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

With annotations only declaring readOnlyHint=false and destructiveHint=false, the description carries real extra load: it names the return payload (resend.dkim_records and their status) despite there being no output schema, states the from_email constraint, and candidly discloses that Proof does not yet send from customer domains so the stored address is unused. It also spells out the account/authentication requirement.

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?

Purpose is front-loaded, followed by output details, the from_email caveat, and a clearly labeled ACCESS block. Every sentence contributes; the only mild cost is the somewhat long authentication sentence.

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 two-parameter mutation with no output schema, the description covers the key unknowns: what is returned (DKIM/SPF records and status), the from_email limitation, and authentication. It could be stronger by stating whether the domain must be verified first or referencing the prerequisite tools, but otherwise an agent has enough to call it.

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 genuine meaning: from_email must belong to this domain, is stored as the preferred sender, and is currently not used for sending. That is semantic value beyond the schema's 'Custom from email address' text.

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 (register a verified domain with Proof's email provider, Resend) which is clearly more specific than the generic 'add_domain' or 'start_domain_verification' siblings. It does not explicitly name any sibling to differentiate itself, so it stops short of a 5.

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 phrase 'Register a verified domain' implies the domain must already be added/verified, and the ACCESS block gives an authentication path plus an alternative (start_login). However, it never states when to call this versus siblings like add_domain, verify_domain, or start_domain_email_verification, leaving the workflow ordering to inference.

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