set_cover_image

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

With no annotations, the description carries the burden of behavioral disclosure. It explains where the cover image is used (homepage, social shares, email header) and notes the return type (updated draft summary). It does not mention side effects like overwriting, but the behavior is largely implied.

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?

Four sentences, front-loaded with purpose, followed by context, argument details, and return. No redundant information. Every sentence adds value.

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?

Given the simplicity (2 params, no output schema), the description covers essential usage: purpose, prerequisite (upload_image), parameter semantics, and return value. It lacks info on idempotency or error handling, but completeness is adequate for this tool.

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?

Input schema has 0% coverage, but the description adds meaning beyond property names. It specifies 'post_id: Draft ID' and 'image_url: Substack CDN URL from upload_image, or any public image URL.' This clarifies valid values and origin requirements.

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 clearly states the tool's action: 'Set the cover (thumbnail) image for a draft.' The verb 'set' and resource 'cover image' are specific and distinct from sibling tools like upload_image or update_draft.

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 provides explicit guidance: 'Use upload_image first to get a CDN URL.' It also clarifies that image_url can be 'any public image URL', not just from upload_image. This helps the agent understand prerequisites and alternatives.

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