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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavioral context: the data is a fixed 500-litre kerosene basket, prices are the lowest published delivered price per county, and a citation page is included. It stops short of describing result size or whether any counties may be missing, which would be the last useful detail for a no-parameter bulk read.

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?

Three tight sentences, front-loaded with the trigger, then the return payload, then a short disambiguation caveat and the sibling pointer. No sentence is filler and nothing is buried.

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?

There is no output schema, so the description carries the return-shape burden and does so: per-county lowest prices, the average, and a citable page. For a no-parameter read tool with full annotation coverage, an agent has everything needed to select and call it correctly.

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?

With zero parameters, the baseline applies, but the description goes further by disclosing the implicit fixed query (500 litres of kerosene, delivered, lowest current published price). This removes the main ambiguity an agent would otherwise have about what the silent dataset call actually measures.

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 verb-and-resource combination ('heating oil prices across Ireland today'), the exact metric returned (every county's lowest current published delivered price for 500 litres of kerosene), and the derived value (national average). It also explicitly separates itself from the per-county sibling, so an agent can route without opening a 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?

Opens with an explicit trigger ('Use this when someone asks about... national average, or which county is cheapest or dearest') and names the alternative for the narrow case ('For one county use tankful_county_supplier_prices'). Both the when and the when-not are present, with the sibling that handles it.

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