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

Annotations already mark the tool as read-only, open-world, idempotent, and non-destructive. The description adds valuable behavioral context: 'Regional figures are forecast-only' and 'Omit both for all regions at the current half-hour,' which explain data nature and no-arg behavior. It does not describe error responses or pagination, but with annotations covering safety, this is adequate.

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 three sentences: purpose, usage, and caveat. Every sentence earns its place; no redundant information. It is front-loaded with the core function.

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 output schema, the description compensates by telling the agent exactly what data is returned (carbon intensity forecast + index band, generation mix) and how to select regions. The forecast-only caveat sets appropriate expectations. It is complete for a two-parameter read-only tool, though it doesn't discuss error cases or data limitations.

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 schema already has 100% coverage with descriptions and examples for postcode and regionid. The description adds the crucial instruction that omitting both parameters returns all regions, which is not in the schema. It also clarifies that either parameter can be passed, reinforcing the schema's optional nature. This goes above the baseline for fully-covered schemas.

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 opens with a specific verb+resource: 'Carbon intensity (forecast gCO2/kWh + index band) and generation mix for a specific Great Britain region.' This clearly distinguishes it from sibling tools like get_intensity (likely national) and get_generation_mix by combining regional scope and both data types.

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?

The description gives clear instructions: 'Pass "postcode"... or "regionid"... Omit both for all regions at the current half-hour.' It does not explicitly name alternatives, but the regional scope and forecast-only caveat imply when this tool is appropriate. A missing explicit reference to sibling tools prevents a 5.

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