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

Annotations are minimal (readOnly=false, not idempotent), so the description carries the burden. It transparently discloses that nothing is charged by this call, payment happens at the hosted link where the user approves the amount, balance updates automatically after payment, and paused trackers resume. These are significant behavioral disclosures beyond what the annotations provide.

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 somewhat long but every sentence carries functional decision guidance. It's front-loaded with the core purpose and payment behavior first, then param selection rules, then the minimum. While dense, the content justifies its length; a rare case where verbosity serves the agent's decision-making.

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 tool involves money handling and has no output schema, so the description must cover return behavior (payment_url, hosted payment link) and the post-payment state changes. It covers currency handling, minimums, suggestion-sourcing, and mutual exclusivity of params. Given the stakes (financial transaction preparation), this is thorough and complete for a prepaid top-up flow.

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% with rich descriptions for both parameters (major vs minor units, examples like 50→50.00, 5000→50.00). The description adds decision guidance not in the schema: when to send which parameter, that they're mutually exclusive, and that amount should match exactly what the user said. This adds genuine routing value beyond the schema.

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 states the verb (prepare top-up), the resource (prepaid credit balance), and the mechanism (returns hosted payment link). It distinguishes itself from siblings by being the only tool that handles payment/prepaid credits, and explicitly clarifies that nothing is charged by this call—a non-obvious behavioral detail that differentiates it well.

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?

Excellent guidance: explicitly states when to pass `amount` (when the user names an amount) vs `amount_minor` (when the user leaves it to you and you pick from suggested_topups from get_credits). It names the alternative source (get_credits), gives exclusions (never convert currencies, never compute, never send both), and references the sibling get_credits in a helpful way. Also states the minimum.

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