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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety profile is covered. The description usefully signals the dataset is a complete, static, fully legislated table — implying no filtering or pagination is needed — but says nothing about response shape (table vs. records) or how multi-year data is laid out.

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?

A single front-loaded sentence with no filler; the resource, dimensions, and coverage scope all appear before any trailing clauses. Nothing is repeated from the title verbatim.

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?

With no parameters, no output schema, and annotations conveying read-only/local behavior, the description covers the essentials. A brief note on the returned structure (e.g., per-year rows keyed by CO2 band and range) would fully close the gap for a lookup tool of this nature.

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?

Zero parameters, so there is nothing for the description to document and the baseline of 4 applies. The phrase 'by CO2 band and electric range, for every legislated tax year' does add useful framing about the dimensions the returned data is keyed on, which goes slightly beyond an empty schema.

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 a specific resource (HMRC benefit-in-kind appropriate percentages for company cars) and its two classification axes (CO2 band, electric range) plus coverage (every legislated tax year). It is distinguishable from a generic fleet-rates tool, though it never explicitly contrasts itself with the adjacent get_hmrc_fleet_rates.

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 only implied: an agent can infer this is the lookup for BiK percentages. There is no explicit when-to-use statement, no mention of which sibling to call for other fleet rate questions, and no note on how to handle a specific tax year (the 'every legislated tax year' phrasing hints the full table is returned, but never says so directly).

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