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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered; the description adds genuine context by naming the two data sources (DESNZ average domestic electricity price and HMRC advisory rate) that drive the calculation. It does not mention caching, vintage of the rates, or whether the result is a single value, leaving some behavioral detail unstated.

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 dense sentence with no filler, and the key concept (cost per mile) is front-loaded. It is a noun-phrase fragment rather than a complete clause, which is slightly less scannable than a verb-led statement but costs no extra words.

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 zero-parameter tool this is close to complete on the input side, but there is no output schema, so the description should say what the caller receives — a numeric pence-per-mile figure, a delta against the HMRC rate, or a structured comparison. As written, the return shape and units must be inferred.

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 tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline of 4 applies. The description correctly implies no inputs are needed by presenting the calculation as a fixed national comparison.

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 states a specific output ("real cost per mile") and a specific comparison (home charging from the DESNZ domestic price versus HMRC's advisory electric rate), which is clearly distinct from siblings like get_charging_cost_gap. It lacks any explicit differentiation from those siblings, so it stops short of a 5.

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: the phrase "at home" and the DESNZ/HMRC framing signal this answers a home-charging cost comparison. There is no statement of when to prefer it over get_charging_cost_gap or get_fleet_charging_costs, and no exclusions or prerequisites.

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