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

Annotations already cover the read-only/idempotent safety profile, but the description goes well beyond them: it discloses that each homepage is fetched once, that bot-blocking sites return blocked=true, the caching window, the rate limits and auth header, and that nothing is stored and no email is sent. This is rich operational context an agent needs.

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?

Front-loaded with purpose and usage, then segmented with 'Input:', 'Limits:' and 'Read-only:' labels, which keeps a long paragraph navigable. The return-value enumeration is lengthy but earns its place given there is no output schema; a small amount of redundancy exists between the homepage-fetch and per-domain-lookup statements.

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?

Even without an output schema, the description enumerates the response shape (results[] with reachable, blocked, finalUrl, statusCode, technologies[], securityHeaders, errors, analyzedAt, cached), which is exactly what an agent needs to consume the result. Rate limits, auth, caching and safety are all covered.

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%, so the schema already documents the single domains parameter, making 3 the baseline. The description adds the cost semantics ('each domain counts as one lookup') but its '1-3 domains or URLs' phrasing omits the email-address input the schema explicitly permits, adding no meaningful semantics beyond the schema.

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+resource ('Website technologies only, for 1-3 sites') and immediately differentiates from siblings by naming email_security and domain_registration as the tools for DNS/email/registration data. An agent can distinguish this from domain_report without opening any schema.

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?

Explicitly states when to use it (lead qualification by technology, competitor research), when not to use it (no DNS, email or registration data) and names the alternatives for those cases. Limits are also spelled out: one lookup per domain, 1-hour cache, 25 free domains/day per IP, paid key via x-api-key.

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