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

Annotations declare readOnlyHint=false and idempotentHint=false, and the description usefully adds that this spends real wallet balance and returns the charged price. It also explains the opt-in idempotency escape hatch, clarifying why the tool is flagged non-idempotent. It stops short of saying whether a failed or repeated purchase can be refunded/reversed or what happens on partial failure.

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?

Three short sentences with zero filler: what it does, what it returns, and the idempotency caveat. The wallet-charge consequence is front-loaded.

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 money-spending mutation with no output schema, the description does name the return value (price charged), which is the key missing piece. It is not complete about parameter selection among the 7 zero-documented fields, but the critical behavioral facts for calling it safely are present.

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 0% across 7 parameters, so the description carries the full burden — yet it explains only idempotency_key, and does so well. The six remaining parameters (sku_id, country, number_type, did_group_id, stock_did_id, includes_origin_calls) get no semantics at all, and the mutual exclusivity of the identifier fields is left entirely to guesswork.

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 phone number') plus the consequential side effect ('charges the wallet'), which separates it in spirit from the quote_number sibling. It never names quote_number or search_numbers, so the agent must infer the distinction rather than being told.

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?

There is no explicit when-to-use / when-not-to-use guidance and no mention that quote_number should be called first to price the purchase or that search_numbers/list_did_numbers supplies the did_group_id/stock_did_id values. With 7 optional parameters and nothing required, the description also gives no hint about which identifier combination selects a purchase.

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