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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely new behavioral facts beyond them: the call is free and nothing is charged, and it reports whether balance and the remaining daily limit cover the cost. It does not describe error behavior or rounding, keeping it below 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?

Two sentences, front-loaded with what is computed and what it is checked against, followed by the usage cue and the free-of-charge note. No filler or redundancy.

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?

There is no output schema, so the description carries the return-value burden and does so: per-format cost, total cost, and coverage against balance and daily limit. Combined with the free-of-charge note and the 100%-covered input schema, an agent has everything needed to call it 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 parameters are already documented, including the format keys, variants, architectures, and the source_url link fee. The description only restates the per-format/total framing and adds no syntax or format detail beyond the schema, 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 (calculates) and resource (credits for a generate_content request), plus the scope: per format, in total, and against both balance and today's daily limit. It is clearly distinguishable from the sibling generate_content, which actually spends credits.

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?

"Use it to answer what a format or a request costs" gives a clear usage context, and the pairing with generate_content is implicit in the wording. It stops short of an explicit when-not or a direct instruction to call this before generate_content, so it is not a full 5.

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