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

Annotations already establish this is a non-destructive, read-only, open-world lookup, so the description needn't restate safety. It does add genuinely useful operational context beyond the annotations: the data comes from two independent blocklists, covers 570,000+ domains, refreshes every 6 hours, and costs 10 credits — the refresh cycle and cost are things an agent cannot learn from structured fields. What's missing is whether a negative result means 'not listed' versus 'unknown'.

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 sentence is well front-loaded, but it opens by restating the title twice ('Phishing Domain Checker: Phishing Domain Checker is...'), which wastes the most valuable position. The database and credit details are compact and worth keeping.

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 no explanation, and the annotations cover the safety profile. However, with zero required parameters, the description should clarify the url-vs-domain choice and whether supplying both is permitted; that gap is material for correct invocation.

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%, so the baseline is 3; the schema already explains that a full URL has its domain extracted automatically and that domain takes a bare hostname. The description adds nothing about the parameters — notably it does not explain that neither parameter is required but at least one must presumably be supplied, which the schema alone leaves ambiguous.

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 states a specific verb and resource: it verifies whether a domain or URL appears in a database of known phishing sites. That is clearly distinguishable from siblings such as SpamDetector, DisposableEmailChecker, or IPBlacklistLookup, which check different artifacts.

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 only implied by the subject matter — an agent can infer it is for checking suspicious domains or URLs. There is no explicit when-to-use/when-not-to-use guidance and no mention of adjacent tools (URLUnshortener, IPBlacklistLookup, SpamDetector) that might be better for a related task.

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