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

Annotations already mark the tool as read-only, idempotent, and non-destructive, and the description adds substantial behavioral context beyond that: the response can include a topUpUrl, an x402 block that may allow an agent to self-top-up, and an x402.video path for one-off purchases. This meaningfully explains how the tool's response is expected to be used.

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 front-loaded with the core purpose, then moves to actionable conditional guidance. Every sentence adds useful information about balance usage, top-up routing, and x402 behavior without redundant filler or repetition of structured fields.

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 does a good job of explaining the returned concepts: balance, plan usage, purchasable packs, topUpUrl, and x402 details. It is slightly vague around what counts as 'low' balance and does not specify units or exact formats, but for a parameterless read-only tool the essential invocation and interpretation context is present.

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 input schema is empty and includes no parameters, so the baseline is 4. There are no parameter semantics to clarify, and the description does not need to compensate for any schema gaps.

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 clearly identifies the resource: the account's prepaid credit balance, plan usage, and purchasable credit packs. It is not a tautology and conveys what information will be surfaced, though it lacks an explicit verb like 'returns' or 'lists' and does not directly contrast with sibling tools such as estimate_run_cost.

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 gives an explicit use case: 'Use it to budget before starting runs.' It also provides conditional next steps for low balance and x402 payment handling. However, it does not name alternative tools or state when not to use it, so it stops short of full exclusionary guidance.

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