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

Annotations declare readOnlyHint=false, openWorldHint=true, and destructiveHint=true, which is unusual for a comparison tool. The description adds critical behavioral context: it costs $0.12 USDC via x402 and requires an x402-aware client, which is a significant operational constraint. However, the destructiveHint=true annotation is not explained in the description—it's unclear what destructive side effect occurs. This is a minor gap given the description does disclose the payment and client requirements.

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?

Three sentences with no waste. The core function is front-loaded, outputs are listed compactly, and the cost/client constraint is saved for the end. Every sentence earns its place.

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?

For a 2-parameter tool with no output schema, the description covers the main inputs, outputs, cost, and client requirement. It doesn't explain the destructiveHint=true annotation or what 'bounded performance evidence' means, but the core information an agent needs to decide whether to call this tool is present. The missing output schema is a structural gap, not a description gap.

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 description coverage is 0%, so the description must compensate. It mentions 'url' and 'baselineJobId' implicitly by saying 'Compare a page against a completed ViewportWitness baseline job'—the page maps to url and the baseline job maps to baselineJobId. However, it doesn't add detail about URL format, whether the URL must be publicly accessible, or how to obtain a baselineJobId. The description adds some meaning but doesn't fully compensate for the 0% schema coverage.

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 clearly states the tool's function: comparing a page against a completed ViewportWitness baseline job. It lists specific outputs (pixel-diff percentages, accessibility issue IDs, error deltas, performance evidence, diagnosis) and distinguishes it from siblings by naming the baseline job requirement. The verb 'compare' plus the resource 'page against baseline job' is specific and unambiguous.

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?

The description implies when to use this tool: when you have a completed baseline job and want to compare a page against it. It doesn't explicitly state when not to use it or name alternatives (check_page, get_report, verify_page), but the baseline job requirement provides clear context. The cost and x402 client requirement are important usage constraints that are disclosed.

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