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

Annotations already declare readOnly, idempotent, closed-world and non-destructive, so the safety profile is covered. The description adds genuine behavioral content beyond that: it enumerates the categories of reference data returned (compass directions, postcode zones, design temperatures, etc.), telling the agent what knowledge it gains.

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 the core purpose, then a dense but purposeful enumeration of the reference datasets, closing with the usage trigger. Every clause carries information, though the long middle list is heavy for a single sentence.

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 no parameters, an output schema present, and rich annotations, the description need not explain return structure or safety. It covers purpose, scope, returned content categories, and the invocation condition, so an agent has everything needed to 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?

The tool takes no parameters, so there is nothing to disambiguate; the baseline for a zero-parameter tool is 4. Nothing in the description misleads about inputs.

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 (lists) and resource (every calculator plus its reference data), and disambiguates what kind of calculators (whole-system sizing vs single-method) so an agent can distinguish it from the sibling calculator tools like heat_pump_size or solar_pv_system_size.

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?

Explicitly gives the trigger condition: 'Call this when a value must be one of a fixed set and you are not sure what the set is.' This is clear context for invocation, though it does not name which sibling to prefer or state exclusions when a value is already known.

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