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

Annotations already declare openWorldHint=true, readOnlyHint=false and idempotentHint=false, so the description's real contribution is the credit cost breakdown (phone 1, email 1, IP 1, signup up to 3) and the note that demo fixtures cost nothing — meaningful operational context an agent can't get from annotations. It still doesn't say what happens on failure or how errors/credits interact.

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 short sentences with the purpose front-loaded and no filler; the credit line is dense but earns its place. Slightly compressed to the point that the return-value clause ("signals and a summary") is vague.

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 non-read-only, open-world tool with no output schema and zero parameter documentation, the description covers purpose and cost but leaves the return shape ("signals and a summary") and two of three parameters unexplained. Adequate but with clear gaps an agent would need filled.

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 0% across three parameters, so the description carries the full burden — yet it only references "phone" incidentally in the pricing line. The roles of country and idempotency_key (format, retry semantics, defaults) are left entirely undocumented.

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 concrete verb and resource ("Check phone") plus the intended outcome ("signup or lead-list quality"), which clearly separates it from check_email and check_ip. The mention of signup overlaps with the sibling check_signup, leaving slight ambiguity about the boundary.

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?

The phrase "for signup or lead-list quality" implies the use contexts, but it never states when to choose this over check_signup or check_email, nor any prerequisites or exclusions. Usage is inferable rather than explicit.

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