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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered; the description adds genuinely useful behavioral context beyond that — prices are fetched fresh from the store, and the comparison deliberately excludes delivery, discounts and retailer prices. This bounds what the output can legitimately be used to claim. It stops short of describing the ranking output format or any failure behavior when prices are unavailable.

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?

Two tight sentences with zero padding; the scope and the per-can/per-litre basis come first, followed by the exclusion caveat that most affects interpretation. Every clause carries 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?

For a read-only, single-optional-parameter tool with no output schema, the exclusion caveats make it usable, but two gaps remain: what the `ids` filter does when empty or omitted, and roughly what the ranking response looks like. Adequate but not complete.

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?

The single parameter `ids` has 0% schema description coverage and the description never mentions it — not its meaning, not that it is optional (0 required), and not what happens when it is omitted (compare all packs vs. none). With low coverage the description was expected to compensate, and it does not.

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 names a specific verb+resource ('Compares available one-time IMPOSSIBREW packs') and states the metric basis (per-can and per-litre arithmetic with fresh store prices), so an agent can distinguish it from the broader comparison siblings. It does not explicitly name compare_beers or compare_alternatives, so sibling differentiation is implicit rather than stated.

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 scopes the result ('rankings cover the same beer and can size only') and rules out delivery, checkout discounts and retailer prices, which implies when the tool is a poor fit for total-cost questions. However, it never names the alternative tool an agent should use for the cheapest overall deal, so the routing decision 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.