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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuinely useful domain context beyond that: a credit pays for one 160-character SMS part, and 153 characters per part in a concatenated message, which is what lets an agent interpret the returned number correctly.

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 tight sentences: purpose first, then the credit-unit semantics, then the endpoint hint. Nothing redundant, nothing padded.

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 zero-parameter read with no output schema, the description supplies the one thing an agent needs to make sense of the result — the credit-to-SMS-part conversion. It stops short of describing the response shape, but the domain semantics effectively cover the interpretation gap that an output schema would otherwise fill.

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 per the baseline there is nothing for the description to disambiguate. The description correctly documents its argument-free nature in the same breath.

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 ('Get the SMS credit balance of the account') and even names the underlying endpoint. It is trivially distinguishable from every sibling, all of which list, send, cancel, or mutate other resources.

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?

Usage is implied rather than stated: an agent can infer this is the balance check to run before sending SMS, but the description never says when to call it, whether it needs to be called before voodoosms_send_sms, or that there is no alternative tool for this data. No exclusions or prerequisites are given.

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