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

Annotations already declare readOnlyHint=false, idempotentHint=false, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds no behavioral context beyond that: it doesn't say what reusing does to the domain, whether it is reversible, whether it affects other workspaces, or what the purposes array changes operationally.

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?

A single short sentence, front-loaded with the action and the source of the id, with zero filler. It is efficient, though arguably terse to the point of under-specification for a mutation tool.

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 non-read-only tool with no output schema and two fully schema-documented parameters, the description covers the id provenance but leaves the core effect of the operation unexplained. An agent can call it correctly, but not predict what reusing the domain does to the account.

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% — both resendDomainId and the purposes enum are fully documented in the schema. The description only echoes the id's source ('from list_resend_domains'), adding no meaning beyond the schema, so the baseline 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?

The description states the resource (a Resend domain already in the account) and implies the action is reusing rather than creating one, which implicitly contrasts with add_mail_domain/register_mail_domain. However, 'use a domain' is a vague verb that never says what reuse actually does — attach it to this workspace, enable sending, etc. It restates the title and only adds the provenance hint.

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?

'from list_resend_domains' implies the caller must first run list_resend_domains to obtain the id, which is useful sequencing guidance. But it gives no when-to-use vs the sibling domain tools (add_mail_domain, register_mail_domain, update_mail_domain) and no exclusions or prerequisites beyond the id source.

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