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

Annotations already declare destructiveHint=true and readOnlyHint=false; the description reinforces this with "Add or replace", making clear the icon/screenshot is overwritten. It also adds constraints the annotations do not: per-slot size limits (256 KB / 1 MB) and that content_type must match the base64 payload. It does not describe failure behavior, but the bar is lower with annotations present.

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 dense sentences with zero filler. The primary action and slot names are front-loaded, with the encoding constraint and sequencing note following.

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?

With no output schema, the description does not say what a successful upload returns, but it covers the action, constraints, ownership and sequencing needed to invoke correctly. The api_key fallback param is left to the schema, which is acceptable.

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 80%, so the baseline is 3. The description adds real value beyond the schema: per-name size limits (icon 256 KB, shots 1 MB), that content is base64-encoded PNG/JPEG/WebP, and the content_type/content correspondence requirement.

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 (Add or replace) and resource (icon or screenshot of an app you own), plus the exact name slots and size caps. An agent can distinguish it from publish_app/update_app/update_app_html 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?

"Call it after publish_app" gives explicit sequencing, and "of an app you own" states the ownership precondition. No when-not guidance or explicit sibling alternative is named, but the context is clear.

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