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

The annotations already establish read-only, idempotent, open-world, and non-destructive behavior. The description adds useful context: it performs an external HTTPS fetch, returns reachability and certificate metadata, and explicitly discloses important non-guarantees around SVG profile compliance, trust-chain validation, trademark ownership, and mailbox display. Minor gaps remain around fetch timeouts/retry behavior, but the behavioral disclosure is well above baseline.

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?

The definition is front-loaded with the core purpose and then layers in scope, return information, and limitations. Most sentences earn their place, though 'No auth or destructive actions' largely repeats annotation information and the five-sentence length is slightly heavier than necessary.

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 present, the description usefully explains what the tool returns: record syntax, URL reachability/content type, certificate subject/issuer/validity dates, and explicit issues. It also sets clear technical boundaries, making the definition complete enough for an agent to invoke it correctly for a BIMI preflight.

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 only one parameter, and the input schema describes it fully with format guidance ('Domain name only, e.g. example.com'). The description does not add further parameter-level meaning beyond implying that a domain is being checked, so the high schema coverage makes 3 the appropriate baseline.

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 a specific verb and resource ('BIMI readiness check') and immediately identifies the concrete checks performed: parsing `default._bimi`, fetching the HTTPS SVG, and parsing VMC/CMC metadata. This clearly distinguishes it from sibling checks such as `check_dmarc`, `check_spf`, and `check_mta_sts`.

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?

It gives a clear use case: 'Use for a technical preflight before a formal BIMI/VMC review,' and defines the boundary by listing what the tool does not certify or validate. It stops short of naming alternative tools or explicitly stating when another sibling should be used instead, so it is strong but not fully exhaustive.

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