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Run a one-shot speed test

vs_speed_test
Read-onlyIdempotent

Run a one-shot Lighthouse lab test on a URL. Returns the performance score (0 to 100) plus Largest Contentful Paint, First Contentful Paint, Cumulative Layout Shift, Time to First Byte, Total Blocking Time and Speed Index (times in milliseconds). When the testing service is busy it answers 503 with code SPEED_TEST_SERVICE_BUSY: try again in a few minutes. No account needed. Docs: https://visualsentinel.com/mcp#tools

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "url": "<url>"
    },
    "name": "vs_speed_test"
  }
}

Input Schema

NameRequiredDescriptionDefault
urlYesFull URL to test, including protocol (https://...).

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.1/5.0
Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description goes beyond them by disclosing the full metric set returned, time units (milliseconds), and the concrete failure mode (503 with the SPEED_TEST_SERVICE_BUSY code plus retry-after guidance) — real operational context an agent needs.

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?

Three tight sentences: purpose and return values first, then the error case, then prerequisites and a docs link. No repetition of the title or schema, and every sentence carries new information.

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

Completeness5/5

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

With no output schema, the description correctly takes on the burden of listing the returned metrics and their units, and it covers the one meaningful failure path. For a single-parameter read-only tool, nothing an agent needs in order to call it and interpret the result is missing.

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?

There is a single parameter with 100% schema description coverage, and the schema already spells out that a full URL including protocol is required. The description adds nothing about the url parameter, so the baseline 3 applies.

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 and resource — 'one-shot Lighthouse lab test on a URL' — which unambiguously separates it from monitor-oriented siblings like vs_monitors_check_now or vs_website_check. The word 'one-shot' signals an ad-hoc measurement rather than a configured recurring check.

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

Usage Guidelines3/5

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

Usage is implied by 'one-shot' and 'on a URL', and the 503 SPEED_TEST_SERVICE_BUSY retry guidance is genuinely useful. However, it never explicitly says when to reach for this instead of vs_website_check, vs_monitors_check_now, or the monitor performance-update tools, so the agent must infer the boundary.

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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