Upload an image

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

The description discloses a meaningful behavior beyond the annotations: 'The returned URL is unlisted rather than secret: anyone who has it can fetch it, even before the post is published.' This adds security-relevant context. The annotations already indicate mutation and idempotency, so the description does not need to repeat those.

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 well structured: first sentence states purpose, second paragraph gives invocation constraints, third gives a security-related caveat. Every sentence earns its place, with no repetition of schema content or annotations.

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 standalone upload tool with no output schema, the description is complete: it explains inputs, constraints, the return value (a URL), and the security model of that URL. It also names the practical use cases ('post body or as a cover image'). No critical calling context 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?

Schema coverage is 100%, so the schema already documents all parameters. The description adds critical cross-parameter semantics that the schema does not: url and path are mutually exclusive ('Give exactly one of url or path'). It also adds global constraints like file type and size limits, which apply to both input modes.

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 a specific action and resource: 'Upload an image to Substack's CDN and get back a URL you can use in a post body or as a cover image.' This clearly establishes what the tool does and why it exists, and it is easily distinguished from the sibling tools, none of which are image upload 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?

The description gives explicit invocation guidance: 'Give exactly one of url or path.' It also enumerates accepted formats (PNG, JPEG, GIF, WebP), rejects HEIC and SVG, states the 10MB limit, and explains how the result is used. This is clear, actionable usage guidance.

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