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

Annotations only declare readOnlyHint/openWorldHint; the description goes further by disclosing the price ($0.002 USDC per successful call), the payment rails (x402 on Base, Solana or Arc) and the billing policy that failed calls are not charged. That is exactly the kind of cost/auth context an agent needs before invoking a paid endpoint.

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 sentences: the first front-loads coverage and scope, the second carries source, output limitation and price. No filler, and each clause maps to a decision the caller has to make.

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?

Output schema exists so return shape needn't be explained, both params are covered by the schema, and the cost model is stated. What remains thin is failure-mode behavior beyond billing (e.g. what happens for an out-of-range year), a minor gap for a 0-required-param read 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 coverage is 100% and the schema already documents the optional 4-digit year and the three division values with 'england-and-wales' as default. The description restates the divisions in prose but adds no format or slug guidance beyond what the schema carries, so 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?

Names the exact resource ('UK public (bank) holidays'), the official source (GOV.UK) and the three supported divisions, plus the optional year scoping. No sibling in the list covers holiday calendars, so the tool is trivially distinguishable from the crypto/market-data tools around it.

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?

'Optionally for one year' and 'dates only, no working-day calculation' tell the agent when this applies and, importantly, what it should NOT be used for (working-day/settlement logic). It stops short of naming an alternative for that excluded case, so it's a strong 4 rather than a 5.

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