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

The annotation readOnlyHint=true already signals a safe read operation, and the description reinforces this with 'Retrieve' and 'GET'. It adds useful behavioral context by noting the result includes line items, which goes beyond the annotation.

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 short clauses that get straight to the point: what it does and the underlying REST call. No filler words or redundant content. Example of concise, high-signal writing.

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 tool with a readOnlyHint annotation and no output schema, the description provides adequate context: it identifies the resource, the identifier, and notes that the response includes line items. It could go further by hinting at the invoice object structure, but it is sufficiently complete for this simple operation.

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?

The schema has 100% coverage: the only parameter invoice_id is described fully ('The invoice id (required)'). The description adds no additional parameter-level detail beyond confirming the parameter is the invoice ID, which is already in the schema, so baseline 3 is appropriate.

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 uses a specific verb ('Retrieve') and resource ('a single invoice by id'), and explicitly mentions 'including its line items', which distinguishes it from list_invoices and other retrieval tools. It is unambiguous.

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?

The phrase 'single invoice by id' clearly signals when to use this tool (when you have a specific invoice ID and need a single invoice), and the REST endpoint adds context. However, it does not explicitly name alternatives or exclusions, so it falls just short of a 5.

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