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

Annotations already declare readOnly/idempotent/non-destructive, yet the description still adds real operational context: history only exists from when logging began, so days before logged_since are unknown rather than unread, and the timezone resolution chain (explicit IANA zone → notification setting → UTC). That is meaningful disclosure beyond the annotations, though it never says what happens if the book_id has no recorded readings.

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 what is returned, then when-to-use, then the data-availability caveat, then examples — a sensible ordering with no filler sentences. The third example is arguably redundant with the schema's default note, but the two non-trivial ones earn their space.

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 present, return-value exposition is unnecessary and correctly omitted; the description covers scope, alternatives, timezone resolution, and the log-start data gap. The only loose thread is that `logged_since` is referenced without being defined, and behavior for an out-of-range `reading_number` is unaddressed.

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 already 100%, so the schema documents the three params; the description still adds value by explaining the fallback behavior behind `timezone` and by exemplifying its format. The `reading_number` default (latest) is only stated in the schema, so the description adds no extra there.

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 — per-day page/progress records for a single reading of one book — and enumerates the exact fields returned (furthest page, page left off, pages read, percent). It explicitly routes the reader to get_reading_history for the book-level metadata, so it is unmistakable against siblings like get_reading_stats or get_reading_trajectory.

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?

Gives four concrete trigger questions ('how far they got each day', 'how many pages on which days', 'how long a book took day by day', 'wants a chart of their progress') and names the alternative tool plus the condition that selects it. No inference required.

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