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

Annotations already mark this as a non-read-only, open-world, non-idempotent operation, but the description adds the key behavioral context: nothing is bought or charged until the user completes Stripe checkout. It also explains what happens afterward (status must become active; whatsapp_connect_link provides the signup link) and the unusual Meta verification flow (phone call, not SMS), all of which is valuable beyond the annotations.

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 dense but front-loaded: it opens with the purpose, immediately states the strict usage condition, then explains the return contract and next steps. Every sentence carries operational information, and the sequence mirrors the workflow an agent must follow.

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?

With no output schema, the description compensates by explaining the returned fields and what to do with checkout_url, then describing the post-checkout state check and WhatsApp signup handoff. Given the open-world, non-idempotent nature of the rental and the unusual verification requirement, it provides everything an agent needs to invoke the tool safely.

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 moderate at 67%, and the schema itself documents phone_number and country while leaving number_type as an enum without prose. The description strengthens parameter semantics significantly by constraining phone_number to a user-chosen result from search_phone_numbers after price disclosure, which is essential for correct invocation, though it does not explain country or number_type beyond what the schema shows.

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 first sentence names the specific action (rent/buy) and resource (a phone number from search_phone_numbers) and explains the intended outcome: giving the user a number to register with WhatsApp. It clearly differentiates the tool from search_phone_numbers and list_phone_numbers by describing the rental action rather than searching or listing.

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 states the critical prerequisite explicitly: call ONLY with a number the user chose from search_phone_numbers results and only after telling them the monthly price. It also names the follow-on tools (list_phone_numbers and whatsapp_connect_link) and the condition for proceeding, leaving little ambiguity about when and how to use this tool.

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