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

Goes well beyond the annotations (readOnlyHint=false, openWorldHint=true, idempotentHint=false) by disclosing the 25 MB limit, the transport-dependent source restrictions, the returned project-relative path and media metadata, the default-on font registration, and the fact that uploading has no side effects on designs or the project icon. Those are exactly the non-obvious traits an agent needs before calling a mutation.

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?

Front-loaded with the action and accepted asset types, then constraints, then the scope disclaimer. Five short sentences, each carrying distinct operational information with no filler.

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 5-parameter mutation with a nested source object and an existing output schema, the description covers the size limit, source transports, font registration behavior and post-upload state. Nothing an agent needs to invoke it correctly appears to be 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 description coverage is 80%, so the schema already explains source, fileType, fontFamily and registerFont in detail. The description mostly restates that content (media types, HTTPS/base64/local path, font registration default) rather than adding new meaning, so the baseline 3 applies.

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?

Specific verb plus resource ('Uploads one ... asset') with an explicit enumeration of the accepted asset kinds. The closing sentence carves out scope against siblings that apply assets to designs or set project icons, so an agent can separate this from apply_* and replace_asset without opening a 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?

States the operating context clearly: 25 MB cap, HTTPS/base64 supported, local paths only in npm/stdio, and 'Upload alone does not apply an asset to a design' which signals a follow-up step is needed. It does not name the sibling to use next (e.g. prepare_asset_upload or the apply_* tools), so routing is implied rather than explicit.

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