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

Annotations already declare readOnlyHint=true, idempotentHint=true and openWorldHint=false, but the description adds the key operational fact that no page is fetched ('address-only', 'without opening the page') and lists the detection heuristics, which is real context beyond the structured fields. It stops short of describing the response shape (verdict, score, reasons) or any limits on URL length/volume, so it is not fully transparent.

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 compact sentences plus a routing clause; the 'no fetch' qualifier and the heuristic list are front-loaded before the usage guidance. No filler, no repetition of the title 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?

For a one-parameter read-only checker the definition covers purpose, scope and sibling routing well. The remaining gap is that no output schema exists and the description never hints at what a result contains (verdict vs. score vs. list of flags), which an agent needs to interpret the response.

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% for the single required url parameter, so the schema already carries the syntax burden and baseline 3 applies. The description adds only the mild constraint that the URL is evaluated statically rather than fetched, with no format or normalization details beyond the schema's 'http(s) URL'.

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 and resource ('address-only check' of a URL) and enumerates exactly what it looks for: lookalike domains, brand names as subdomains, punycode, IP hosts, suspicious TLDs. The parenthetical 'without opening the page' plus the explicit contrast with scan_url lets an agent distinguish it from its only sibling without opening either schema.

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 a concrete trigger ('Use before clicking or recommending a link') and names the alternative with the condition that selects it ('For prompt-injection detection use scan_url'). Both when-to-use and when-to-use-something-else are explicit.

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