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

With no annotations, the description carries the full burden and does disclose the key traits: 'Read-only' (safety) and 'passive' (no active probing), plus the external source (crt.sh). It omits rate-limit/pagination behavior and result size expectations, so it falls 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?

One front-loaded sentence with the verb first; the em-dash aside efficiently explains the mechanism. Slightly dense, but every clause earns its place with no filler.

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 lookup with no output schema and no annotations, the description covers purpose, source, passivity, and safety. The only gap is that the return format (a list of domains) is left implicit, which is acceptable given the phrasing.

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 coverage is 100% and the single 'domain' param is self-evident. The description restates 'the given domain' but adds no format rules or edge-case guidance beyond the schema's example, so baseline 3 applies.

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?

Specific verb ('Find') plus a precise resource ('other registrable domains that share a TLS certificate'). It names the mechanism (common SAN) and data source (crt.sh), which clearly distinguishes it from siblings like cert_transparency and ct_org_search that operate on the target's own certs.

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 phrase 'passive link to sibling properties' implies the investigative use case, but the description never states when to prefer this over cert_transparency/ct_org_search or any exclusion conditions. Usage is inferable but not explicit.

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