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

Beyond the readOnlyHint/openWorldHint annotations, it discloses that it only reports what it could genuinely read and lists what it could not, that it sees source not the rendered screen (so contrast, hover/focus, runtime variables are out of reach), and that it is free and unmetered. This is rich behavioural context the annotations do not carry.

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

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

It is front-loaded with the triggering scenario and the return shape before the args block, and every sentence carries usable information (what it measures, what it returns, its limits). It runs somewhat long for a single-parameter tool, but there is little outright waste.

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

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

With no output schema, the description steps in and describes the return value precisely (findings worst-first with value, percentile, corpus median, and the change to make). It also covers inputs, limits, and the alternative for rendered-DOM checks, leaving nothing essential unstated.

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

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

Schema coverage is 0% for the single 'code' param, so the description must compensate: it specifies accepting CSS, JSX/HTML with class attributes, or both, notes Tailwind utilities are read on the default scale, and warns to 'paste the real thing, not a summary.' That adds substantial meaning beyond the bare string schema.

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

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

The description states a specific verb and resource — measure a code paste and return graded numbers — and enumerates the exact dimensions audited (type ladder, spacing grid, radii, transition timing, container width). It explicitly differentiates itself from siblings: 'the one tool here that reads YOUR work instead of someone else's product,' and routes DOM-dependent checks to get_score(domain).

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

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

It gives a concrete trigger ('when the user says a screen looks generic, cheap or off and you need to know WHY') and an explicit alternative with its condition ('for a public URL, get_score(domain) reads those from the live DOM instead'). It also states what the tool cannot cover, steering the agent away when those are the goal.

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