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

Discloses behavior beyond the annotations: 512 KB read cap, no JavaScript execution, public-only scope, per-call pricing, free failed calls, and it explicitly reconciles the readOnlyHint=false annotation ('not marked read-only because each call is charged'). This is exactly the context annotations can't 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?

Front-loaded with what is fetched, then limits, then usage, then pricing. The long field enumeration is justified because no output schema exists and it substitutes for one. No sentence is disposable.

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?

With no output schema, the description compensates by enumerating return fields, and it covers limits, safety semantics, cost, and usage. An agent has everything needed to call it 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% and the single 'url' param is already documented, so the baseline is 3. The description adds mild value by reinforcing the 'public' constraint and giving a concrete example (url=https://github.com/), clarifying expected format.

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 (fetch head) and resource (public web page) and enumerates exactly what metadata is returned, including OpenGraph and Twitter-card fields. An agent can immediately distinguish this from siblings like html_to_text, url_parse, http_status, or extract_rendered (explicitly noted: never runs JavaScript).

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 concrete use cases — building link previews, labelling citations, previewing how a URL unfurls before sharing. Clear context, but no explicit when-not-to-use or named alternatives (e.g. why not html_to_text or http_status), so it stops short of full routing guidance.

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