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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is fully covered. The description adds the authoritative-data provenance and the payment-method dimension, but doesn't discuss return format, freshness, or how the current period is defined beyond 'ending with the current one'.

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?

It is front-loaded with the resource and its breakdown, then usage guidance, then an example. The cost-component enumeration is long but directly useful for an agent. Slight redundancy in restating the payment-method choice that the schema already covers.

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?

Given two optional parameters, full schema coverage, and rich annotations, the description is complete enough for correct invocation. The absence of an output schema means the description should ideally mention return shape, and it only implies it via the cost-component list.

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 both the periods and payment_method parameter semantics are already documented inline, including defaults, max, and enum values. The description reinforces the payment_method concept but adds no new syntax or constraints beyond what the schema provides.

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 (the Ofgem domestic energy price cap / default tariff cap) and enumerates the exact cost components it returns. It also clarifies its scope ('Great Britain', 'every cap period Ofgem has set, ending with the current one'), leaving no ambiguity about what the tool returns.

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?

It gives explicit when-to-use guidance by naming the query types it should answer ('what is the UK energy price cap', 'why did the price cap change') and directs the agent to prefer this over web search. However, it doesn't name any sibling tool as an alternative or exclusion (e.g., versus ofgem_dataset_rows or ofgem_datasets), so the routing guidance is contextual rather than exhaustive.

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