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

Annotations already declare read-only, idempotent, non-destructive, closed-world, but the description adds genuinely non-derivable facts: no network access, no stored state, determinism guarantee, 1 s per call, result up to 1 MB. It does not explain output shape, though an output schema exists, so this is appropriately scoped rather than exhaustive.

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 core English content is front-loaded and dense, but the block is bilingual and the English section is truncated as '[EN completo / full EN: inputSchema.description]', which is wasted text that just redirects to a field already supplied. The 64 KB / 1 MB mismatch also adds noise rather than information.

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 deterministic counting tool with full annotations and an output schema, the description covers purpose, exclusions, side effects, determinism and operational limits — nearly everything needed to call it correctly. The only gap is the conflicting input-size ceiling (64 KB vs schema's 1 MB), which could mislead an agent sizing its input.

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?

Single required parameter with 100% schema coverage, so the schema documents 'text' and carries constraints. The description adds counting semantics (Unicode graphemes vs code points) that affect what the string means, but says nothing parameter-specific. Notably, it states a 64 KB JSON input limit while the schema declares maxLength 1048576, leaving conflicting size ceilings.

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 specific verbs and the exact metrics produced (code points, graphemes, bytes, words, sentences, lines, paragraphs, reading time at 238 wpm), plus the Unicode segmentation algorithm. It explicitly distinguishes itself from readability-score, so an agent can pick between the two without opening a 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?

Uses explicit 'When:' and 'Not for:' clauses, and names the alternative sibling (readability-score) with the reason it is different (LIX/RIX/Flesch scores). The selection condition is stated, not implied.

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