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

Annotations already declare readOnlyHint=false and openWorldHint=true with destructiveHint=false, so the mutation/network profile is known. The description adds substantive context beyond that: request_plan files a request and 'never grants', topup/checkout return a hosted Stripe URL, and the call is a resolution path for 402s. It does not cover idempotency or whether filing duplicate requests is harmful.

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?

A single dense paragraph that front-loads what the tool returns, then walks through the three optional modes in call order. Every clause carries information, though the enumeration of return fields is slightly heavy for a tool whose schema is the only other structure available.

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 return-value burden and does so by listing the returned fields and framing the call as a 402 resolution. All three optional parameters are explained. Minor gap: no statement on how the three modes interact if passed together, or what happens on a rejected request.

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?

Schema coverage is 100%, so the baseline is 3, and the description exceeds it: it gives the EUR unit for topup_amount, clarifies checkout_request_id must reference an already-approved plan, and supplies a fuller enum set for request_plan (starter|pay_per_use|pro|max|enterprise) than the schema's abbreviated 'pro|max|…'.

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 names a specific verb+resource pairing (billing status plus self-service payment) and enumerates exactly what is retrieved: wallet balance, pay-per-use flag, licence tier, per-action prices, purchasable plans, unpaid approved plans. It is unambiguous and easily distinguished from every sibling tool in the list.

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?

It clearly states the trigger conditions for each mode: passing checkout_request_id to COMPLETE an approved plan, topup_amount to TOP UP, request_plan to REQUEST. It also names the concrete problem this solves (resolving a 402 licence/credit error). It stops short of naming alternative tools or when *not* to call it, but no sibling competes for this purpose.

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