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

With no annotations, the description carries the full burden. It does disclose the costs and the payment-method prerequisite, which is valuable for a spend action, but says nothing about charge timing, reversibility, refunds, or what happens if the payment method is missing/invalid. For a money-moving tool this leaves meaningful behavioral gaps.

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 purpose, then the tier table in a single compact sentence, then the prerequisite. Every clause carries information; nothing is redundant.

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 two-parameter tool with full schema coverage and no output schema, the description supplies the pricing context and the payment-method precondition an agent needs to call it correctly. Error/refund behavior is the only remaining gap, and that is comparatively minor.

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, but the description adds real meaning beyond the schema: it maps each enum tier to a dollar amount and bonus percentage, turning opaque enum values into decision-relevant economics. It adds nothing for idempotency_key, which the schema already explains well.

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 ('Purchase committed-use credits') plus the scope of the transaction, including exact tier economics. An agent can distinguish this from setup_billing, get_billing_status, and get_pricing without opening any schema.

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 names a prerequisite ('Requires an active payment method') and enumerates the available tiers, which implies when the tool applies. However, it never says when to choose this over setup_billing or get_pricing, and states no exclusions or preconditions beyond the payment method.

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