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

The annotations already declare readOnly, idempotent, non-destructive and closed-world, and the description restates the side-effect profile in prose. What it genuinely adds beyond structured data is the determinism guarantee and concrete operational limits (64 KB JSON input, 1 s per call, 1 MB output), which an agent cannot infer from annotations.

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 content is well structured with labeled sections (Cuándo / No para / Efectos secundarios / Límites), but it is duplicated verbatim in Spanish and English, roughly doubling the length with no new information in the second block.

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 present, annotations covering the safety profile, and 100% parameter coverage, the description fills the remaining gaps: purpose, alternative routing, determinism, and hard operational limits. Nothing an agent needs to invoke it correctly is missing.

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%, so every parameter (ikm, salt, info, digest, length) is already documented with types, defaults and bounds. The description only restates that inputs are hex-encoded and digests are SHA-256/512, adding little semantic detail beyond the schema; 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?

States a specific verb and resource ("Deriva material de clave con HKDF (RFC 5869)"), names the underlying standard, and lists the exact inputs (hex IKM, salt, info, SHA-256/512). It explicitly distinguishes itself from the sibling kdf-pbkdf2, so an agent can select it without opening the schema.

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?

Contains explicit "Cuándo" (when a key must be derived from a password/secret with known parameters) and "No para" (not PBKDF2 — use kdf-pbkdf2) sections. The routing condition to the alternative is stated directly rather than implied.

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