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

Annotations already declare readOnlyHint=false, openWorldHint=true and idempotentHint=false, so the mutation profile is covered. The description adds two useful facts beyond them — that funds come from the wallet balance and that a valid session token is required. It does not warn about the non-idempotency consequence (a retry can charge twice) or what happens on insufficient balance, which matters for a purchase tool.

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?

Two short sentences, front-loaded with the core action and scope; no filler or restated title. It could be slightly tighter by folding the token requirement, but nothing is wasted.

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?

An output schema exists, so return values need not be described, and annotations cover the safety profile. Still, for a wallet-charging, non-idempotent purchase the description omits failure modes (insufficient balance, invalid/expired token) and the double-purchase risk, leaving an agent without guidance for the most consequential edge cases.

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 description coverage is 100% — token, orderId and packageCode are each documented in the schema, including the pointer to verify_agent_otp. The description adds no parameter-level detail (e.g., how to obtain packageCode or that orderId must reference an active order), so the baseline of 3 applies.

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 ('Purchases a ... top-up package') and clarifies the funding source ('using the wallet balance'), which separates it from get_topup_packages (read) and purchase_esim_with_wallet (a different purchase). It does not explicitly name those siblings, so an agent must infer the boundary rather than being told it.

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?

Usage is only implied: 'compatible top-up package' hints the agent should first resolve available packages (get_topup_packages) and that the package must match the active order, but no explicit when-to-use, prerequisites, or alternative is named. No guidance on the relationship to purchase_esim_with_wallet either.

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