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

Annotations only carry readOnlyHint=true and openWorldHint=false, so the description carries the burden of real behavioral disclosure — and it delivers far beyond it: the endpoint is free and safe to poll, requires only any valid `vq_key_` key with no scope, reports an account-wide balance shared across jurisdictions, and exposes the bySource durability split (subscription credits forfeit at period end, payg persists) plus nextExpiry semantics. No contradiction with annotations; this is exactly the kind of contextual cost/safety/expiry detail that structured fields cannot express.

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 longer than average (~150 words plus a curl example), but every section earns its place: purpose statement up front, then cost/safety, auth, cross-jurisdiction semantics, balance durability, and a concrete example. Bolded labels and clear paragraph breaks make it scannable. It is slightly verbose relative to the minimal feature surface, but nothing is redundant.

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-only tool with an output schema present, the description is effectively complete. It covers authentication requirements, cost implications, polling suitability, ledger semantics across jurisdictions, and the durability/expiry behavior an agent must understand before trusting the number. The output schema covers return-value shape, so the description need not restate it.

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 has zero parameters and 100% schema coverage, so the baseline per the rubric is 4. There is nothing for the description to add about parameter syntax or meaning, and it correctly omits fabricated parameter details. The description instead clarifies the implicit scoping — the balance is keyed to 'this API key's account' — which is reasonable compensation for the absence of an account parameter.

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?

The opening sentence states a specific verb plus resource — 'Returns the credits this API key's account can spend right now.' This clearly differentiates it from all 21 siblings, which are act/section/search/citation tools; the only near-relative, get_pricing_in, is about cost rates, not balance. An agent can identify this as a read-only balance endpoint without opening any schema.

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 description gives explicit use cases ('safe to poll for low-balance alerting or to pre-flight a batch job') and explains the relationship to the US credits endpoint, including that both read the same ledger. It does not name a concrete alternative MCP sibling to prefer in some cases, but for a zero-parameter read tool with no close sibling, the context provided is clear and actionable.

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