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

The bracketed '[write, queued, priced]' adds real signal beyond the annotations: this is an asynchronous job that consumes credits. It also discloses a non-obvious behavioural trait — no text is drawn, so the title must be added in an editor afterwards — plus the polling target. It does not explain the destructiveHint=true cost/irreversibility angle beyond the single word 'priced'.

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?

A single dense paragraph with the operation class front-loaded in brackets and the poll target at the end. Nearly every clause carries information, though the run-on parenthetical about framing is heavier than it needs to be.

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 7-parameter mutation with no output schema, it covers the core prompt semantics, aspect handling, the async polling loop and the returned image_id/image_uuid. The remaining gap is the family of optional nested-object parameters, which are delegated rather than described.

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 description coverage is 0%, so the description must carry the load: it defines thumbnail_prompt (meaning plus a 1100-character limit), image_aspect (all four values plus the 16:9 default) and constrains character_uuids to 1-3 READY characters. However, image_title, image_ai, character_assets and image_environment are left entirely to a pointer at create_character_image, so several parameters remain undocumented here.

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 (create a thumbnail picture for a character) and scopes it precisely: a video/post cover framed with 1-3 ready characters. It differentiates itself from create_character_image by deferring 'other options' there, so an agent can tell the two apart.

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?

Explicitly says to poll with get_character_image and points to create_character_image for the shared option set, which routes the agent correctly through the workflow. It stops short of stating when NOT to use it (e.g. a plain image with no thumbnail framing) — that is only implied by the framing language.

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