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

Annotations already establish a safe, idempotent, closed-world read, and the description goes well beyond them: it states what the result contains (BS 6700 minimum, a rounded recommended size, reheat time), the 45 l/person/day basis, the reheat formula (volume x temp rise / 14.3 x kW), and the counterintuitive coil-sizing caveat. No contradiction with annotations.

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?

Front-loaded with purpose and method, then the occupancy rule, the reheat logic, and the outputs, so an agent gets the essentials first. It runs long, though; the coil-versus-boiler paragraph is educational context rather than something needed to call the tool correctly.

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?

With six parameters, an output schema present, and full schema coverage, the description supplies everything else an agent needs: method provenance, the occupancy fallback for new builds, the formula dependencies, and the advisory caveat. Return values are covered by the output schema and echoed usefully here.

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 baseline is 3, but the description genuinely adds meaning the schema lacks: the 45 litre daily multiplier behind 'N', the explicit statement that a larger heat pump yields a smaller cylinder, and the reheat-time relationship touching kW, storage and cold-water temperatures. It does not restate parameter syntax, which is fine given 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 names a specific verb and dual resource: working out a dwelling's daily hot water demand and the cylinder size a heat pump needs. It also cites the governing method (MIS 3005-D 3.5.2, BS 6700/MGD 007), which cleanly separates it from siblings like heat_pump_size and room_heat_loss.

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 frames the tool as 'for understanding a system and checking a quotation' and warns that installed work still requires a certified installer's design and survey, which tells the agent the outputs are advisory not prescriptive. It gives the condition for switching input basis (new build → bedroom count), but never names a sibling tool or an explicit when-not-to-use case.

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