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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld). The description adds genuinely useful context beyond them: no wallet or payment required, no arguments needed, and — critically — that the payload is a sample rather than live validation output, which prevents the agent from treating the data as real.

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 sentence, front-loaded with the [FREE] marker and immediately explaining the sample-response purpose. Every clause carries information; nothing is redundant.

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 zero-argument preview with no output schema, the description supplies the essentials: it is free, needs no wallet or parameters, and returns a shape-demonstrating sample. It does not detail the fields present in the sample, but 'see the shape' adequately signals the caller should inspect the response itself.

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?

The schema has zero properties and 100% coverage, so the baseline is 4. The description reinforces this by stating 'no arguments needed', which is accurate and prevents an agent from hunting for missing inputs.

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 exactly what the tool returns: a sample response for email_validate, provided free so the caller can inspect the response shape before paying. It names the sibling (email_validate) it previews, so an agent can distinguish it from the paid tool and from other *_preview siblings.

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 clearly frames the use case: call this first to see the shape, before spending 0.006 USDC on the real email_validate. There is no explicit 'do not use for real validation' exclusion, but the 'sample response' wording and the reference to the paid tool make the boundary unambiguous.

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