drop zone that opens the file chooser. Dialogs are dismissed by default and every one is reported in the result, as are new console errors.

Sign in

fl_login signs the agent in with the passwords you already have, without ever showing them to the agent: it only learns which fields were filled. Passwords and one-time codes are masked in every tool result, including captured request bodies and page reads, and a recorded flow stores a reference to the login, never the secret. On a multi-step sign-in (e-mail page, then password page) the agent calls it on each step.

Sources are tried in the order of FLINTRAIL_PASSWORDS (default browser):

| 1password[:path] | 1Password CLI signed in, or OP_SERVICE_ACCOUNT_TOKEN in Flintrail's environment for unattended use. | One-time codes stored in 1Password are filled too. A service account never asks for Touch ID;