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

Goes well beyond the annotations: credit cost per call, credit refunded on unknown, no automatic retries, the try_again-timeout recovery path and the 5-minute no-recharge window, plus the legal constraint on verified addresses. Annotations declare destructive/openWorld but say nothing about billing or idempotency-in-practice; the description supplies all of that.

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?

Information-dense and front-loaded with the credit warning, which is the highest-stakes fact. Slightly long, and the enumerated return fields pad it, but with no output schema those details earn their place.

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 carries the return-value burden and does so completely: state values, reason, score range, flags, did_you_mean, and parsed components. Combined with cost, retry, and compliance guidance, an agent has everything needed to call and interpret this tool.

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 100%, so the baseline is 3, but the description adds semantics the schema cannot convey: the timeout parameter's failure mode ({status: 'try_again'}) and the fact that repeats within 5 minutes aren't charged. It does not describe the smtp or accept_all flags, leaving those to the schema.

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+resource ('verifies one email address') and the singular scope implicitly distinguishes it from the sibling emailable_create_batch, which handles many addresses. An agent can tell what this does and when it is the wrong choice without opening a schema.

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?

Gives real usage context: it spends credits, test keys spend none, terms permit verifying opt-in subscribers, and it explains the retry path when the reply is {status: 'try_again'}. It does not explicitly say 'use emailable_create_batch for multiple addresses', so the alternative is left to inference from 'one email address'.

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