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

Annotations cover safety hints (idempotent, non-destructive), and the description adds genuinely non-obvious behavior: the 40-book cap, the want-to-read default, that already-present books are left in place and reported back, and that 'read' books publish to the activity feed subject to profile privacy defaults. Nothing contradicts the annotations; the idempotent hint is consistent with 'left where they are'.

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 purpose and cap, then selection guidance, then side effects, then an example — a sensible order with no filler. The activity-feed/privacy sentence is long and heavily nested, which costs a little readability.

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?

An output schema exists, so return values need no explanation; the description still covers inputs, cap, default shelf, duplicate handling, downstream visibility, and the sibling alternative. For a 2-parameter mutation tool this is complete enough to call correctly on first try.

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% (including the status enum and the 40-max on book_ids), so the baseline is 3, but the description adds that status applies uniformly to every newly added book and that absent status means want-to-read. The concrete JSON example reinforces the shapes. Minor gap: it does not explain what happens when book_ids exceeds 40.

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 first sentence states a specific verb (adds), resource (books to the reader's library), scope limit (up to 40, one call), keying (by ReadingRate book id), and default shelf behavior. It is clearly distinguishable from add_book_to_library (single, unmatched books) and lookup_books_batch (matching, not adding), both of which it names.

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?

Explicit routing: use after lookup_books_batch has matched a shelf or list and the reader has chosen which books to add; unmatched books go through add_book_to_library once the title is confirmed. It also states the tool never adds to the catalog, an explicit when-not condition.

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