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

Annotations already establish readOnlyHint=true and openWorldHint=false, so the agent knows this is a safe, self-contained calculation. The description usefully adds that rates come from dated sources and are UK-specific and that an own price can override them, but says nothing about rounding, currency output, or staleness handling.

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 core output units (per charge, per mile, per 100 miles) are front-loaded before the tariff qualifiers. Minor parenthetical load but nothing that wastes the reader's time.

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 output schema, the description carries the return-shape burden and does describe the three cost figures produced. It is largely sufficient for a 4-parameter pure calculation tool, with only currency and precision left unstated.

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 description coverage is 100%, so battery_kwh, start_percent, end_percent and price_pence are already self-documenting. The description adds only the meaning of price_pence as an optional own price, which the schema also states, 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?

States a specific computation (cost of one charge, per mile and per 100 miles) against named UK tariff bases, so the agent knows exactly what is produced. It does not, however, name or contrast itself with plausible siblings such as charging_time or charger_size, so differentiation relies on the name alone.

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 mention of home cap, public and rapid prices suggests this is the costing tool rather than a timing or sizing tool. There is no explicit when-to-use/when-not statement and no routing to siblings such as charging_time for duration questions.

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