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

Annotations already cover read-only, idempotent, non-destructive, and closed-world behavior, so the bar is lower, and the description still adds real value: it discloses the language-fallback behavior (a language with no page on the site is simply omitted, so the list can be shorter than the catalogue's languages) and explains that `url_language` signals when `url` is in a different language than requested. These are non-obvious output-shape traits an agent would otherwise get wrong. It stops short of saying what happens when the id does not resolve.

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 the core action, then the returned fields, then the one edge case, so the agent can stop reading early. It is a single dense sentence plus a short follow-up with no filler. The long parenthetical about missing language pages is heavy but carries genuine information rather than padding.

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 no output schema, the description correctly takes on the job of enumerating return values, and it does so field by field, including the caveat about truncated language lists. Annotations handle the safety profile, so nothing critical is missing. The remaining gap is error behavior for an unknown id and any indication of whether print-edition data can be large.

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% and both parameters carry thorough descriptions, including the enum labels and the default-language rule, so the baseline of 3 applies. The description largely restates the language behavior the schema already documents ('in the chosen language', 'url_language says when url is in another language than the one asked') rather than adding new syntax or format detail. It introduces `url_language`, but that is a return field, not a parameter.

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 ('Get the catalogue facts of one title') and enumerates the exact fields returned: title, subtitle, author, original title, cover address, print editions, and per-language title page addresses. That is far more specific than the name alone. However, it never distinguishes itself from close siblings such as get_book_facts, get_book_outline, or get_summary, so an agent cannot tell from the description alone which of these overlapping 'book facts' tools to pick.

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 contains no when-to-use, when-not-to-use, or alternative-tool guidance at all. It does not say that an id must first come from search_books (that hint lives only in the schema), nor does it steer the agent away from get_book_facts or toward get_book_outline for chapter-level content. Any usage context must be inferred entirely from the sibling list.

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