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

Annotations already declare readOnly, idempotent, non-destructive, so the safety profile is covered. The description adds context annotations cannot: a per-day quota that varies by plan and the prescribed behavior when the quota is exhausted. It does not say whether the screenshots are cached or how long generation takes.

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?

A single sentence with zero waste: the trigger condition, the render specifications, the quota constraint, and the fallback all appear in tight, front-loaded order.

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 an output schema present, the description need not describe return values, and annotations cover safety. It covers trigger, scope, viewport details, and quota, but leaves the quota-exhausted alternative tool unnamed, a small gap for an agent deciding what to call next.

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 100%, so both parameters (site_id format and the page default) are already fully documented in the schema. The description only adds 'one page', which is the schema default restated, so baseline 3 applies.

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?

States a specific verb and resource — screenshots of one page — and specifies the two viewports rendered (desktop 1280 px top-to-bottom, phone 390 px). An agent can distinguish this visual-preview tool from HTML-oriented siblings such as get_site or get_stats without opening any schema.

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?

Gives explicit timing guidance ('use after create_site or a bigger change') plus a fallback path when quota is exhausted ('review the HTML instead'). The fallback does not name the concrete sibling tool (get_site), so the agent must infer the target, which keeps it just below a 5.

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