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

The description adds useful behavioral context by stating the return format is b64_json and that a default model member is auto-picked. Annotations already indicate the operation is not read-only. It does not disclose rate limits, costs, failure behavior, or what happens when the default model cannot be used.

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?

The description is two short sentences with no filler. Both sentences carry useful information: the core action/output type and the default routing behavior.

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 moderately simple image-generation tool, the description covers the core purpose and output format, and the schema fills in defaults. It is less complete on return envelope details, model selection options, and any caveats about n or size, but it is not misleading and provides a minimal viable contract.

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 schema provides names and defaults for prompt, n, size, model, and timeout_seconds, so the description does not need to repeat them. It does add meaning to the model parameter by explaining that an empty model defaults to auto-picking a FastImg member, but it does not clarify accepted sizes, prompt constraints, or timeout semantics.

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 the specific action ('Generate images') and the output encoding ('b64_json'), making the tool's purpose unambiguous. The sibling list includes video, chat, TTS, and other modalities, so this tool is clearly the image-generation entry point.

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 description implies usage when image generation is needed, and 'Default auto-picks FastImg member' tells the agent that model selection can be omitted. However, it does not explicitly contrast with sibling tools like nine_fanout or nine_video, nor does it explain when to override the default model.

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