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

Annotations already establish the safety profile (read-only, idempotent, open-world, non-destructive). The description adds a genuinely useful behavioral fact they don't carry: it is free within a daily allowance and then costs $0.001 USDC per call via x402, which is relevant to invocation decisions.

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?

Two tight sentences with the purpose front-loaded ahead of the pricing note. Nothing is wasted, though the pricing clause is tangential to 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 an output schema covering return values, full annotation coverage for safety, and complete parameter documentation, the description only needs to add purpose and any special behavior. It does both (purpose + pricing), leaving usage routing as the notable gap.

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 single parameter has 100% schema description coverage ("ONS parish code"), so the schema does the documenting. The description adds nothing beyond it and only implies the input is a parish lookup; baseline 3 fits when the schema carries the load.

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 states the concrete return ("The ward(s) and local authority for a civil parish"), making it clear this is a reverse lookup from parish to its containing ward and local authority. It does not, however, distinguish itself from nearby siblings such as uk-geography__ward or uk-planning__parish, so the boundary between them is left to inference.

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?

There is no statement of when to use this tool versus the many sibling geography lookups, no prerequisites, and no exclusions. The only guidance offered is billing, not usage context, so an agent gets no routing help.

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