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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the agent knows this is a safe read operation. The description adds the 'PAID' cost context and the 'compact derived snapshots' nature, which is useful. However, it doesn't disclose what 'compact derived' means, whether it returns a comparison table or just multiple snapshots, or any rate limits. With annotations covering safety, a 3 is appropriate.

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?

One sentence with the cost front-loaded and the core function stated clearly. It's efficient and doesn't waste words. The pricing info is arguably a separate concern but earns its place as a critical usage constraint.

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 read-only comparison tool with one parameter, the description is mostly adequate. The main gaps are: what 'compact derived snapshots' means, what the output looks like (no output schema), and whether horse strings need pre-resolution. The pricing is disclosed, which is good. Given the tool's simplicity, a 3 is reasonable.

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 0%, so the description must compensate. The description explains the 'horses' parameter implicitly: it's an array of 2-5 horse identifiers. However, it doesn't specify what format the strings should be (names, IDs, URIs) or whether they need to be resolved first. The sibling 'resolve_horse' suggests resolution might be needed, but the description doesn't clarify. Baseline 3 is fair since the description adds some meaning but leaves format ambiguity.

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 clearly states the tool compares compact derived snapshots for two to five horses, which is a specific verb and resource. It distinguishes itself from siblings like horse_snapshot (which likely returns a single horse's snapshot) and horse_changes (which tracks changes), though it doesn't explicitly name an alternative.

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?

The description implies usage: when you need to compare snapshots of multiple horses. It doesn't explicitly state when not to use it or name alternatives, but the sibling list and the phrase 'compact derived snapshots' provide some context. The pricing note is a usage consideration but not a when-to-use guideline.

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