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

Annotations already declare destructiveHint=true and idempotentHint=false, so the safety bar is partly met; the description earns credit for explaining exactly why mutation is risky — partial updates plus 'null or empty clears one' means an omitted-versus-empty distinction that is easy to get wrong. It also flags the compliance constraint that postal address is required for marketing. It does not cover permission requirements or what breaks when fields are cleared, which keeps it from a 5.

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?

Two sentences, both earning their place: the first enumerates the mutable surface, the second states the update/clear semantics. The field enumeration is dense and would read faster as a list, but nothing is redundant or padded.

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 9-parameter, all-optional mutation tool with no output schema and a destructive hint, the description covers purpose, mutability surface, and clear semantics — the core things an agent needs. Gaps remain around id resolution (which domain ids are valid, sending vs receiving) and the consequence of clearing the marketing postal address, but the essentials are present.

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 67% and several properties (senderName, companyName, mailingAddress) have no schema text at all, so the description does real work by mapping each named field to its role. Most valuable is 'the part before @ and the domain' for the support/marketing pairs, which is the only place the local-versus-domainId split is explained. It does not clarify what a 'receiving' vs 'sending' domain id implies in practice.

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 uses a specific verb-plus-resource framing ('Set who email comes from') and enumerates exactly which identity fields are under its control — sender name, company name, postal address, support/marketing local parts and domains, reply domain, DMARC address. That makes it clearly distinct from domain-level siblings like update_mail_domain or read-only siblings like get_email_setup. It stops short of explicitly naming an alternative, which keeps it at 4 rather than 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 description explains what the tool does operationally ('Only the fields given change; null or empty clears one'), which implies a partial-update patch workflow, but it never states when an agent should reach for this versus update_mail_domain, update_support_inbox, or update_workspace_settings. There are no stated prerequisites (e.g., that domain ids must already exist via list_resend_domains/register_mail_domain).

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