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

Annotations already cover the read-only, non-destructive, closed-world profile, but the description adds real behavioral context beyond them: no DNS existence check, no certificate validation, and a concrete failure mode (external DNS without a webmail record yields a non-resolving URL). It does not address caching or whether the result is ever persisted, keeping it short of 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 dense sentences with zero waste: the purpose is front-loaded, followed immediately by the critical non-measurement caveat and its consequence. Every clause earns its place.

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 one-parameter, read-only tool with no output schema, the definition covers what it returns, that it is constructed rather than verified, and the failure case. It stops short of noting that some hosts use alternative webmail hostnames, but is otherwise sufficient to call 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?

Schema description coverage is 100% for the single 'domain' parameter, so the schema already documents it. The description adds the substitution template (webmail.<domain>), which shows how the value is used, but nothing about accepted formats or subdomain handling beyond what the schema implies.

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+resource: it constructs the webmail access URL for a domain using the conventional https://webmail.<domain>/ pattern. The explicit 'Das ist keine Messung' clause pre-empts confusion with validation-type siblings, though no sibling is named directly.

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?

Usage is implied rather than stated: the description tells you it produces a link, and the caveat about external DNS implies this is a convenience builder, not a checker. There is no explicit 'use this when / use X instead' routing to siblings such as get_email_status or check_domain_availability.

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