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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds non-redundant behavioral facts: no network request, no payment, no existence check. It also discloses the tool's central limitation (it does not confirm the vendor is tracked), which is exactly the context an agent needs to avoid over-trusting a positive result.

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 sentences, front-loaded with the cost signal ('Free.') and the offline/format-only constraint before the alternative. No filler; each clause delivers a distinct, decision-relevant fact.

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, annotation-rich tool with no output schema, the description covers purpose, boundaries, cost, and the rerouting case. It hints at a return ('see how it will be matched') but doesn't describe the match output, a minor gap given there is no output schema to lean on.

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% and the schema already enumerates the accepted formats (name, slug, domain) with examples. The description restates 'name, slug or domain' without adding format syntax or validation rules beyond the schema, 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?

States a specific verb (validate/check) and resource (vendor name, slug or domain) plus the scope of the check ('well-formed' and 'how it will be matched'). It also names and routes away from the sibling list_vendors, so an agent can distinguish it from vendor_status/list_vendors without reading schemas.

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?

Explicitly states the condition for this tool (format-only, offline) and names the alternative plus its selecting condition ('use list_vendors to confirm the vendor is actually tracked'). The boundary between format validation and existence verification is unambiguous.

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