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

Annotations already cover the safety profile (readOnly, idempotent, openWorld, non-destructive), and the description adds genuinely non-obvious behavior: a 0.01 USDC charge on Base x402 per call and an X-Provider-Key header for customer-owned credentials. That is real cost and auth disclosure an agent cannot infer from structured fields. It does not say what the fingerprint is (hash algorithm, stability across fetches) or how failures surface, which keeps it off a 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?

Two tight sentences with the core purpose front-loaded and cost/auth detail second, with no filler. The trailing 'where needed' is loose and leaves the agent guessing when the header actually applies.

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 read-only tool with annotations and no output schema, the description covers cost and auth but leaves the deliverable underspecified — an agent cannot tell what 'content fingerprint' looks like in the response or how to interpret a reachability failure. Adequate to call, thin for interpreting results.

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 description coverage is 0% for the single 'url' parameter, so the description must carry the burden, and it only implies that 'url' is the 'source' being checked. It adds no format expectations, no statement about redirects or auth-protected URLs, and no indication of whether the URL is fetched live or read from cache.

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?

Names a specific action and resource pair ('check source reachability and content fingerprint') and explicitly bounds it against a nearby misreading ('not independent fact checking'). It stops short of 5 because 'source' is never tied to the single 'url' parameter, and the sibling list contains close neighbors (api_safe_url, api_page_change) that the description does not distinguish itself from, even though fingerprinting overlaps with change detection.

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?

Provides one useful exclusion — this is not fact checking — which tells an agent when the result should not be trusted as verification. There is no positive guidance on when to pick this over api_safe_url or api_page_change, and no mention of prerequisites such as needing a funded wallet before the first call.

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