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

The description discloses the API key requirement, the quota consumption ('Each call spends one image of the monthly quota'), the watermark differences between paid/free tiers, and the output format (PNG shown inline at 1x). These go beyond the annotations (which already note readOnlyHint=false, so a non-read operation is expected). The quota cost and auth requirement are valuable extra context.

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 compact and front-loaded. The core purpose is stated in the first sentence, followed by side effects, then usage guidance. No filler or tautology; every sentence provides new information.

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 tool with no output schema, the description clearly states the return format (PNG inline), the auth requirement, the quota cost, and the watermark distinction. It also mentions the template source from list_templates or save_template via the parameter schema. The guidance to prefer og_image_url for specific cases completes the picture for correct invocation.

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%, and each parameter (size, fields, template) already has thorough descriptions within the schema. The main description adds nothing beyond the schema (it only says 'with the given text'), so it does not exceed the baseline for high-coverage schemas.

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 opens with a precise verb and object: 'Render one of the user's saved templates with the given text and return the PNG'. It also names the sibling og_image_url and explains when to choose it instead, so it clearly differentiates from the family of tools.

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 guidance is given: 'Use it when the user wants the actual file, e.g. to host it themselves' and 'For an og:image tag that needs no hosting, or for a 2x retina file, prefer og_image_url.' This directly addresses when-to-use and when-not-to-use, with a named alternative.

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