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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behaviour: the call is free, nothing is spent, and it also returns the balance. It does not cover any rate limits or response shape, which keeps it below a 5.

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 outcome, then modes, then the routing instruction, then an example — a sensible order with no filler. The sentence fragmenting ("Free. Tell the person…") is terse to the point of being slightly clipped, but every clause carries information.

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 read-only estimation tool with no output schema, the description covers what is returned (cost and balance), what it costs to call (nothing), and when to call it. It stops short of describing the return structure or how the estimate relates to actual charges, which is the only remaining gap.

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 75%, so the baseline is 3, but the description adds real meaning by establishing that shots/locales/colours are alternative modes ("or with locales… or with colours…") rather than combinable inputs — something the per-field "Instead:" notes only imply. It also restates the shots default. The example payload reinforces the call shape.

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 — returns what an image set would cost — and immediately enumerates the three cost modes (shots, locales, colours) plus the balance. It distinguishes itself from generate_image_set, localize_image_set and recolour_image_set by framing itself as the pre-flight check for those tools.

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?

Explicitly instructs: "Tell the person the cost before generate_image_set, localize_image_set or recolour_image_set." That names the alternatives and the condition that selects this tool over them. The default behaviour (plan's ticked shots) is also stated, so the no-argument path is unambiguous.

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