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

Annotations already cover the safety profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false, openWorldHint=false), so the bar is lower. The description still adds genuinely useful behavior: it normalizes http/https, www prefix, and trailing slash before matching, and it discloses the return payload (short URL plus QR code when found). Missing is what happens on a miss, which with no output schema leaves a small gap.

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 short sentences, front-loaded with the core purpose, then matching behavior, then return value. No filler or redundancy; every sentence contributes.

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 lookup with no output schema, the description covers purpose, URL-matching semantics, and the shape of the result. The only omission is the not-found behavior, which is implied ('if found') but not stated explicitly.

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 100% for the single 'url' parameter, so the schema carries the parameter documentation. The description adds matching semantics (variation tolerance) rather than per-parameter meaning, which qualifies as marginal added value over structured data — baseline 3.

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 and resource ('Check whether a URL already has a b59.link short URL') and even names the service domain, so an agent immediately understands this is a lookup rather than a creation. It does not explicitly name the sibling it contrasts with (shorten_url), but the 'already has' framing makes the distinction inferable.

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 — you would call this to de-duplicate before shortening — but the description never states when to prefer this over shorten_url or get_stats_url, nor any preconditions or exclusions. The context is guessable but not spelled out.

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