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

Annotations only cover read-only and open-world; the description goes well beyond by disclosing the paid x402 flow ($0.003 USDC per successful call, accepted on Base, Solana, Arc) and that failed calls are not charged — critical invocation context. It also names the upstream source and license (UK Trade Tariff API, OGL v3) and the post-Brexit coverage boundary.

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?

Three sentences, front-loaded with the purpose and return fields, then the search mode, then coverage and cost. No filler; each sentence carries distinct information (scope, search semantics, price/coverage).

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?

An output schema exists, so return-shape explanation isn't required, yet the description still previews the fields plus the payment and coverage constraints an agent needs before calling. Nothing material for correct invocation 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%, so the baseline is 3, but the description adds interpretive value: it ties `origin` to preferential-rate lookups for a given country and clarifies that `q` drives heading search rather than code lookup. It does not add format/syntax detail beyond the schema examples.

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 precise verb+resource ('Returns UK Global Tariff data for a 6–10-digit commodity code') and enumerates the payload (description, third-country duty, VAT, measures, preferential rates). The 'UK only, post-Brexit schedule' scope line separates it from tariff_us even without naming that sibling.

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?

Explains the two call modes clearly: pass a commodity code for tariff data, use `q` for text search that returns matching headings. It also scopes coverage to the UK schedule and notes the pricing model. It never names tariff_us or states when-not to use this tool, so it stops short of an explicit alternative routing.

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