govuk-mcp

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

Annotations already provide readOnlyHint=true, destructiveHint=false, idempotentHint=true, and openWorldHint=true, covering safety and idempotency. The description adds valuable context beyond annotations: it discloses the external API used ('postcodes.io public API'), authentication requirements ('no key required'), and typical use cases (government service direction). It does not contradict annotations, as 'look up' aligns with read-only behavior.

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 front-loaded with the core purpose in the first sentence, followed by usage guidelines and implementation details. Each sentence earns its place: the first defines the tool, the second explains when to use it, the third gives practical examples, and the fourth discloses the API source. There is no redundant or verbose content, making it highly efficient.

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 the tool's low complexity (single parameter, read-only operation) and rich annotations (covering safety and idempotency), the description is complete. It explains the purpose, usage context, and external API, and since an output schema exists, it does not need to detail return values. The description adequately complements the structured data without gaps for this straightforward lookup tool.

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 input schema has 100% description coverage, fully documenting the single 'postcode' parameter with examples and constraints. The description does not add any parameter-specific information beyond what the schema provides, such as format details or edge cases. With high schema coverage, a baseline score of 3 is appropriate, as the description relies on the schema for parameter semantics.

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 clearly states the specific action ('look up a UK postcode') and the resource being retrieved ('local authority, region, constituency, and other administrative geography'). It distinguishes this tool from sibling tools like 'govuk_search' or 'govuk_list_organisations' by focusing exclusively on postcode-based administrative geography lookup, not general search or organization listing.

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 explicitly states when to use this tool: 'Useful for determining which council area, parliamentary constituency, or NHS region a postcode falls within' and 'Commonly used to direct users to the correct local service on GOV.UK (e.g. council tax, planning, waste).' It provides clear context about its purpose in government service navigation, distinguishing it from more general search tools among its siblings.

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