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 bar is met structurally; the description adds genuinely new context beyond them: the response is identical for every account, no permissions are required, and supplying an unrecognised font or out-of-range size causes rejection with nothing saved. It does not describe response shape in detail, but that is the only gap.

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?

Three sentences, front-loaded with the enumeration of the resource, then the usage imperative, then the behavioral caveats. The list of covered domains is dense but each item earns its place by defining the catalog's scope; no filler sentences.

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?

No output schema exists, and the description compensates by naming what the returned catalog contains and noting it is account-invariant. Combined with the preconditions ('nothing is saved', 'needs no permissions'), an agent has everything needed to call it correctly and interpret the result.

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?

The tool takes zero parameters, so the baseline is 4 — there is nothing for the description to disambiguate. The enumeration of catalog contents implicitly tells the agent what the keyless call will yield, which is a small value-add.

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?

A specific verb ('list') plus a precisely scoped resource ('every design value Unplain accepts'), followed by an enumeration of exactly what is returned (fonts, theme presets, cover layouts, footer styles, AI art styles, colour format, typography/spacing ranges). Nothing among the siblings (create_brand_theme, generate_cover_image, etc.) overlaps with this catalog role, so the agent can route unambiguously.

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?

Gives an explicit sequencing instruction — 'Call this BEFORE styling an ebook' — with the reason (values must come from what exists rather than be guessed). It does not name when-not to call it or an alternative, but no sibling supplies this catalog, so there is little ambiguity left.

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