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

Annotations already cover readOnly/idempotent/closed-world, and the description still adds substantial behavior: the insufficient_snapshots edge case (status "insufficient_snapshots" and diff: null, explicitly not an empty diff), required attributions ('Powered by CoinGecko', source_terms), and the data_contract envelope on every answer.

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 the core diff purpose and return shape before trailing boilerplate. It is dense with many clauses, and the disclaimer/attribution/paid-route tail is longer than a caller strictly needs, but nearly every sentence carries functional information.

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?

No output schema exists, so the description carries the return burden and does so thoroughly: the top-level shape, per-row fields (country_code, source_quality, changes[] with field/basis/from/to), the empty-result edge case, and the appended contract envelope. Nothing needed to call and interpret it 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 coverage is 100% and both params are documented there, so baseline is 3. The description goes further by supplying default behavior ('defaults to the two newest') and directional meaning (from = earlier, to = later), which the schema only implies, plus a concrete worked example.

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 ('What changed') and resource ('two frozen, immutable sovereign-reserve snapshots'), and the diff framing cleanly separates it from the sibling get_sovereign_reserves (which returns a single snapshot). An agent can pick the diff tool 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?

It gives clear trigger context ('between two snapshots', 'defaults to the two newest') and a when-not rule ('for bulk, history, backfills and scheduled pulls use the CSVs and datapackage.json'). It does not name the sibling get_sovereign_reserves as the alternative for single-date reads, so routing between those two is left to inference.

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