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

Annotations already declare openWorldHint=true, idempotentHint=true and destructiveHint=false, so the description only needs to add context — and it does: accepted MIME types, a 5 MiB ceiling, and the requirement that the URL be public https. It does not mention auth requirements or what happens on fetch/size failure, which keeps it below the top mark.

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?

Two sentences, zero filler. The hard constraints are front-loaded and the sibling preference plus return value follow in a single compact sentence.

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-parameter tool with no output schema, the description covers source constraints, formats, size limit and the returned assetId — everything needed to invoke it correctly. Only failure-mode behavior (unreachable URL, oversized image) is left unstated.

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 50%: idempotencyKey is documented in the schema, but url carries only format/maxLength. The description compensates by constraining url to a public https source and tying the size/format limits to it, adding real meaning beyond the schema's type declaration.

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 (Upload), resource (image), supported formats (JPEG, PNG, WebP), size cap (5 MiB) and source constraint (public https URL). It also explicitly names the sibling upload_image, so an agent can distinguish the two without opening either schema.

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?

"Prefer this to upload_image" gives a clear routing preference to the only relevant sibling. It does not state the inverse condition (e.g. use upload_image for local files or non-URL sources), so the alternative's trigger is only implied.

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