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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description usefully discloses that line items are included in the response and gives the underlying endpoint, but says nothing about missing-id behavior (404), permissions, or response shape beyond 'its lines'.

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 very short sentences, front-loaded with the resource and return content, with the endpoint as supplementary context. Nothing is wasted.

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 carries some return-value burden and does state that lines are included, which is the key expectation for a single-bill fetch. It is nearly complete for a one-parameter read, with only error/edge behavior left unstated.

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% and the single id parameter is fully documented, including its origin ('from billy_list_bills'). The description adds no format or constraint detail beyond what the schema already provides, so the baseline 3 applies.

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 ('One supplier bill by id') and adds the return scope ('with its lines'), so an agent knows this is a single-record fetch rather than a list. It doesn't explicitly name siblings, but 'get' vs 'list' and 'bill' vs 'invoice' are clear enough from the name and description.

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: fetch one bill when you have its id, as opposed to enumerating. There is no explicit when-to-use/when-not guidance or a named alternative such as billy_list_bills in the description itself (that reference lives in the schema).

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