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

Annotations already declare readOnly=false, idempotent=true, destructive=false, closed-world, so the safety profile is covered; the description's genuinely additive fact is that dates default to today and can be nulled when unknown. It does not say what happens to existing startedOn/finishedOn values when the status changes, whether progress is reset, or what error surfaces if userBookId is not in the library - relevant for a mutation.

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, front-loaded with the verb and the state values, with no filler. Deduction because "Dates default to today" uses a vague collective noun instead of naming startedOn and finishedOn, forcing the agent to map it back to the schema.

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?

For a 4-parameter mutation with 25% schema coverage, no output schema, and four annotations, the definition is adequate but thin: it explains date defaulting yet omits the status/date interaction, the source of userBookId, and failure conditions. Enough to attempt a call, not enough to call it confidently in edge cases.

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 only 25% (only status is described), so the description must compensate: it usefully clarifies the enum states and the default/null semantics of the two date fields. However it never explains userBookId (presumably from list_library/get_library_book) or which date belongs to which transition, leaving half the parameter meaning implicit.

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?

"Moves a library book to TO_READ, READING or COMPLETED" gives a specific verb (moves), a specific resource (library book), and the exact state machine values, so an agent knows it is a status mutation. It falls short of 5 because it never names the sibling it is not - notably log_reading_progress, which also changes reading state - so the boundary must be inferred.

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?

There is no when-to-use or when-not-to-use guidance; the description never mentions log_reading_progress, rate_book, or get_library_book, despite a large sibling set with overlapping concepts. The only conditional instruction is about passing null for dates, which is a parameter rule rather than a selection rule.

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