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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds substantial non-obvious context: this tool does not perform the check itself, it returns a paid x402/MPP request body (POST /v1/email/verify/batch) with the response schema and an example, priced at $0.25 per call, and bad input is refused before any charge. That is exactly the kind of behavioral disclosure annotations cannot convey.

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?

Front-loaded with the usage trigger and the sibling routing, and every clause carries information. The middle sentence is dense with semicolon-joined clauses about the payment flow, which slightly burdens scanning but remains informative rather than 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 payment-request-returning tool with no output schema, the description fully explains what is returned (the request body, response schema, example), what the agent must do next (POST with an x402/MPP client), the cost, and the validation guarantee. Nothing needed to invoke it correctly is missing.

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 there is only one parameter, so the schema already documents items, maxLength, maxItems (25) and duplicate removal. The description reiterates 'up to 25 addresses' but adds no syntax or format detail beyond the schema; 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 (check) and resource (email batch) with explicit scope (up to 25 addresses in one POST) and names the sibling it is not ('For one address use check_email_address'). An agent can distinguish it from check_email_address without opening either schema.

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?

Gives an explicit trigger ('before a mailing or import'), an explicit limit (up to 25), and an explicit alternative with the selecting condition ('For one address use check_email_address'). When-not guidance is effectively covered by routing the single-address case elsewhere.

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