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

The description goes far beyond the readOnlyHint:false annotation by disclosing the watermark, the 20-per-day draft limit, the one-credit unlock flow, the 'you cannot unlock it' restriction, persistence to the user's files, and the returned artifact id. It also warns against retrying, which is exactly the kind of behavioral nuance an agent needs.

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 dense and front-loaded with the most important behavior, and every sentence adds operational guidance. It loses a point because the size list largely duplicates the schema's parameter descriptions, and some of that repetition could have been trimmed while preserving the unique behavioral warnings.

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 8 parameters, a nested props object, and no output schema, the description is remarkably complete: it specifies required fields, template-specific inputs, defaults, return value, and failure/retry policy. The remaining parameter details (align, gradient, appearance, saved_template_id) are already documented in the schema, so nothing critical is missing.

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?

Although schema coverage is already 75%, the description adds crucial semantics: template_id 'simple' and 'blog' each have specific props keys, title is required, and 'Leave brand_kit_id out to use the space's default brand.' It also reinforces size options with contextual placement details, giving the agent everything needed to construct valid parameters.

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 uses a specific verb-resource pair ('Render one free branded image draft...') and clearly distinguishes this from siblings by covering image generation for social/link previews while siblings like generate_pdf, generate_email, and generate_substack_post target other output types. It also enumerates the exact output formats, making the tool's scope unambiguous.

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 gives strong context on when to use it (every size is tied to a posting destination), and it adds direct behavioral instructions like 'Do not call this again to retry.' It does not explicitly name alternative tools for other content types, but the context is clear enough for an agent to select this tool for image drafts.

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