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

Annotations declare idempotency and non-destructiveness, and the description goes well beyond them: it discloses the domains:write permission requirement, that ownership is not verified, that DNS is not modified, and that reporting does not begin until the user publishes the rua address. These are exactly the side-effect boundaries an agent needs before invoking a mutating 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?

Four sentences, each front-loaded and carrying distinct information: purpose, return value, idempotency, and permission/side-effect boundaries. No filler or repetition.

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 compensates by describing the return (the rua reporting address and account entry). Combined with the permission, idempotency, and non-side-effect disclosures, an agent has everything needed to call this correctly.

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?

Only one parameter, and schema description coverage is 100% ('The domain to add, e.g. example.com'), so the schema already carries the semantics. The description's 'Takes a domain' adds nothing beyond it; baseline 3 is appropriate.

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 (add a domain), scopes it to the signed-in user's MailVakt account, and names the domain's purpose (DMARC report collection). This distinguishes it clearly from siblings like list_domains or diagnose_domain.

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?

Implies the usage context (enrolling a domain for DMARC collection) and explicitly addresses the repeat-call case ('Repeated calls for the same account and domain return the existing entry'). It does not name an alternative tool or state when not to use it, so it falls short of a 5.

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