Validate a list of email addresses

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

The description goes well beyond the readOnlyHint annotation by explaining that Unknown means the mailbox could NOT be checked due to specific causes, that the call is slow by nature, requires an API key with a 25-address free limit, and that parallel calls fail. These are behavioral traits not captured in the annotations.

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?

The description is longer than average but every sentence serves a distinct purpose: purpose, Unknown semantics, API key, performance, and concurrency. It is front-loaded with the primary function in the first sentence and structured logically, earning its length.

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?

Given the tool's complexity (batch validation with nuanced Unknown handling and concurrency limits), the description covers all operational aspects: usage, auth, performance, and result interpretation. The output schema covers return details, so the description complements it well.

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 already fully documents the only parameter 'emails' with description 'The addresses to check, at most 50 per call' (100% coverage). The description's mention of 'up to 50' repeats the schema but adds no new parameter-specific meaning, so baseline 3 is appropriate.

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 clearly states the tool checks up to 50 email addresses in one call and returns per-address verdicts plus a summary, using a specific verb ('Check') and resource. It distinguishes from sibling tools like validate_email by emphasizing the batch nature ('up to 50') and is unambiguous.

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 provides explicit usage context: split longer lists into chunks of 50, never run parallel calls because the API serialises jobs, and how to handle Unknown results (present separately, don't reject). However, it doesn't explicitly name alternative tools like validate_email for single addresses, so it's not a full 5.

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