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

Annotations already cover safety (readOnly, idempotent, non-destructive) and open-world access, so the bar is lower, yet the description adds real value: live mail-server lookups, that every reason is returned including paid-plan-only ones, and that 'nothing sent is stored.' It omits auth requirements and rate limits, which keeps it from a 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?

The purpose is front-loaded in the first clause and the paragraph is dense with useful detail rather than filler. The phrase 'the way signupbouncer checks a new signup' is mildly redundant, and the run of output details could be tightened, but overall it earns 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?

With an output schema present, the description need not enumerate return fields, yet it helpfully names verdict, score, reasons, normalised inbox, network, and suggestion. Combined with annotations covering safety, an agent has enough to call this correctly; only error/rate-limit behavior is unaddressed.

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% with only two parameters, so the baseline is 3. The description adds a little meaning, noting the IP's network is used and that the email is normalised (Gmail dots and +tags folded), but that detail concerns output rather than adding syntax or constraints for the 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 gives a specific verb and resource: 'Checks one email address, and optionally the signup's IP address,' and frames the scope as a signup-time check. It is easy to separate from siblings like lookup_burner_domain or explain_reason, but it never explicitly names or contrasts those alternatives, so it stops short of a 5.

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?

Usage is implied by 'the way signupbouncer checks a new signup' and by the optional IP parameter, so an agent can infer this is the signup-gating check. However there is no explicit when-to-use/when-not guidance and no routing to explain_reason or lookup_burner_domain, leaving the agent to infer the boundary.

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