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

No annotations are provided, so the description carries the burden and does disclose a non-obvious trait: a free org's monthly allowance is granted on this read, once per UTC month — a side effect an agent would otherwise not expect from a 'balance' call. It does not state auth requirements or whether reads are rate limited, leaving a modest gap.

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?

Front-loaded with the core action in the first sentence, then units, then the free-org caveat, then the exclusion. Every sentence answers a distinct question an agent would ask; nothing is padded.

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 an output schema present, the description needn't describe return values, and it covers the two things the schema cannot: unit scaling and the monthly allowance side effect. Without annotations, a note on permissions or read safety would round it out, but the essentials are there.

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, which is the baseline-4 case. The description's unit explanation (1 USD = 1,000,000 micro-USD) actually pertains to the response rather than inputs, but it is still the kind of semantics an agent needs to interpret results correctly.

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: 'Return the current org's prepaid router credit balance.' No sibling tool covers billing or credits, so an agent can distinguish this immediately; the scope ('current org', 'prepaid router credits') is unambiguous.

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?

Explicitly rules out an adjacent intent: 'Credit packs are purchased on the billing page, not through this tool,' which prevents the agent from hunting for a purchase tool here. It lacks a positive statement of when to reach for this versus account_brief, so it stops short of full when/when-not coverage.

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