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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false. The description adds value beyond them: that the call is free, that the result splits free versus paid credit, that a top-up URL is included, and that x402 connections without an API key have no balance. The last point is an edge case an agent would otherwise misread.

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?

The description is front-loaded with 'Free.' and the return content, then the usage triggers, which is the right order. The Japanese sentence repeats the English content, which adds bulk, but it looks like a deliberate bilingual client choice rather than 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?

There is no output schema, so the description carries the return explanation itself, and it names both balance parts and the top-up URL. For a zero-param read-only tool, nothing that an agent needs in order to call it correctly is missing.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline is 4. Schema coverage is 100% and the empty object is self-explanatory.

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 the prepaid credit balance (free and paid parts) of the API-key account, plus the top-up URL. That is clearly distinct from the sibling verification/lookup tools, which resolve or validate addresses, companies, and invoice issuers.

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 two concrete triggers, 'before a large batch' and 'after an insufficient_credits error', so an agent knows when to call it. There are no named alternatives or exclusions, but for a zero-param status check the guidance is nearly complete.

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