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 adds genuinely new behavioral facts — deterministic output with 'no AI inside', nothing logged, and a warning/confidence envelope — which matter for trusting the result. The return-field list partly duplicates the output schema, keeping it just short of a 5.

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?

Dense but front-loaded: the capability, mode list, and example come first, with the response/determinism note second. The response-field enumeration is somewhat compressed into a run-on list, but no sentence is wasted.

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 full annotation coverage and an output schema present, the description needs only to convey purpose, mode semantics, and trust characteristics — all of which it does. The one substantive gap is routing guidance relative to the near-synonym sibling amount_to_words, which is left to inference.

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 all three parameters are already documented in the schema, giving a baseline of 3. The description adds a little value by illustrating the 'to' enum in a worked example and noting Arabic-Indic digit acceptance is handled, but it introduces no new syntax or constraints for 'language' beyond what the schema states.

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 description states a specific transformation (number → words) and enumerates the three variants it produces (cardinal, ordinal, year) across 56 languages, with a concrete worked example. It does not, however, distinguish itself from the sibling amount_to_words, which an agent could plausibly confuse with this tool.

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?

Usage is only implied through the example (number 2026, language 'en', to 'year'), with no explicit statement of when to pick this over amount_to_words or parse_number/format_number. It also doesn't point to list_languages for discovering valid language codes despite advertising 56 supported languages.

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