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

No annotations are provided, so the description correctly carries the full burden. It explicitly discloses the read-only nature ('Read-only'), the return unit (micro-USD with the conversion factor spelled out), and the roadmap context that writes are not yet available. This provides meaningful behavioral context beyond what structured metadata would cover.

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 tight sentences with zero waste. The first sentence front-loads the core purpose, and the second adds the essential unit detail and read-only disclosure. Every word earns its place; there is no filler or repetition.

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?

For a zero-parameter, read-only tool with an output schema covering the return value, the description covers all that's needed: what it does, the unit of measurement, the scope, and the read-only constraint. The only minor gap is not describing edge cases (e.g., behavior with zero balance), which is a negligible concern for such a simple tool.

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 has zero parameters, and the rubric grants a baseline of 4 for this case since there is nothing to document. Schema coverage is trivially complete at 100% with an empty object. The description correctly needs no parameter explanation.

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 and resource: 'Return the current org's prepaid router credit balance.' This is unambiguous and clearly distinguishes itself from all sibling tools, none of which deal with credits or balances. The scope ('current org') and object ('prepaid router credit') are precisely defined.

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 purpose sentence inherently signals when to use it — when an agent needs the org's credit balance. The 'Read-only — no write tool yet (credit purchases land in Phase 2 via Stripe)' line clarifies what it is NOT (a purchase/credit tool), providing useful exclusionary context even though no direct alternative tool is named.

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