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selenium-mcp

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start_browser

Launches the Chrome, Firefox, or Edge browser session that all other tools control, so call it first. Downloads the matching driver automatically and returns session status.

Instructions

Start a browser session (Chrome, Firefox, or Edge) that all other tools act on. Call this first. Only one session can be started this way: call stop_browser before starting another, or use session_create to run several browsers at once. The matching driver is downloaded automatically by Selenium Manager. Returns the session status.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
browserNoWhich browser to launch: chrome (default), firefox, or edge. It must be installed on this machine.chrome
headlessNoRun without a visible window (default false). Use true for CI or background runs, false when the user wants to watch.
windowSizeNoInitial window size, e.g. { width: 1440, height: 900 }. Change it later with the window tool's resize action.
browserArgsNoExtra command-line flags for the browser, e.g. ['--incognito'] or ['--lang=de']. Default none.
scriptTimeoutMsNoMaximum time an asynchronous script may run, in milliseconds (default 30000).
pageLoadTimeoutMsNoMaximum time a page load may take before navigate fails, in milliseconds (default 30000).

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed8 schema fields changedv0.3.0
    • addedInput schema / properties / browser / description
      Added value: +"Which browser to launch: chrome (default), firefox, or edge. It must be installed on this machine."
    • addedInput schema / properties / browserArgs / description
      Added value: +"Extra command-line flags for the browser, e.g. ['--incognito'] or ['--lang=de']. Default none."
    • addedInput schema / properties / headless / description
      Added value: +"Run without a visible window (default false). Use true for CI or background runs, false when the user wants to watch."
    • addedInput schema / properties / pageLoadTimeoutMs / description
      Added value: +"Maximum time a page load may take before navigate fails, in milliseconds (default 30000)."
    • addedInput schema / properties / scriptTimeoutMs / description
      Added value: +"Maximum time an asynchronous script may run, in milliseconds (default 30000)."
    • addedInput schema / properties / windowSize / description
      Added value: +"Initial window size, e.g. { width: 1440, height: 900 }. Change it later with the window tool's resize action."
    • addedInput schema / properties / windowSize / properties / height / description
      Added value: +"Window height in pixels (240-4320)."
    • addedInput schema / properties / windowSize / properties / width / description
      Added value: +"Window width in pixels (320-7680)."
  2. First observedv0.1.2

TDQS

A4.4/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations, so the description carries the full disclosure burden. It discloses two valuable non-obvious behaviors: the session is a singleton that all other tools act on, and the driver is auto-downloaded by Selenium Manager. It stops short of failure modes (e.g. browser not installed beyond the schema note) or auth/permission requirements.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Five tight sentences, front-loaded with the core action and the ordering instruction before the constraints and alternatives. Every sentence carries a distinct, useful fact.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 6-parameter, all-optional tool with no output schema, the description covers the key invocation contract (call first, singleton, driver auto-fetch) and notes the return value ('Returns the session status.'). Slightly light on what the returned status contains, but adequate.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so every parameter is already documented in the schema with defaults, ranges, and examples. The description adds no syntax or format detail beyond what the schema provides, so the baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb (Start) and resource (a browser session) with the concrete browser options, and explicitly distinguishes itself from stop_browser and session_create in the same breath. An agent can identify this tool without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicit sequencing guidance ('Call this first'), the singleton constraint, and named alternatives with the conditions that select them ('call stop_browser before starting another, or use session_create to run several browsers at once'). Nothing is left to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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