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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so safety is covered. The description adds real context beyond that: an explicit price ('PAID: US$0.02 per successful autonomous x402 call, or 1 Horse Truth direct credit') and the ordering/return shape ('newest first, each with the fields that changed and when'). It doesn't mention rate limits or whether the credit is charged on failure, so it stops short of a 5.

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?

Four sentences, each doing distinct work: cost, payload, when-to-use, alternatives. The pricing constraint is front-loaded where a cost-sensitive agent will see it, and there is no repetition or filler.

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 must carry the return shape, and it does state the unit (change events with changed fields and timestamps, newest first). It stops short of describing pagination behavior or whether an empty result is possible for a horse with no material changes, which is the only remaining gap for this otherwise well-specified tool.

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%, including naming conventions, the (NZ) suffix rule, the resolve_horse fallback, and the 1-100 range with default 50, so the schema does the heavy lifting. The description only reinforces ordering ('newest first') and adds no syntax or format detail the schema lacks, so the baseline 3 applies.

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 description states a specific verb and resource: 'Return material Horse Truth change events for one horse,' with scope ('newest first,' 'each with the fields that changed and when'). It explicitly distinguishes itself from horse_snapshot (current state) and horse_explanation (reasons), so an agent can separate it from siblings 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?

Usage is stated both positively ('Use this tool when you want the history of material changes for one horse') and by exclusion, naming two sibling tools and the conditions that select them (current state, reasons behind it). Nothing 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.