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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=false, and destructiveHint=false, so the safety profile is fully covered. The description adds the underlying Fortnox endpoint (GET /3/invoices/{DocumentNumber}), which helps an agent map fields, but it doesn't add return shape, error behavior, or lookup ambiguity details beyond the annotations and output schema.

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 sentences, front-loaded with the action and resource, followed by a compact API reference. No wasted words.

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 an output schema present and rich annotations, the description doesn't need to explain return values. It sufficiently identifies the tool and its lookup key. It could be slightly more complete by distinguishing itself from PDF and document-variant siblings, but nothing critical is missing.

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 there is only one parameter. The description repeats the DocumentNumber placeholder via the API path, adding only minor value over the schema's 'identifies the invoice'. Baseline 3 is appropriate when the schema already documents the parameter.

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+resource combination ('Retrieve a single invoice') so the agent can distinguish it from list_invoices or find_customer_invoices. However, it doesn't differentiate it from close siblings like get_invoice_pdf or read_invoice_document, which also target a single invoice record.

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: you call it when you have a DocumentNumber for one invoice. There is no explicit when-to-use vs when-to-use-something-else guidance, and no mention of alternatives such as list_invoices for discovery or get_invoice_pdf for the PDF form.

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