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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false. The description adds valuable context beyond annotations by disclosing the external API dependency: 'Uses the postcodes.io public API (no key required).' This informs the agent about network reliance and lack of authentication requirements, exceeding the baseline for annotated tools.

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 compact and front-loaded: the first sentence states the core purpose, the second gives use cases, the third provides an implementation detail. Every sentence contributes original value with no repetition of schema or annotations.

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?

For a simple, read-only lookup with one parameter, an output schema, and comprehensive annotations, the description fully covers the essentials: what it does, when to use it, what it returns, and external API details. No critical gaps remain.

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 description coverage is 100%, including a descriptive example and length constraints. The description itself only refers to 'UK postcode' without adding new parameter syntax or format details. Baseline 3 is appropriate; the schema carries the semantic weight.

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 tool's function: 'Look up a UK postcode to retrieve its local authority, region, constituency, and other administrative geography.' This specific verb+resource+output combination unambiguously distinguishes it from all sibling tools, none of which perform postcode lookups.

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 provides strong usage context: 'Useful for determining which council area, parliamentary constituency, or NHS region a postcode falls within. Commonly used to direct users to the correct local service on GOV.UK.' While it does not explicitly mention alternatives or when-not-to-use, the context makes the intended use clear, and no sibling tools offer postcode lookup.

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