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

Beyond the readOnly/idempotent annotations, it discloses genuinely useful behavior: missing PTR is reported separately from a PTR that fails to confirm, the `fcrdns` field carries the verdict, and remediation belongs to the address block's operator. That is result-shape and failure-mode information the agent cannot get from 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 core operation is front-loaded in sentence one, but the body repeats the same idea twice ('a PTR alone proves nothing' / 'Only the round trip is evidence') and spends a clause on Google/Yahoo bulk-sender framing that is motivational rather than operational.

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 no output schema, the description still conveys the return shape well: a PTR value plus a confirmed/not-confirmed verdict, with missing-PTR as a distinct state. Exact field names beyond `fcrdns` and the absence of a hostname-based variant explanation are the only gaps.

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?

There is a single parameter with 100% schema coverage, and the schema already says it must be an address and not a hostname. The description restates the IPv4/IPv6 acceptance and adds no syntax, format, or edge-case detail (e.g. CIDR or private ranges) beyond the schema.

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 first sentence names the exact resource (PTR record for an IPv4/IPv6 address) and the paired forward lookup, which distinguishes it from hostname-oriented siblings like check_dns and check_cname. It does not explicitly name an alternative sibling, but the IP-only scope 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?

Usage context is clear: this is the check for FCrDNS compliance against bulk sender requirements and Gmail's IPv6 rejection policy, so an agent knows when it matters. There is no explicit 'use X instead of this' routing, which keeps it short of a 5.

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