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

With no annotations, the description carries the full behavioral burden and does so richly: 2048px source cap and 15MB limit both rejected with HTTP 400, 4x ceiling, single-url (not array) result shape, credit hold/refund semantics, 0.5 credit cost, and the async job contract returning {id, status} to be polled via getApiJob.

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?

Purpose and limits are front-loaded, and nearly every sentence carries distinct constraints or routing info. It is on the long side, with some repetition of schema-documented request_id behavior, but 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 tool with no output schema and no annotations, the description supplies the return shape, polling path, failure modes, cost, and credit lifecycle. An agent has everything needed to invoke and follow up correctly.

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 100%, so both image and request_id are already documented in the schema. The description adds the 2048px constraint and the listGenerations retrieval angle, but this largely overlaps the schema text, 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?

States a specific verb and resource ('Upscale an image') with concrete scope: up to 4x, capped at 2048px on the longest side. It also explicitly distinguishes itself from siblings createImage and editImage, so an agent can route correctly 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?

Gives an explicit when-to-use ('raise the resolution of an image you already have') plus named alternatives with their differing purpose ('createImage and editImage make new or changed images instead'). Nothing is left to inference about tool selection.

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