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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, openWorldHint=true), and the description adds real value beyond them: the call persists a record AND kicks off external verification against MX, SPF, DKIM, DMARC and blacklists. It does not disclose what happens on a duplicate domain, whether verification is asynchronous, or whether the record is created before verification passes — the main failure modes for a write tool.

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?

Front-loaded and short, but the second sentence largely repeats the first: 'Registers a new sending domain under the workspace, saves it' restates 'Connect a new sending domain'. The verification-detail clause earns its place; the re-registration clause does not.

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 single-parameter mutation with no output schema, the description covers what the tool does and its verification side effect, which is the core need. It stops short of stating what the caller gets back (verification status? record id?) or how to handle a domain that already exists, which matters because verification results are the whole point of the call.

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?

There is one required parameter and 0% schema description coverage, so the description carries the full burden — but it says nothing about what the 'domain' string should look like (bare apex domain vs subdomain vs full URL, max 300 chars, normalization). The one sentence about 'a new sending domain' just restates the parameter name.

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 concrete verb and resource ('Connect a new sending domain to the workspace') plus the immediate consequence ('initiates DNS and deliverability verification'), which lets an agent separate it from read-only siblings like list_domains or check_domain. It never names a sibling explicitly, so the differentiation is inferential rather than stated.

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 word 'new' hints this is for onboarding a domain not already present, but there is no explicit when-to-use guidance, no mention of when to prefer check_domain instead, and no prerequisites (workspace role, domain ownership, prior setup). An agent must guess the routing between add_domain, check_domain and list_domains.

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