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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the trailing 'Read-only' adds no credit. However, the description goes beyond them by disclosing the conditional low_credit field with purchase guidance when under an hour remains, plus the set of returned fields in the absence of an output 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?

Two dense but front-loaded sentences: the primary return payload comes first, the conditional low_credit behavior second, and the trailing 'Read-only' is the only expendable clause. No filler.

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 full burden of describing return values and does so thoroughly (spendable dollars, spendable_seconds and its meaning, held amounts, pricing, recent purchases, first_purchase, conditional low_credit). An agent knows exactly what to expect from a call.

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 imposes no burden and the baseline is 4. There is nothing further the description could meaningfully add on this dimension.

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 ('Returns the prepaid Mac credit') and enumerates the concrete contents of that resource (spendable dollars, spendable_seconds, held amounts, pricing, purchases). This clearly separates it from siblings like get_mac_checkout, get_mac_job, and get_mac_session.

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 tool, and the 'under an hour left' clause hints at a pre-flight check before exec_mac or start_mac. But no explicit when-to-use, prerequisites, or named alternatives are given.

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