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

With no annotations, the description carries the full behavioral burden. It usefully explains that reserved_now credits are held by in-flight runs and partially refunded, that recent is limited to the 20 newest runs, and that last_30_days is per-UTC-day. 'Free' indicates the cost profile, and the API-shape note adds context. It stops short of explicitly stating read-only or no side effects, but the informational framing strongly implies a safe query.

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 a single, dense sentence followed by two short fragments. It front-loads the core purpose and efficiently packs field semantics without fluff. The structure is slightly run-on, but every element earns its place.

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?

The tool has zero required parameters, an output schema exists, and the description already enumerates the returned fields, their meanings, and the cost profile. An agent has everything necessary to invoke this tool correctly. The only missing piece is sibling routing, which is covered under usage guidelines.

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?

Schema description coverage is 0%, and the description does not mention api_key at all. The parameter is optional with a default of null, so the gap is less severe, but an agent still cannot tell why or when to provide an api_key (e.g., overriding the default key). The description fails to compensate for the coverage gap.

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 immediately states the tool's purpose: reporting credits and spending. It enumerates the exact fields returned (credits, spent_total, reserved_now, runs_total, last_30_days, recent), making the resource and output clear. It does not explicitly differentiate from the sibling 'balance', but the detailed field list gives enough specificity.

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?

No guidance is given about when to use this tool versus 'balance' or 'list_runs'. The description mentions the data returned, but does not state contexts, exclusions, or alternatives. 'Same shape as GET /v1/usage. Free.' is technical detail, not usage direction.

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