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

Annotations already declare readOnly, idempotent, and non-destructive, so the bar is lower. The description adds real behavioral context beyond them: it is unmetered/no-credit, requires sign-in, and explains png fallback behavior. It does not cover pagination or response shape, but the output schema exists.

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, front-loaded with the core purpose, then cost, then format/auth caveats. No filler or repetition; it is tight though slightly dense.

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 read-only download tool with full schema coverage and an output schema, the description covers auth, billing, and format behavior adequately. Nothing essential is missing, though return/download mechanics are left to the output schema.

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?

With 100% schema description coverage the baseline is 3, and the description meaningfully exceeds it by explaining the 'format' parameter semantics: png returns the lossless sibling when one exists and falls back to jpg. That is genuine meaning the schema's one-line 'Default jpg' does not convey.

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 ('Full-resolution download link for a StockCake image... or one of your edited images') and scopes it to full-resolution CC0/edited assets, which implicitly separates it from get_image, resize_image, and upscale_image. It does not explicitly name a sibling to contrast against, so it stops short of a 5.

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 supplies useful context ('Unmetered — no credits') and a prerequisite ('Requires a free StockCake account'), but gives no explicit when-to-use/when-not guidance or alternative routing versus the sibling tools. Usage is implied rather than directed.

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