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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, yet the description still adds real behavioral context: no network, no stored state, determinism, and concrete limits (64 KB JSON input, 1 s per call, 1 MB result). The grapheme-based width semantics is a further behavioral detail absent from the schema keywords.

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 action, then cleanly labeled clauses (When / Not for / Side effects / Deterministic / Limits) with zero filler. It loses a point only because the whole block is duplicated in Spanish and English, roughly doubling length for a single-purpose tool.

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, so return values need no explanation, and all four parameters are fully described in the schema. Combined with stated time, size and input limits and explicit safety/determinism notes, nothing an agent needs before calling 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%, so the parameters are already documented and the baseline is 3. The description still contributes the unit that matters most: width is measured in graphemes, not code units or bytes, which changes how an agent sizes its inputs.

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: 'Pads text to a target width in graphemes on the left, right or both sides with a fill string.' It also names the scope (graphemes, three sides) and distinguishes itself from the nearest sibling by ruling out text-truncate, so an agent can pick it without opening any 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?

Explicit 'When:' clause ('text has to be transformed, compared or measured exactly instead of approximated') and explicit 'Not for:' clause naming the alternative text-truncate. Both the trigger condition and the exclusion are spelled out rather than inferred.

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