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

Annotations already declare readOnly/openWorld/non-destructive, so the bar is lower; the description adds genuinely new behavior information: the underlying data sources (CT logs plus common-hostname brute force) and a 100-credit cost per call. The cost disclosure is especially useful for an agent weighing this against free DNS siblings. It does not discuss timeouts or rate limits beyond what the schema's deepScan note implies.

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 tight sentences, front-loaded with the core capability and ending with cost. The opening 'Subdomain Finder: Subdomain Finder discovers' redundantly repeats the tool name, a small but real waste in an otherwise efficient definition.

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 3-parameter read-only tool with full schema coverage, rich annotations, and an output schema, this is close to complete: method, cost, and output shape are all touched. Minor gaps remain around what the scan-completion status signifies and why one would pick a specific limit value, but nothing critical for correct invocation is missing.

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 all three parameters are documented in the schema, including deepScan's tradeoff. The description mentions the CT-log/common-name versus full-wordlist distinction only indirectly, adding essentially nothing beyond the structured parameter docs. Baseline 3 applies.

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 ('discovers active subdomains and their resolved IP addresses') plus the technique used (certificate transparency logs and common hostnames), which clearly separates it from plain DNS lookups in the sibling list. It never names an alternative sibling (e.g., DNSLookup, DomainAvailability) explicitly, so it stops short of full differentiation.

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?

When-to-use is only implied by the stated purpose: an agent can infer this is for enumerating subdomains of a target domain. There is no explicit 'use this instead of X when Y', no prerequisite note, and no statement about when not to use it (e.g., for a single known hostname, where DNSLookup would be cheaper than 100 credits).

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