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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered structurally. The description adds behavioral context by listing the returned field set (rating histogram, series linkage, audience), which hints at output richness, but says nothing about rate limits, storefront scoping effects, or error behavior for invalid IDs. Adequate but not rich beyond the annotation baseline.

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?

Two sentences with zero waste: the fetch scope and returned fields are front-loaded, followed immediately by the sibling-routing clause. Nothing is redundant or padded.

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 covering return values and annotations covering the safety profile, the description only needs to convey purpose and routing — both of which it does fully. Nothing an agent needs to call this tool correctly is missing.

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 all three parameters (ID, Country, Lang) are fully documented in the schema. The description names them but adds no format or semantics beyond what the schema already states, so the baseline 3 is appropriate when the schema carries the burden.

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 ('Fetch') and resource ('Apple Books ebook details') keyed to ID/Country/Lang, then enumerates the returned fields (ISBN, page count, publisher, audience, rating histogram, series linkage). It explicitly distinguishes itself from the sibling tools that find, review, or recommend, so an agent can tell it apart from apple_books_book_reviews and apple_books_book_similar without opening a 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?

Names three specific alternatives with their distinct roles: apple_books_search to find titles, apple_books_book_reviews for reviews, apple_books_book_similar for recommendations. This tells the agent not just when to use this tool but which sibling to route to instead, which is exactly the routing guidance required.

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