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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so safety is covered. The description adds genuine behavioral context that the check is two-part — presence of the BIMI record plus whether the DMARC policy is strict enough for display — which is a real dependency an agent cannot get from the annotations or schema.

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?

One tight sentence, front-loaded with the primary action and parenthetically clarifying the domain term. No filler, no redundancy with the schema or annotations.

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 read-only diagnostic, the description covers what is inspected and the DMARC dependency that governs the outcome. It stops short of hinting at result shape, but with no output schema and such a simple surface, little more is needed.

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?

Single parameter with 100% schema description coverage, including the note that URLs and email addresses are accepted, so the schema carries the parameter semantics fully. The description adds nothing about the domain argument, making the baseline 3 correct.

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?

Names a specific verb (check) and resource (BIMI record), then glosses it in plain language ("logo in the inbox") so the agent knows the domain concept being tested. The mention of the DMARC-strictness prerequisite differentiates it from the sibling check_dmarc/check_dkim tools, which test those records 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 only implied: the agent can infer this is the right tool when the question is whether a brand logo will display in the inbox. It never states when to prefer this over check_dmarc, nor any prerequisites or exclusions. Adequate but leaves routing to inference.

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