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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the description is free to add business semantics, which it does: both wallets are returned, and a zero balance in one currency may still permit registration via automatic conversion. It does not mention auth requirements beyond 'authenticated customer' or any caching/freshness of the balance, keeping it short of 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?

Three short sentences: purpose first, response contents second, then the one non-obvious caveat about conversion affecting register_domain. Nothing is redundant and the important nuance is not buried.

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?

An output schema exists, so return-value documentation is unnecessary, and the description covers the non-obvious cross-tool consequence with register_domain. The only real omission is the role of the `currency` argument, which is a minor but genuine gap for a one-parameter read tool.

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 single `currency` parameter has 0% schema description coverage, so the description must carry the load. It mentions 'the requested currency' but never explains what the parameter actually controls, given that both USD and EUR balances are returned anyway, and it does not state the accepted values (USD/EUR) or the USD default.

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 ('Return the authenticated customer's prepaid balance') plus the scope ('for domain purchases') and the contents of the response (USD and EUR wallets). No sibling tool overlaps with balance retrieval, so it is unambiguous which tool to pick.

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?

The reference to register_domain implies this is a pre-registration funds check, and the conversion note explains how to read a zero balance. However, it never explicitly says 'call this before register_domain' or when not to call it, leaving the usage pattern to inference.

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