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

Annotations already declare read-only, idempotent, non-destructive, closed-world, so the safety recap is redundant. The description earns credit for the operational limits not in structured fields: 64 KB JSON input, 1 s per call, 1 MB output ceiling, plus explicit determinism.

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?

Front-loaded with the definition, then labeled sections for when/not-for/side effects/limits, which is easy to scan. It is however fully duplicated in Spanish and English, roughly doubling length without adding information for a single reader.

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 the description needn't explain return values, and annotations plus full schema coverage handle safety and parameters. Determinism and size/time limits close the remaining gaps an agent would need before calling.

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 coverage is 100% with an enum on input_encoding and defaults documented, so the schema carries parameter semantics fully. The description only loosely gestures at bytes-vs-text, adding no format or validation 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 with the exact variant (URL-safe Base64, RFC 4648 §5, no padding, JWT usage), which is exactly what distinguishes it from sibling base64-encode. An agent can route between the base64 family without opening schemas.

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?

Explicitly names when to use it and names the excluded alternative (base64url-decode) in a labeled 'Not for' clause. Minor internal noise: 'convertir bytes o texto a esta codificación, o desde ella' ('or from it') blurs the boundary the next sentence draws, but the exclusion is clear enough to act on.

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