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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, establishing the safety profile. The description adds valuable context beyond annotations: 'The host is sent only to URLhaus; LiveDataLink never connects to it,' which discloses data flow privacy. It also mentions 'bounded list,' hinting at result limits, but doesn't elaborate on return shape.

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 concise sentences with no wasted words. The first sentence states the core purpose and output; the second adds a relevant privacy note. Information is front-loaded and each sentence earns its place.

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 simple lookup tool with full schema coverage and annotations covering safety, the description covers the essential aspects: host types, output nature (bounded list), and data flow. It doesn't mention error handling or rate limits, but these are not critical for a read-only, idempotent lookup. The only minor gap is lack of explicit sibling differentiation, but that is partly addressed by the name and the description's specificity.

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 both host and limit are documented in the schema. The description adds minimal extra meaning: it reinforces the accepted formats for host ('IPv4 address, hostname, or domain') which partially overlaps with the schema's description. Since the schema already covers parameters, the description's contribution is marginal – hence the baseline 3.

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 clearly states the action ('Look up'), the resource ('IPv4 address, hostname, or domain in URLhaus'), and the outcome ('return a bounded list of associated malware URLs'). It distinguishes from siblings by focusing on 'host' lookups, which is unique among the urlhaus_lookup_* tools.

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?

The description implies the use case (looking up a host) but does not explicitly mention alternatives or exclusions. While siblings like urlhaus_lookup_url exist, the description doesn't say 'use this instead when you have a host' – the guidance is left to inference from the name and description.

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