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

With no annotations, the description does real work: it discloses pricing (€0.004 per shot), the billing/auth requirement (API key in Authorization: Bearer), and a public-URL-only constraint. It stops short of failure behavior, rate limits, or how the image is returned (binary vs URL), which matters for a paid external render service.

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?

Front-loaded with purpose, features and cost before the parameter block; every sentence carries information. The 'Parameters —' list is dense but scannable, with only minor redundancy between the capability sentence and the parameter list.

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 9-parameter, unannotated tool with a 0%-coverage schema and an output schema that covers return values, the description supplies auth, cost, scope limits and full parameter semantics. The remaining gap is operational behavior on errors or timeouts.

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% and every parameter is an untyped string|null, so the description carries the entire burden and does it well: it gives types, ranges (width 320–3840, wait 0–8000), defaults (1280x720, png), enum values (png|jpeg|webp, scale 1|2), and the meaning of selector, full_page and dark.

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?

Opens with a specific verb+resource: 'Rendered screenshot of any public URL', then enumerates capabilities (viewport, full page, element by CSS selector, dark mode, retina 2x). An agent knows exactly what the tool produces, though it never names a sibling such as og_image, html_pdf or browser to differentiate.

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 phrase 'any public URL' implies a scope limit (no authenticated or private pages), but there is no explicit when-to-use / when-not-to-use guidance and no routing to alternatives like og_image or url_to_markdown. The agent must infer selection from the feature list alone.

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