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

Annotations already declare readOnlyHint=true, which the description is consistent with. Beyond that it adds real context: the upstream source (Correos de México catalog), the granularity of results (every neighborhood with settlement type), and a per-call cost of $0.003 USDC via x402 — a payment requirement an agent must know before invoking. It stops short of describing error/not-found behavior, response encoding, or rate limits.

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?

A single front-loaded sentence that leads with the verb and resource, then the returned fields, then pricing. No filler, and nothing is buried behind the cost note.

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 one-parameter read-only lookup, the description covers source, output fields, and payment model — enough to call it correctly without an output schema. Missing only failure-mode behavior (invalid or unknown postal code) and response shape, which are minor for a tool this simple.

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?

There is exactly one parameter and the schema already documents it at 100% coverage ('5-digit postal code'). The description adds only the country scope (Mexican) implied by context; no format edge cases, leading-zero handling, or validity constraints are supplied. Baseline 3 applies when the schema carries 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?

States a specific verb+resource: 'Mexican postal code lookup from the national catalog of Correos de México', and enumerates exactly what it returns (state, municipality, city, colonia with settlement type). No sibling in the list handles postal codes, so the purpose is unambiguous even though the tool name 'cp' and title 'cp' are opaque.

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?

Usage is only implied: an agent infers it should call this with a 5-digit Mexican postal code. There is no statement of when to prefer it, prerequisites, or exclusions, and no alternative is named. The visible per-call price does at least signal a metered, pay-per-use tool.

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