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

With no annotations provided, the description carries the full burden and does a good job: it states what is returned (cap per region, fuel, payment method), units (pence per kWh and pence per day including VAT), metadata (source_url and verified_at), and the period behavior including advance availability of the next quarter. It does not explicitly state 'read-only' or discuss rate limits, but 'Returns...' makes the read nature clear enough.

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 description is information-dense with no filler. Every sentence earns its place: the opening sentence defines the use case, the middle sentences specify output units and period behavior, and the final sentence excludes the wrong tool. It is slightly longer than minimal but all content is directly useful to an agent.

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 read-only data lookup with five optional parameters and no output schema, the description covers the input intent, the output format, the units, the temporal behavior, and the boundary against sibling tools. Nothing an agent needs to decide whether to call it or interpret its result is missing.

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 60%, and the description adds meaningful context beyond the schema: it explains the default period behavior, that the next quarter is usually on file, the unit semantics, and that values include VAT. It also clarifies what the three payment methods refer to and ties the return values to unit rates and standing charges. It does not add much for the region parameter beyond what the schema already states, but overall it compensates well for the coverage gap.

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 names a specific verb ('Use this when...'), the resource ('Ofgem default tariff cap'), and the exact scope: current UK energy price cap, unit rate, standing charge, and payment method comparisons. It also distinguishes itself from individual supplier tariffs, so an agent can reliably separate it from get_uk_energy_tariffs.

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 explicitly states when to use the tool ('when a user asks what the current UK energy price cap is, the default tariff unit rate or standing charge in their region...') and explicitly excludes the alternative use case ('Not for individual supplier tariffs') while naming the sibling tool to use instead (get_uk_energy_tariffs). This is strong routing guidance.

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