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

With no annotations, the description carries the full burden and does: HTTP method (HEAD), redirect policy (follows up to five), what is not fetched or charged (no page body), absence of robots.txt enforcement, partial-failure semantics (405/403 on HEAD does not imply GET failure), and pricing ($0.001 USDC via x402). This is exactly the non-obvious behavioral context an agent cannot infer from the schema.

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 purpose and the economic rationale, and every sentence adds a fact an agent would otherwise get wrong (redirect limit, no body fetched, robots not checked, HEAD-vs-GET divergence, price). Dense but not padded; the em-dash header list in the middle sentence is slightly listy but earns its space.

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 one-parameter, no-annotation, no-output-schema tool, the description covers purpose, return fields, cost, redirect behavior, and the main reason a caller might misinterpret the result. Nothing an agent needs in order to call it correctly or interpret a non-2xx 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?

Schema coverage is 100% and there is only one parameter (url), already documented in-schema as 'Public http(s) URL to check'. The description adds no syntax or format guidance for the parameter, so the baseline 3 applies — the schema does all the parameter work here.

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 (check if a URL is reachable via HEAD) and pins down the exact scope: status code, final URL, content-type, content-length, last-modified, no body. It explicitly distinguishes itself from the sibling read_url_as_markdown, which is the tool an agent would otherwise confuse it with.

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?

Gives an explicit when-to-use rule ('before read_url_as_markdown ($0.003) to confirm a URL resolves and is HTML'), an explicit exclusion ('doesn't check robots.txt'), and an explicit failure-mode caveat about HEAD-rejecting servers. The agent knows exactly when this tool is worth its $0.001 and when a non-2xx here is not conclusive.

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