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

Annotations only declare readOnlyHint=true; the description adds real behavioral value by enumerating the returned line fields (product, description, quantity, unit price, amount, tax) and the payment associations, which is important since there is no output schema. It omits error behavior (e.g., missing id), auth scope, and rate limits, keeping it below a 5.

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 tight sentences with the payload contents front-loaded and no filler. The trailing 'GET /invoices/{id}' is mildly redundant given the tool name but efficiently encodes the REST mapping.

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 single-parameter read with an annotation covering safety and the return shape described in prose (no output schema), an agent has what it needs to invoke it correctly. Only the absence of error/empty-result behavior keeps it from being fully complete.

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 only one parameter and schema_description_coverage is 100%, so the schema already documents the id and its pattern; the schema even names billy_list_invoices as the source. The description adds no format or constraint detail beyond 'by id', 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 resource (one invoice) and its scope (by id), and enumerates exactly what comes back, so it is clearly distinguishable from billy_list_invoices and billy_get_bill. It stops short of naming a sibling to route against, but the 'one invoice by id' scoping makes the boundary obvious.

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 only implied: 'by id' signals the agent should call this once it holds an invoice id, and the schema points at billy_list_invoices as the source. There is no explicit when-to-use statement, no when-to-prefer-billy_list_invoices guidance, and no mention of prerequisites.

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