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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered elsewhere. The description adds genuinely new behavior: it performs an outbound fetch of the asset and is metered at 10 free calls/day then $0.002/call, payable via x402 payment metadata or an X-API-Key. It still omits failure modes (redirects, timeouts, large-file behavior), which matters for an external fetch tool.

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 as a direct question, then the checks performed, then a single dense clause covering free tier, paid fallback, payment mechanism and API-key alternative. Every sentence carries information an agent needs; nothing is repeated from the annotations or schema.

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 an output schema present, return values need no explanation, and the annotations cover safety and idempotency. The description closes the remaining gaps an agent would otherwise hit, namely cost/auth per call and the exact validation criteria. Minor residual gap is fetch behavior (redirects, timeouts) for an open-world network call.

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 schema itself explains that `image` is "an image URL directly" versus `url` as a page URL. The description reinforces this by framing the check around the page's og:image being fetched, adding slight meaning beyond the schema. It does not state precedence when both are supplied or what happens when neither is, which is why this is not a 5.

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?

"Does the page's og:image work in link previews? Fetches it and checks status, type, file size, pixel dimensions and aspect ratio (1200×630)" gives a specific verb and resource plus the exact criteria evaluated, so an agent knows precisely what is being returned. It does not name a sibling it should be preferred over (e.g. broken_links or http_link_parse), which keeps it just short of a 5.

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?

Usage is implied by the phrasing: an agent infers it should call this when validating a page's social-preview image. There is no explicit when-to-use/when-not statement and no guidance on choosing between the `url` (page) and `image` (direct image URL) modes, nor when a sibling like broken_links would be the better call.

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