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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description goes further by disclosing hard limits (64 KB input, 1 s per call, 1 MB result), determinism, and the 2,000 code point cap, which are not in the annotations. It stops short of describing error behavior, keeping it just below the top.

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?

Well front-loaded: what it returns first, then when/when-not, side effects, determinism, limits. The bilingual ES/EN duplication roughly doubles length, which is justified for a bilingual catalog but is verbosity an agent must skip past.

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?

An output schema exists, yet the description still summarizes the key return values (bytes, code points, graphemes, totals). Combined with limits, exclusions and determinism, an agent has everything needed to call it correctly.

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?

With one parameter at 100% schema coverage, the baseline is 3. The description adds a meaningful semantic constraint not fully conveyed by the schema: the 'Max 2,000 code points' limit, which clarifies the unit of the maxLength bound.

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 concrete verb and resource: lists each character with its code point, UTF-8 bytes, UTF-16 units and totals. It also names the sibling it is not (text-invisible-scan), so an agent can distinguish it from the invisible-character tools 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?

The 'When' clause gives a positive trigger (exact conversion to/from the encoding without guessing) and the 'Not for' clause names text-invisible-scan by name. Both the inclusion and exclusion conditions are explicit.

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