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

Annotations already cover readOnly/idempotent/openWorld/non-destructive, but the description adds real value: the per-call quota cost (1 of 1,000/month) and, critically, the caveat that these are snapshot observations that do not prove live links. That reliability caveat is exactly the kind of context an agent cannot infer from structured fields.

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 purpose and return shape before the cost and caveat. Nothing is wasted, though the return-shape sentence overlaps somewhat with the existing output 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 read-only lookup with an output schema and full annotation coverage, the description supplies everything an agent needs: what is queried, what comes back, what it costs, and how much to trust the result. Return values are already covered by the output schema.

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%, so domain, sort, limit and release_id are all documented in the schema and the baseline is 3. The description's mention of 'host count' and 'authority score' loosely echoes the sort enum values but adds no syntax or format detail beyond the schema.

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 ('Look up referring domains (backlinks) for a single target domain') plus the data source (Common Crawl webgraph). The 'single target domain' scope also implicitly separates it from multi-domain siblings like gap_analysis and from the time-series sibling backlink_changes.

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 'single target domain' scoping and the explicit quota cost, which tells the agent this is a metered call worth budgeting. However, it never states when to choose this over backlink_changes or gap_analysis, and there are no explicit exclusions.

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