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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=false), so the bar is lower. The description adds genuinely new behavioral context the structured fields lack: a per-call cost of US$0.02 or one Horse Truth credit, tied to 'successful' calls, which matters before an agent invokes it repeatedly.

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?

The payment line is well front-loaded and earns its place, but the idea 'two to five horses' is repeated three times across the first, second, and fourth sentences. The middle sentence largely duplicates the opening one, so the text is longer than the information content requires.

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 no output schema, the description carries the burden of explaining returns, and 'compact derived snapshots' leaves the content and shape of the comparison unspecified. Safety, cost, and routing are covered, but an agent cannot anticipate what measures come back or in what form.

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 the baseline is 3: the schema already documents the horses array, its 2-5 bounds, form-guide formatting, country-suffix handling, and the resolve_horse fallback. The description only restates the count range, adding no syntax or naming detail beyond the schema.

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 ('Compare compact derived snapshots for two to five horses') with the cardinality bound explicit. It also names the sibling it does not replace (horse_snapshot for a single horse), so an agent can separate the two without opening either 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?

Gives an explicit when-to-use condition ('when you want two to five horses side by side on the same derived measures') and a named alternative with its own trigger ('Use horse_snapshot instead for a single horse'). The 2-5 vs 1 boundary is unambiguous.

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