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

Annotations only flag this as non-read-only, open-world, and non-idempotent; the description adds the material fact that it consumes credits from the connected BillionVerify account, which is real cost information an agent needs. It does not mention rate limits or what happens on cache hits beyond force_refresh's schema note.

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, front-loaded with the purpose, then the mechanics, then the cost. No filler or repetition of the tool name.

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 3-parameter tool with no output schema, the description covers purpose, mechanism, and the credit cost, which is enough to invoke it correctly. It stops short of hinting at the result shape (e.g., deliverable vs undeliverable), which would help an agent interpret the response.

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%, so email, check_smtp, and force_refresh are already documented. The description reinforces check_smtp ('optionally the mailbox over SMTP') but says nothing extra about force_refresh or caching behavior beyond the schema, 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 ('Verify whether a single email address can receive mail') and enumerates the actual checks performed (syntax, domain mail servers, optional SMTP). The word 'single' cleanly distinguishes it from the sibling verify_batch_emails.

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?

Scope is implied by 'single email address' versus the obvious batch sibling, and the description notes that SMTP checking is optional, but it never explicitly says when to pick this over verify_batch_emails or when to enable check_smtp. Usage must be inferred rather than read.

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