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

Annotations already declare readOnlyHint=true, openWorldHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds real behavior beyond that: it follows redirects, returns final_url, and explains that favicon_source distinguishes a page-declared icon from the /favicon.ico guess.

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?

Three tight sentences, front-loaded with what is extracted, then redirect behavior, then the sibling routing hint. Every sentence carries distinct information with no filler.

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 correctly compensates by naming the return fields (final_url, favicon_source) and the redirect-following behavior. For a one-parameter read tool with rich annotations, 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?

There is a single parameter and schema description coverage is 100%, so the schema already documents the url param fully; the description's phrasing mirrors the schema text rather than adding format/validation detail. Baseline 3 is appropriate when the schema does the work.

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?

The description gives a specific verb (fetches/extracts) and a concrete resource list (title, description, canonical URL, Open Graph/Twitter Card tags, favicon) and frames it as 'the data behind link previews', which immediately separates it from the sibling favicon tool that deals only with icons.

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?

It explicitly routes the agent to redirect_trace when per-hop redirect detail is needed, which is a clear when-to-use-this-instead signal. It does not similarly differentiate from favicon or security_headers, so the guidance is solid but not exhaustive.

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