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

Annotations already declare readOnly, idempotent, non-destructive, openWorld, so the safety profile is covered. The description adds genuinely non-schema information: a free tier of 10 calls/day, then $0.001/call, and the two accepted auth mechanisms (x402 via params._meta["x402/payment"] or HTTP, or X-API-Key credits), plus the serving host. That is meaningful operational context an agent needs before invoking.

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?

Two sentences, both front-loaded: capabilities first, then cost/auth. No filler, and the pricing sentence is dense with actionable detail rather than padding. Nothing could be removed without losing information an agent needs.

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?

An output schema exists, so return values need not be described, and the description covers inputs at a high level plus the auth/payment path. The only gap is that it doesn't say how the optional split and contains parameters interact with the base CIDR computation, which matters for a three-parameter 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 coverage is 100% and each parameter already carries a description and example, so the baseline is 3. The description only echoes the parameters as capabilities ('contains-check and subnet split') without adding syntax, format, or interaction rules (e.g. whether split returns multiple CIDRs or what happens when contains is combined with split).

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 the resource (IPv4/IPv6 subnet math) and enumerates the exact computations it performs: network, range, size, netmask/wildcard, private/reserved type, contains-check, and subnet split. An agent can tell this is a pure calculator and not an address-owner lookup like the sibling ip_owner.

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 capability list — you call it when you need subnet arithmetic or a contains test — but there is no explicit when-to-use/when-not guidance and no routing to or away from siblings such as ip_owner or address_validate. The only 'when' signal is the pricing threshold, which is about cost, not applicability.

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