smth — Browser MCP Server

https://calpano.github.io/smth/

A local MCP server that gives Claude a full browser: screenshots, DOM inspection, WCAG color analysis, and page interaction — all via a persistent Puppeteer/Chromium session running in Docker.

What it does

smth exposes 16 MCP tools across three areas:

See — read and analyze any web page without interacting with it browser_see_visual · browser_read_text · browser_see_dom · browser_see_fonts · browser_see_colors · browser_see_color_pairs

wAct — drive the browser like a user browser_launch · browser_goto · browser_click · browser_hover · browser_type

Compare — capture and diff DOM snapshots browser_remember_dom · browser_doms · browser_dom_compare · fetch_dom_content · browser_list_devices

Requirements

• Docker (Desktop or Engine)
• Claude Code (or any MCP client supporting Streamable HTTP or SSE transport)

Installation

git clone https://github.com/maxvolkel/smth.git
cd smth
docker compose up -d

The container starts on port 3000. Add .mcp.json to any project that should use it:

{
  "mcpServers": {
    "smth": { "type": "http", "url": "http://localhost:3000/mcp" }
  }
}

Then restart Claude Code. The mcp__smth__* tools will be available immediately.

Optional: expose local files

To let smth open local HTML files from a project directory:

PAGES_DIR=/path/to/your/project docker compose up -d --force-recreate

Files are mounted read-only at /pages. Open them with their host path or file:///pages/filename.html.

Usage examples

Example 1 — Audit a page for accessibility contrast issues

browser_launch url="https://example.com"
browser_see_color_pairs

Returns every text/background color pair on the page with WCAG 2.2 contrast ratios and AA/AAA pass/fail flags. Instantly shows which combinations fail accessibility requirements.

Example 2 — Extract page content without nav/footer noise

fetch_dom_content url="https://example.com/article/123"

Opens the URL, automatically finds a peer page on the same site, computes what's shared (navigation, header, footer, sidebar), and returns only the unique content of the target page. No manual CSS exclusions needed.

Example 3 — Check what a page looks like on mobile

browser_launch url="https://example.com"
browser_list_devices
browser_see_visual device="iPhone 15 Pro"

Emulates an iPhone 15 Pro (viewport, pixel ratio, user-agent) and returns an embedded screenshot directly visible to the model.

Example 4 — Inspect a form before filling it

browser_launch url="https://example.com/login"
browser_see_dom lens=["code"]
browser_type id="username" text="myuser"
browser_type id="password" text="mypass"
browser_click selector="input[type=submit]"
browser_read_text

The code lens shows form fields, names, actions, and event handlers without layout noise. After filling and submitting, browser_read_text reads the resulting page.

Example 5 — Diff a page before and after an interaction

browser_launch url="https://example.com/settings"
browser_remember_dom name="before" lens=["text"]
browser_click selector="#enable-toggle"
browser_remember_dom name="after" lens=["text"]
browser_dom_compare a="before" b="after"

Shows exactly which text appeared or disappeared after clicking the toggle — useful for verifying that a UI action had the expected effect.

Tool reference

Full documentation for every tool lives in doc/smth.md.

Tool	Read-only	Description
browser_list_devices	yes	Lists device names for browser_see_visual
browser_launch	no	Starts a session, opens a URL
browser_goto	no	Navigates to a new URL in the existing session
browser_read_text	yes	Page text as Markdown + interactive element selectors
browser_see_fonts	yes	All fonts grouped by family, size, weight
browser_see_colors	yes	All computed colors with usage counts
browser_see_color_pairs	yes	Text/background pairs with WCAG contrast ratios
browser_see_dom	yes	Lens-filtered compact HTML or text search
browser_click	no	Clicks an element by id or CSS selector
browser_hover	no	Hovers and returns a before/after diff
browser_type	no	Types text into a form field
browser_remember_dom	no	Saves a named DOM snapshot
browser_doms	yes	Lists all saved snapshots
browser_dom_compare	yes	Diffs two snapshots or subtracts a background
fetch_dom_content	no	Opens a URL and returns only its foreground content
browser_see_visual	yes	Screenshot (viewport or full page, any device)

Container management

docker compose up -d                              # start
docker compose down                               # stop
docker compose up -d --build                      # restart after code changes
docker compose logs -f                            # tail logs

After a --force-recreate, the MCP session token is invalidated. The first tool call will get a Session not found error — just retry once; Claude Code re-establishes the session automatically.

Accessing a local dev server

Docker cannot reach the host's localhost. Use host.docker.internal instead:

# Wrong
browser_launch url="http://localhost:4000/"

# Correct
browser_launch url="http://host.docker.internal:4000/"

Privacy

smth runs entirely on your local machine. It does not transmit any data to external services. The browser session is isolated inside a Docker container. No telemetry, no analytics, no network calls beyond what the pages you visit make themselves.