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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the safety profile needs no restating. The description instead adds genuinely non-obvious behavior: the IANA-registry boundary and the `registered: false` fallback for unknown codes, plus a precise definition of the `cacheable` field tied to RFC 9110 §15.1. Return-shape detail is deferred to the output schema, which is appropriate.

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?

Three sentences, front-loaded with the purpose and return fields, then the coverage boundary, then the one definitional clarification that would otherwise confuse an agent reading the `cacheable` field. Nothing is padded or repeated from 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 one-parameter lookup with an output schema, an existing schema description, and full annotation coverage, the description supplies exactly the missing pieces: registry scope, out-of-registry behavior, and field semantics. Nothing an agent needs to call it correctly is absent.

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% and the single `code` parameter already carries its own description and example (418), so the schema does the work. The description adds no format constraints beyond what the schema states (e.g. nothing about range validation or string-vs-number coercion). Baseline 3 applies.

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?

Specific verb plus resource ('Look up detailed information about any HTTP status code') with an explicit enumeration of what is returned (class, name, description, cacheability, causes, handling). No sibling tool overlaps this resource, so the agent can distinguish it immediately from the security-scanning siblings.

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 scope boundary is stated clearly: every code in the IANA registry is covered, and anything outside it comes back as `registered: false` rather than described — which tells the agent what happens for odd inputs like 999 or 0. There is no explicit 'use X instead' routing, but no sibling covers this resource, so the omission is minor.

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