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

Beyond the readOnlyHint/openWorldHint annotations, it discloses cost behavior ('free, consumes no quota'), authentication requirements, and the precise unit semantics of the returned balance. These are meaningful behavioral traits an agent cannot derive from the annotations or the empty schema.

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?

One dense sentence that front-loads what is returned, followed by a compact parenthetical for cost, endpoint, and auth. No redundant or filler text.

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?

With no output schema, the description carries the return-value burden and does so fully: key identity, balance with unit conversion, quota buckets, and environment. Combined with auth and cost notes, 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?

The tool takes zero parameters, so the schema is trivially complete and the baseline is 4. The description correctly adds no parameter discussion because there is nothing to pass.

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 description names a specific verb (returns) and enumerates the exact resources returned: the calling key, ledger balance, per-bucket quota remaining, and environment. It distinguishes itself from sibling tools like account, credits_checkout, and credits_packs by scoping to the caller's own key rather than account-level or purchase operations.

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?

It gives clear selection context — 'Free, and consumes no quota' tells the agent this is the zero-cost way to check balance/quota — and states the auth prerequisite (connected account or dv_sk_ key). It stops short of explicitly naming an alternative tool or a when-not condition, so it is clear context without exclusions.

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