Get a book cover

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

Beyond the readOnlyHint annotation, the description reveals that the tool returns an image, that images over 1 MB are refused due to context window protection, and that the coverUrl is an alternative. This adds useful behavioral constraints without contradicting the annotation.

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?

The description is three sentences long, front-loaded with the core purpose, and each sentence provides essential information with no redundant text. It is concise and well-structured.

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?

While the parameter is fully covered and the tool is simple, the description is vague about the exact output format (e.g., raw image bytes, MIME type) and error behavior for missing covers or invalid IDs. Without an output schema, more detail on the return value would improve completeness.

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 100% with a descriptive parameter comment. The description adds value by stating that book IDs originate from other tools and that null IDs indicate no cover, which is not present in the schema. This extra context enhances understanding.

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 'Fetches the cover of a book and returns it as an image,' using a specific verb and resource. It is distinct from sibling tools (search, list, stats) which perform different operations. The mention of where book IDs come from further clarifies the tool's role.

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 provides clear context: book IDs come from other tools, and for images over 1 MB, it advises using the coverUrl from the book entry instead. This effectively tells the agent when to use the tool and when to avoid it, though it does not name an alternative tool explicitly.

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