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

With no annotations, the description carries the full burden. It usefully discloses that the data is FREE and priced in p/kWh inc VAT under Agile Octopus, but it does not define the 'upcoming' horizon, update cadence, or behavior when no slots are available.

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?

The definition is a single, front-loaded sentence with the key cost signal 'FREE' first. Every element earns its place by specifying tariff, unit, VAT inclusion, count, and region scope.

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 simple one-parameter read tool with no output schema, the description supplies enough context to invoke it correctly. It could be more complete by specifying the return fields or the time horizon for 'upcoming', but the core invocation requirements are clear.

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 the single postcode parameter is already documented as setting the electricity region. The description reinforces that the postcode determines region, but adds no syntax or format detail beyond the 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?

The description states a specific resource and scope: the cheapest 3 upcoming half-hour electricity slots for a UK postcode region. It implicitly distinguishes itself from siblings like all_slots and daily_average by returning only the cheapest upcoming slots, but it does not explicitly name alternatives.

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 implied: call this when you want the cheapest imminent half-hour slots rather than all slots or averages. However, there is no explicit guidance on when to choose this over best_window, all_slots, daily_average, or export_rates.

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