# Show HN: Fab – a fast browser CLI for AI agents > Source: > Published: 2026-09-29 20:27:39+00:00 Ever noticed how painfully sluggish it is for agents to use a web browser? `fab`'s sole mission is to fix that. A beacon of perf in the hellscape of clunky agentic browser tooling. It's also cheaper and more accurate than the alternatives. "Customize my October training block": two session swaps among look-alikes, a new court day, save. Same LLM (mercury-2.5), real time. Left: the LLM driving chrome-devtools-mcp, 35.0 s. Right: fab, 2.7 s. Your agent says what it wants done, in words. fab does the clicking, typing, reading and signing in, and hands back the result. | | LLM + chrome-devtools-mcp | fab | |---|---|---| | tasks passed | 73.1% | **92.5%** | | median time per task | 40.7 s | **16.6 s** | | cost per task | $0.0133 | **$0.0044** | A blind suite of 40 tasks on 20 unseen apps, same planner LLM, strict outcome checks, measured 2026-09-24. How it's measured: [`bench/LOG.md`](https://github.com/ianks/fast-agentic-browser/blob/main/bench/LOG.md) ("Blind gate: held-out-3"). ``` fab do "open news.ycombinator.com and tell me the top story and its points" fab do "search for Ada Lovelace and tell me when she was born" --url en.wikipedia.org fab do "log in to github.com" # your saved login, 2FA included fab do "what's the total of refunded orders?" --url localhost:3000/orders # exact, across every page fab do "pay with {{card number}}, expiry {{expiry}}, CVC {{CVC}}, but don't place the order" fab do 'sign up as "qa+1@acme.dev" with {{new password}}' --url localhost:3000/signup fab do '{"do": "every job on all pages", "url": "jobs.example.com", "records": {"properties": {"title": {"type": "string"}, "salary": {"type": ["number", "null"]}}}}' fab close ``` Agents declare what they want back as JSON Schema and get exactly that shape ([docs/output.md](https://github.com/ianks/fast-agentic-browser/blob/main/docs/output.md)); words are for people. No, and there never will be. For the first time in history, the dreams of the Unix philosophy are finally being realized by LLMs. One-shotting bash pipelines is viable now. This is an **extremely** powerful way to shape data, and MCP prevents that. Just let the LLMs use bash, sheesh. **One output format.** Every command prints JSON Lines: a `start` event, a `record` per result, and one `end` with the answer or a typed error, on success and failure alike ([docs/output.md](https://github.com/ianks/fast-agentic-browser/blob/main/docs/output.md)). **Scrape in plain English.** Ask for data from a list and fab streams a record per item as it pages through it, opening each item's page when a field lives there: ``` fab do "every job on all pages: title, company, and the salary from its page" --url jobs.example.com \ | jq -c 'select(.t=="record").data' > jobs.jsonl ``` fab writes the request's logic as a small program (loops, conditions, limits; `--print` shows it, `fab run` runs one). The items, fields, pages and detail pages are handled by code in the page: an LLM maps each page layout to its fields once, detail pages are fetched in parallel, and values are copied exactly. The eval (`bench/scrape`, 20 cases on generated sites, 1,746 records: numbered pages, Next buttons, Load more, endless scroll, two-row records, 3-level nesting, client-rendered details, sponsored items, missing fields) is exact on all 20 in most runs, with an occasional miss where the LLM maps a field wrong; 1,000 products with their pages take about 20 s. **Secrets never reach your agent.** - Logins come from 1Password, Bitwarden or the macOS Keychain, and are typed only on the site they're saved for. - `{{plain words}}` stands for anything else in your password manager. Passwords, codes, card numbers and keys stay`••••` everywhere, and fab asks you once per site before it uses a card, an identity or a key. - A `{{new password}}` is generated and saved before it's typed. - `fab secrets` shows what's connected. ``` cargo install --locked fast-agentic-browser fab doctor ``` High-level steps need an `OPENROUTER_API_KEY` (in the environment or `~/.config/fab/env`). **Agents** drive fab through the CLI: - **The CLI skill:**`fab --skill` prints it,`fab --skill --install` adds it to Claude Code, and`--install codex|agents|` puts it elsewhere. Commands on one session (`-s NAME` , default`default` ) share one browser window and its logins. - **Concurrency:** requests that overlap (several`fab do` at once) run side by side in tabs of the same browser, sharing its logins; each caller's next step continues on its own tab (name callers with`FAB_CLIENT` ). At most`FAB_POOL_MAX` tabs (default 4) — more requests wait their turn — and idle tabs close after`FAB_POOL_IDLE` seconds (60). - **Durable tasks:** a request survives a crash.`fab tasks list` shows them,`fab tasks resume ID` continues one from its last confirmed step, and`fab tasks resolve` settles a step whose outcome is uncertain (did the click land?) before anything is repeated. **Browser.** fab uses your default browser (Chrome, Edge, Brave, Firefox, …) in a window you can watch, with its own profile. `--headless` hides the window, `--browser firefox` picks one, and `--connect auto` works in your running Chrome. `fab-bench` runs the suites, races and the decision dataset (`fab-bench bench --mode agent --suite bench/heldout3.toml`), and the scrape eval (`fab-bench scrape run`). `scripts/demo` opens a live browser with a terminal wired to it, for trying requests by hand. The history of every change and its measured effect is in [`bench/LOG.md`](https://github.com/ianks/fast-agentic-browser/blob/main/bench/LOG.md). Licensed under either of [Apache License, Version 2.0](https://github.com/ianks/fast-agentic-browser/blob/main/LICENSE-APACHE) or [MIT license](https://github.com/ianks/fast-agentic-browser/blob/main/LICENSE-MIT) at your option.