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

Beyond the annotations (destructive and idempotent already flagged) it discloses the $1.99/month charge billed daily from the wallet, the $2 paid-credit requirement with the caveat that trial credit only covers calls, that no human sign-in is needed, and the 24-hour deduplication window. That is substantive behavioral context an agent cannot get from the structured fields.

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?

Four dense sentences, front-loaded with the action, then cost, then failure recovery, then idempotency rules. No filler and no repetition of the tool name or title.

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 paid, destructive, open-world mutation this covers cost, funding, error path, and retry safety well. It stops short of saying how to find an available number first (search_numbers) or what the call returns, though the latter is mitigated by the response being self-evident and the schema warning constraints.

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 67% and the two described fields already document the businessId default and idempotencyKey retry semantics. The description restates the dedup/idempotency behavior and gives no guidance on the 'number' parameter beyond the schema's E.164 pattern. Baseline 3 is appropriate.

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 uses a specific verb and resource ('Order a phone number for the business') and prices it, which separates it from sibling reads like ringkit_list_numbers and ringkit_search_numbers. 'Step 7: Get a phone number' is slightly softer than 'order', but the pricing detail immediately disambiguates it as a purchase.

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?

It places the tool in a numbered provisioning flow, states the funding precondition, and names the exact fallback: on a 402 funding_required call ringkit_top_up, have the human pay the checkoutUrl, then retry. It also tells the agent when a new idempotencyKey is warranted versus reused.

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