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

Annotations cover safety (readOnlyHint, openWorldHint), and the description adds substantial context beyond them: the 8 KB GET cap, redirect following, the exact returned fields, refusal of private/local/logged-in URLs, and the billing semantics for failed calls under both x402 and pack credits.

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-loads the core purpose in one short sentence, then layers return values, restrictions and pricing. The two sentences on payment/refund mechanics are the longest block but are genuinely decision-relevant for a paid tool; the paragraph is dense but nearly every 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 2-param read tool with no output schema, the description covers what comes back (status, ok, final URL, content type, length, timing), what's disallowed, and the cost/failure model. An agent has everything needed to decide and invoke correctly.

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 the url constraint and the HEAD/GET enum are already documented in structured fields. The description restates 'public http(s) URL' and the HEAD-default/GET-8KB fallback without adding format or behavior details the schema lacks, so baseline 3 applies.

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?

Opens with a specific verb and resource ('Check if a URL is up'), then names the exact mechanism (HEAD with GET fallback). It clearly separates itself from heavier siblings like page_extract and deep_brief by framing itself as a cheap pre-fetch check.

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 usage context: 'A cheap uptime, health or broken-link check before a heavier fetch,' which implies the alternative without naming it. It stops short of naming sibling tools or stating when NOT to use it, so it's clear but not exhaustive.

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