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

Goes well beyond the annotations: it discloses input validation failures (HTTP 400 for oversized pixels and for >15MB), the credit hold/refund lifecycle, and the async contract (returns {id, status}, poll getApiJob). It also clarifies the result is a single image result with a url, not an array — a detail annotations cannot convey.

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-loads the operation and its hard limits, then moves to alternatives, tagging, async flow, and credits — a sensible ordering. It is dense and mostly earns its length, though the parenthetical '(not an array)' and the repeated 2048px cap could be trimmed.

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 an async mutation with no output schema, the description supplies everything an agent needs: input constraints, cost, result shape, and the follow-up call (getApiJob) to poll. Nothing material is left to inference.

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 the schema already documents both parameters; the description adds the consequence of violating the pixel constraint (HTTP 400 rejection) and explains the practical purpose of request_id (tagging for later retrieval via listGenerations, type image). That is useful reinforcement but largely overlaps the schema text.

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 and resource ('Upscale an image') plus scope: up to 4x, capped at 2048px on the longest side. It explicitly separates itself from createImage and editImage, so an agent can route without opening any 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?

Gives an explicit when-to-use rule ('raise the resolution of an image you already have') and names the alternatives that make new or changed images instead. The preconditions for using it (source below 2048px, under 15MB) are stated as rejection conditions, which is effectively a when-not rule.

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