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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and closed world, so the safety profile is covered; the description's 'no side effects' restates that. What it genuinely adds is deterministic-output assurance and concrete operational limits (64 KB JSON input, 1 s per call, 1 MB result), which are not in any structured field.

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 ES/EN structure is well front-loaded and the 'Cuándo / No para / Efectos / Límites' layout is easy to scan. However, the entire block is duplicated verbatim in English, roughly doubling token cost with no added information for an agent that reads either half.

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 an output schema exists (so return values need no explanation), the definition covers selection criteria, sibling disambiguation, determinism, and resource 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 coverage is 100% and both parameters are documented with type and range (digits min 1, max 17). The description only restates that the number is rounded to N significant figures and does not add format, edge-case, or precision-loss guidance beyond the schema, so the 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?

The first sentence names a specific verb and resource ('Rounds a number to N significant figures') and states the output form (plain and scientific notation). It is immediately distinguishable from round-number, which is named explicitly as the decimal-place alternative.

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?

There is an explicit 'When:' clause (results that must be exact/reproducible rather than estimated) and an explicit 'Not for:' clause pointing at round-number with the exact distinction (N decimal places, half_up/half_even exact decimal arithmetic). No inference is required to choose between the two.

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