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

Annotations cover the safety profile (readOnly, non-destructive, closed-world), and the description adds substantial behavioral context: nightly capture cadence, independent third-party source, coverage of domains with no payment history, and the up threshold definition (>=0.95). It doesn't mention caching, staleness windows beyond 'nightly', or error behavior, which keeps this below 5.

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 the core question (was the domain up) and the nightly-vs-live distinction is placed well, but the pricing/key paragraph is dense and interrupts the flow between purpose and the sibling pointer. Multiple tiers of payment detail in a single run-on sentence dilute the signal.

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?

Complete for a 2-param read tool with no output schema: purpose, data source, cadence, coverage edge case (domains with no payment history), the up definition, and the sibling alternative are all present. Return field names (share, latency p50/p95, sample counts, last-probe time) are enumerated, which compensates for the lack of an output schema.

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?

With 100% schema description coverage, the schema already documents both domain and api_key. The description adds payment semantics not in the schema (free pk_free_ key at 10 calls/day shared, $0.001/call via x402, 1 free MCP trial/day), which is useful pricing context but not parameter-format guidance. Baseline 3 is appropriate.

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 and frames it as a third-party nightly probe of domain uptime. It explicitly contrasts itself with the sibling verify_before_pay ('for a check taken at query time use verify_before_pay'), so an agent can distinguish the two without opening either 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?

Clear context: historical nightly liveness series, not a live check, and the alternatives are named (verify_before_pay for query-time checks). It does not spell out exclusions like when historical liveness is unhelpful (e.g., brand-new domains with no probe history), but the when-to-use routing is strong.

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