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

The annotations already carry readOnly=false, openWorldHint=true, and idempotentHint=true. The description meaningfully expands on these: the tool opens but does not pay, the hosted_url is a short-lived human-facing page, card details never touch the Forum, repeated calls return the same page, and credits appear in account_status only after payment completes.

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 main outcome is front-loaded and the paragraph is dense without being bloated. However, several sentences restate parameter details that already exist in the schema, causing slight redundancy. The behavioral sentences earn their place, but the repetition keeps it just below a top score.

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?

The description explains the full flow: prerequisites (secret), constraints (USD and $10 minimum), idempotent behavior, the fact that no tool pays, and the post-condition of credits appearing in account_status. Since an output schema exists, the return shape does not need to be spelled out here.

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 already documents all three parameters at 100% coverage, including cents, the $10 minimum, USD-only acceptance, and the secret's origin. The description largely repeats this parameter-level information rather than adding new meaning beyond the schema, so it stays at the baseline 3.

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 action and result: 'Opens a top-up for the account and returns the payment intent with hosted_url'. It also clarifies that the tool only initiates the payment while a human completes it, which clearly differentiates it from the sibling read and account tools.

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 clear operational context: use the secret from sign_up, the data key cannot open a payment, repeated calls while a page is open return the same page, and credits later appear in account_status. It does not explicitly name alternative tools, but no sibling competes with this payment-opening purpose, and the secret/data-key distinction provides a useful when-not.

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