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

Annotations already declare readOnlyHint=false, idempotentHint=true and destructiveHint=false, so the write/replace semantics are largely covered. The description adds real value with the accepted formats and the 4 MB ceiling, but does not say whether a non-square image is rejected or cropped, or whether an existing cover is replaced.

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 tight sentences with no filler. The purpose and the usage surfaces come first, then the hard constraints, which is exactly the information an agent needs up front.

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 three-required-parameter mutation with no output schema, the description covers the payload constraints well but not the identifiers it requires (game, key) or the replacement behavior for an existing cover. It is adequate rather than complete.

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 only 33%: image_base64 is documented, but game and key are bare strings with no explanation anywhere. The description adds constraint detail for the image (PNG, JPEG, WebP, GIF, 4 MB) that goes beyond the schema, but leaves the two other required parameters completely unexplained.

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 pairs a specific verb (upload) with a specific resource (the square cover) and enumerates the surfaces it feeds (cabinet, sharing card, coin image). That distinguishes it from the image-related siblings set_entry and set_trailer, though it never names those siblings directly.

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?

Usage is implied: this is the tool for the cover image, as opposed to entries or trailers. There is no explicit statement of when to use it versus set_entry/set_trailer, and no prerequisites such as the game needing to exist first.

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