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

With no annotations, the description carries the burden and does well: it discloses the conditional dependency (solar share requires spare solar production) and the output caveat that omitting extraEvCost suppresses the payback estimate. It does not describe the shape of the returned savings series, though an output schema exists to cover that.

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?

Four sentences, front-loaded with the computation and then the data-gathering rules. The sentence 'Use to compare EV and petrol/diesel running costs' largely restates the opening and is mild redundancy, but nothing else is wasted.

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 20-parameter calculator with an output schema, the description covers the collection policy, the conditional requirements, and the one output caveat, which is everything an agent needs to invoke it correctly. Units and defaults are handled by the schema, so no gap remains.

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 parameter meanings are already given; the description adds genuine value beyond that by explaining the cross-field condition (solarDirectShare > 0 requires spareSolar) and the optionality semantics of extraEvCost. It could have said more about how the three charging shares interact, which keeps it from a 5.

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?

Opens with a specific verb and resource: compares annual running cost of an EV against a petrol/diesel car in Cyprus, with the charging split (solar, home grid, public) and outputs (yearly savings, payback on extra purchase cost). This clearly distinguishes it from siblings like compare_rent_vs_buy or compare_investment_vehicles without needing to open the schema.

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?

Gives concrete invocation guidance: ask for annual distance, consumption, fuel price, charging shares and tariffs, and require spare solar production when solar charging is selected. It also says to reuse supplied facts and only ask for missing essentials. It does not name sibling alternatives or say when NOT to use it, so it falls short of a 5.

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