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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the description's main added value is disclosing the shape of the response: items, taxes, PO and receipt matching, and payments. It also usefully notes that the same endpoint serves credit notes, which an agent would otherwise not assume. It omits error/not-found behavior, which keeps 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 compact fragments with the resource and IDN scoping front-loaded, and no filler. The trailing 'GET /invoices/{idn}' duplicates information already implied by the tool name, a minor redundancy, but overall the text is tight and scannable.

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 a single fully documented parameter, no output schema, and annotations covering safety, the description does the one thing that matters: it summarizes what the agent will actually receive (line items, taxes, matched POs/receipts, payments). A note on lookup failure or on the credit-note IDN namespace would make it 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?

Schema description coverage is 100% and the schema already explains that IDN is the company's own document number rather than the internal id. The description only restates 'by IDN' without adding format, source, or failure semantics. Baseline 3 is correct when the schema does the explanatory work.

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 ('One supplier invoice or credit note by IDN') and enumerates the payload contents (items, taxes, PO/receipt matching, payments). The 'One ... by IDN' framing implicitly separates it from precoro_list_invoices, though the sibling is never named. The added 'GET /invoices/{idn}' endpoint line anchors the resource unambiguously.

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: an agent can infer you call this when you already hold a document IDN and need detail, versus precoro_list_invoices for browsing. There is no explicit when-to-use/when-not statement, no mention of behavior when the IDN is unknown, and no guidance on obtaining an IDN in the first place.

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