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

Adds substantial behavior beyond the single destructiveHint=false annotation: synchronous size/format/SSRF/image-safety validation, per-kind content rejection rules, and the side effect that the uploaded URL gets written to the target object. This is exactly the kind of context annotations don't cover.

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?

No wasted words and the login prerequisite plus core action are front-loaded. It is a dense semicolon-separated block, which reduces readability, but every clause carries necessary 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 2-param tool with no output schema, the description covers prerequisites, validation behavior, content restrictions, side effects, and the sibling fallback. Return-value specifics are the only notable gap, which is acceptable given the lack of an output schema.

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 both parameters are already well documented, making 3 the baseline. The description adds some meaning (moment-image restricted to JPEG/PNG/WebP, avatar content rules), but most of that is behavioral rather than extending parameter definitions.

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?

States a specific verb+resource: mirror a public HTTP(S) image to a stable URL. It explicitly distinguishes a sibling case (organization logo should use fetch_organization_logo), so an agent can tell this apart from alternatives without opening schemas.

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?

Gives concrete when-not guidance (avatar with QR codes/contact info is rejected, replace the image and don't retry the same one) and points to the alternative tool for organization logos. It doesn't differentiate from the other upload-prep siblings, but the routing and exclusion guidance is clear and actionable.

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