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

Beyond the annotations (readOnly, openWorld, idempotent, non-destructive), the description adds concrete behavioral details: 'Verified source: IANA RDAP. 4-hour cache,' and 'registrant info where publicly available,' which warns about variable data availability. It also specifies the return payload fields and provides a feedback fallback mechanism. This is exactly the kind of contextual behavior an agent needs that annotations alone do not convey.

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 description is appropriately front-loaded: purpose first, then usage routing, then behavioral details, then feedback boilerplate. It stays readable despite the long fallback sentence, but 'Read-only. No side effects. Idempotent.' partially duplicates annotations, and the feedback block adds length. Still, every section serves a functional purpose for an agent.

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 single-parameter read-only tool with an output schema, the description is complete: it explains the source, cache behavior, return fields, public-data caveat, and when to use alternatives. It also tells the agent how to escalate if the response is inadequate. Nothing essential 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?

The schema fully documents the single 'domain' parameter with 100% coverage, so the baseline is 3. The description repeats the same format guidance ('domain: Domain name without protocol e.g. example.com not https://example.com'), adding no meaning beyond the schema. It correctly emphasizes the required format, but does not elevate the parameter understanding.

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 opens with a specific verb and resource: 'Fetch domain registration details via IANA RDAP (the modern structured replacement for WHOIS).' It names the exact data returned (registrar, dates, nameservers, registrant info) and explicitly distinguishes itself from domain_fetch_ssl_certificate_chain and domain_fetch_dns_records. An agent can immediately tell what this tool does and what it is not.

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 gives explicit routing guidance: 'Use this when you need registration metadata. Use domain_fetch_ssl_certificate_chain instead when you need certificate history. Use domain_fetch_dns_records instead when you need live DNS resolution.' This names both the condition to select this tool and the alternative tools with their conditions, leaving no inference to the agent.

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