Fastbrowser

fastbrowser Skill

fastbrowser-cli is a command-line client for the FastBrowser HTTP server, which keeps a persistent MCP connection to a Chrome browser alive so commands incur minimal latency. Each command maps 1-to-1 to a FastBrowser tool and returns the tool's response on stdout.

Invocation

Run the CLI directly via tsx:

npx fastbrowser_cli@latest <command> [flags]

Typical Workflow

1. Query the accessibility tree for specific nodes with query_selectors (first match per selector by default; pass -a, --all for every match).
2. Act on an element by its accessibility selector: click, fill_form, press_keys. The selector can be a direct uid reference (e.g. #1_42, fastest path) or any CSS-like selector (e.g. button[name="Submit"]), which is resolved to a uid internally.

Snapshot output looks like:

uid=1_0 RootWebArea "Example Domain" url="https://example.com/"
  uid=1_1 heading "Example Domain" level="1"
  uid=1_2 link "More information..." url="https://www.iana.org/..."

Page Management

# List all open browser pages
npx fastbrowser_cli@latest list_pages

# Open a new page at a URL
npx fastbrowser_cli@latest new_page --url https://example.com

# Close a page by its numeric id
npx fastbrowser_cli@latest close_page --page-id 1

# Navigate the current page to a URL
npx fastbrowser_cli@latest navigate_page --url https://example.com

# Restart the daemon - run this if pages opened by the bridge were closed manually
# and the MCP connection has broken
npx fastbrowser_cli@latest server restart

Configuration

Global flags accepted by every command:

The daemon binds to one backend at startup. If it is already running with a different backend, the CLI refuses the request and prints the exact restart command to switch.

# Use chrome-devtools-mcp for one command
npx fastbrowser_cli@latest --mcp-target chrome_devtools list_pages

# Switch the running daemon to a different backend
npx fastbrowser_cli@latest --mcp-target chrome_devtools server restart

Selector Language

The selector syntax is modelled on CSS selectors, adapted for accessibility tree structures.

Role selector

Matches nodes by their accessibility role.

button
link
combobox
searchbox
heading
WebArea

Universal selector

Matches any node.

*

UID selector

Matches a node by its exact unique identifier.

#4
#1_3

Attribute selectors

Attribute selectors match values inside node.attributes. The special virtual attribute name maps to node.name.

link[href]
button[disabled="true"]
link[href^="https"]
link[href$=".com"]
link[href*="example"]
button[name~="Submit"]
heading[name="Welcome"]
link[name="Click \"here\""]

Combinators

WebArea link
main > button
label + textbox
link ~ link
heading, button
RootWebArea > link[href^="https"]

Positional pseudo-classes

Narrow a match by position within the parent's children array. Indexing is 1-based; the root node never matches a positional pseudo-class.

link:first-child
button:last-child
menuitem:nth-child(2)

Functional pseudo-classes

Take a comma-separated selector list inside parentheses. The argument list itself supports the full selector language and may be nested.

:has() walks descendants of the candidate node (excluding the node itself). Relative leading combinators (e.g. :has(> link)) are not supported.

:is(heading, button)
link:not(:first-child)
*:has(button)
*:not(:has(link))
main > *:not(button)

Examples

Sample accessibility tree:

uid=1 WebArea "Main Page"
  uid=2 main
    uid=3 heading "Welcome"
    uid=4 link "Click here" href="https://example.com"
    uid=5 button "Submit" disabled="true"
  uid=6 navigation
    uid=7 link "Home" href="/"
    uid=8 link "About" href="/about"

Example queries on it:

• link matches all the links (uid=4, uid=7, uid=8)
• navigation > link matches only the links that are direct children of navigation (uid=7, uid=8)
• link[href^="https"] matches links with an external href (uid=4)
• button[name="Submit"] matches the submit button by name (uid=5)
• *[disabled="true"] matches any disabled element (uid=5)
• heading, button matches both headings and buttons in one query (uid=3, uid=5)
• #7 matches a node by its UID (uid=7)
• link:first-child matches uid=4 and uid=7 (first child of main and navigation)
• link + button matches uid=5 (button immediately after a link)
• :is(heading, button) is equivalent to heading, button (uid=3, uid=5)
• link:not([href^="https"]) matches the relative-href links (uid=7, uid=8)
• *:has(button) matches ancestors of a button (uid=1, uid=2)

