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

Annotations cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true), and the description adds genuinely new behavior: per-IP rate limiting with a small check budget, no auth requirement, and output parity with the full API. It doesn't say what happens when the rate limit is hit (error shape), which keeps it below 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?

Three sentences, all load-bearing: first the action and key-free constraint, then the rate-limit and output-parity caveats, then the upgrade path. Front-loaded with the differentiator and zero 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?

No output schema exists, and the description compensates by stating that the same deliverability verdict and flags are returned as the full API. Auth expectations, rate limits, and the escalation path are all covered for a simple single-param tool.

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 a single required 'email' parameter, so the schema already carries the semantics. The description only implies one address at a time, adding little beyond what the schema documents; 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?

States a specific verb and resource ('Verify one email address') plus the distinguishing scope: demo mode with no account or API key. An agent can separate it from verify_email, verify_batch, and verify_email_demo's siblings without opening 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?

Gives explicit when-to-use (evaluating Verifly before registering, no key needed) and routes to alternatives by condition: register_account for a key + credits, then verify_email for real volume. Exclusions and escalation path are both stated.

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