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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds value by disclosing what comes back (lines, taxes, payments), which matters since there is no output schema. It says nothing about behavior for missing IDs or permissions, so it clears the lowered bar without exceeding it.

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, zero filler, with the most useful information (what the response contains) placed first. The raw HTTP path adds little for an agent but is short enough not to be harmful.

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 is the only source on return values, and it only names three payload components at a high level. For a simple single-record read this is minimally adequate, though field-level expectations or not-found behavior are absent.

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 coverage is 100% for the single required 'id' parameter, so the schema already carries the semantics. The 'GET /purchase_invoices/{id}' fragment confirms the id is a path identifier but adds no format or constraint detail beyond 'Purchase invoice id.'

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 pair (fetch one purchase invoice) and enumerates the payload (lines, taxes, payments), which distinguishes it from jurnal_list_purchase_invoices without needing to open either schema. It never names the sibling explicitly, but the singular scope 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?

Usage is only implied by the singular 'One purchase invoice' versus the plural list tools. There is no explicit statement of when to use this over jurnal_list_purchase_invoices, and no prerequisites or error conditions are mentioned.

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