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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety and external-lookup behavior are covered. The description adds useful context about the returned payload (narrator, duration, audio preview, ebook link), though this overlaps with the output schema and omits auth/rate-limit behavior. Modest added value over structured fields.

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 tightly written sentences with zero filler; the core capability is front-loaded and the routing guidance follows immediately.

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?

Complete for a detail-lookup tool: annotations cover the safety profile, an output schema documents the return shape, schema coverage is full, and the description provides sibling routing. Nothing an agent needs to invoke this 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 ID format (numeric example), Lang example (en-US), and Country default (US) are already documented in the schema. The description only names the parameters, adding no syntax or constraint detail beyond what the schema provides. Baseline 3 is appropriate.

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), resource (public Apple Books audiobook details), an explicit keying scheme (ID, Country, Lang), and enumerates the returned content (narrator, duration, audio preview, ebook link). It also distinguishes itself from apple_books_book and apple_books_audiobook_search by name.

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?

Explicitly routes the agent: use apple_books_audiobook_search to discover audio editions and apple_books_book for ebook details, implying this tool is for direct lookup by known ID. It doesn't state negative conditions (e.g., what happens with an invalid ID), but the alternative-selection guidance is clear and actionable.

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