PostalDataPI MCP Server

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It states the tool checks validity, implying a read-only operation, but lacks details on error handling, rate limits, or authentication needs. It adds some context by specifying the default country, but more behavioral traits could be included.

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 well-structured and front-loaded with the purpose, followed by usage guidelines and parameter details. Every sentence adds value, with no wasted words, making it efficient and easy to parse.

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, no annotations, and an output schema that covers return values, the description is mostly complete. It explains purpose, usage, and parameters well, but could benefit from more behavioral details like error cases or limitations, though the output schema reduces the need for return value explanation.

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 description adds significant meaning beyond the input schema, which has 0% description coverage. It explains that 'postal_code' is 'The postal code to validate' and 'country' uses 'ISO 3166-1 alpha-2 country code' with a default of 'US,' clarifying semantics that the schema alone does not provide.

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 purpose with a specific verb ('Check') and resource ('postal code'), and distinguishes it from siblings by focusing on validation rather than metadata retrieval or lookup. It explicitly mentions 'exists in a given country,' which differentiates it from tools like 'search_by_city'.

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 explicit guidance on when to use this tool: 'when someone wants to verify if a postal code is valid before processing an order, form, or address.' This clearly defines the context and distinguishes it from alternatives like 'get_postal_code_metadata' or 'lookup_postal_code,' which likely serve different purposes.

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