Inspection

• query_selectors is the most efficient way to get specific elements or data from the page. Use it instead of take_snapshot whenever possible.
• By default, query_selectors returns the first match per selector (cheaper, less output). Pass -a, --all when you need every match — pair it with --limit to cap results per selector.
• Use --wa, --with-ancestors to include each match's ancestor chain in the result, and --wc, --with-children to include the descendant subtree of each match.

# Query the accessibility tree returning the FIRST match per selector (--selector is repeatable)
npx fastbrowser_cli@latest query_selectors --selector "button" --selector "link"

# Include ancestor nodes in the result
npx fastbrowser_cli@latest query_selectors --selector 'heading[level="1"]' --with-ancestors

# Include the descendant subtree of each match
npx fastbrowser_cli@latest query_selectors --selector 'main' --with-children

# Per-selector control over withAncestors / withChildren via JSON
npx fastbrowser_cli@latest query_selectors \
  --selectors-json '[{"selector":"button","withAncestors":true},{"selector":"link","withChildren":true}]'

# Pass --all to return every match per selector; --limit caps results per selector (0 = unlimited)
npx fastbrowser_cli@latest query_selectors --all --selector "button" --selector "link" --limit 5

# Include ancestor nodes in the result
npx fastbrowser_cli@latest query_selectors --all --selector 'heading[level="1"]' --with-ancestors

# Per-selector control over limit / withAncestors / withChildren via JSON (with --all)
npx fastbrowser_cli@latest query_selectors --all \
  --selectors-json '[{"selector":"button","limit":3,"withAncestors":true},{"selector":"link","limit":0,"withChildren":true}]'

# Take an accessibility-tree full page snapshot of the current page - very expensive, prefer targeted queries when possible
npx fastbrowser_cli@latest take_snapshot

Interaction

# Click by a direct uid reference (fast path - no accessibility-tree lookup)
npx fastbrowser_cli@latest click --selector "#1_42"

# Click by any CSS-like selector - resolved to a uid internally
npx fastbrowser_cli@latest click -s 'button[name="Submit"]'

# Fill a single form field - selector can be a uid (#1_7) or any CSS-like selector
npx fastbrowser_cli@latest fill_form -s 'textbox[name="Email"]' -v "hello@example.com"

# Press a comma-separated sequence of keys (literals and named keys both work)
npx fastbrowser_cli@latest press_keys --keys "Tab, Tab, Enter"
npx fastbrowser_cli@latest press_keys --keys "Hello, Tab, Enter"

Batch Execution

Run several commands from one invocation — each line is parsed and executed like a standalone command, reusing the persistent HTTP server session.

Syntax rules:

• One command per line, same syntax as the standalone CLI (e.g. click -s '...')
• Shell-style quoting (single and double quotes)
• Blank lines are ignored
• Lines starting with # are comments

By default, the batch stops at the first failing line (shell set -e semantics). Pass --no-stop-on-error to log failures and continue; the process still exits non-zero at the end if any line failed.

# From a file
npx fastbrowser_cli@latest batch ./demo.fbs

# Piped on stdin
cat demo.fbs | npx fastbrowser_cli@latest batch

# Inline script
npx fastbrowser_cli@latest batch --script $'press_keys --keys "Enter"\nclick -s 'button[name^="Tout effacer"]''

# Continue through failures
npx fastbrowser_cli@latest batch --no-stop-on-error ./demo.fbs

Example demo.fbs:

# open a page and interact
new_page --url https://example.com
fill_form -s 'textbox[name="Email"]' -v 'hello@example.com'
press_keys --keys "Tab, Enter"

Command Reference

Output & Errors

Tool output is written to stdout — one line per response content part. On failure the CLI writes fastbrowser-cli error: <message> to stderr and exits with code 1.