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

Annotations already declare readOnlyHint=true, so the safety profile is known. The description adds valuable behavior: it notes the tool is 'Free' (cost implication) and specifies the return value ('credits left'), which is absent from the schema (no output schema). It does not contradict annotations; it supplements them with cost and output semantics.

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, concise sentence that front-loads the key facts: 'Free', 'Returns credits', and usage timing. Every word carries value; there is no filler or redundancy.

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?

Given the tool's simplicity (no parameters), the readOnlyHint annotation, and the absence of an output schema, the description fully equips an agent: it explains what the tool returns, its cost, and when to call it. Nothing necessary for correct invocation is missing.

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?

There are 0 parameters, and schema coverage is 100% (trivially). With no parameters, the description need not explain any. The baseline for 0-param tools is 4, and the description correctly provides no spurious parameter details. It correctly implies the tool requires no input.

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 states a specific verb ('Returns') and a precise resource ('credits left on this API key'), making the tool's purpose unambiguous. It clearly distinguishes this from the sibling tools (aid, appeal, etc.), which serve different functions. No ambiguity exists.

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 explicitly provides when to use the tool: 'Call it before a long run of billed calls.' This gives clear context for invocation. While it doesn't mention alternatives, none are needed since this is a distinct utility, and the guidance is actionable.

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