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

Annotations already declare readOnly/idempotent/no-open-world, so the bar is lower, but the description still adds real operational context: totals and VAT are auto-computed, output is validated, it consumes exactly one metered call, and OAuth sign-in with an InvoiceVerify account is required. It does not say what happens when validation fails (error vs. returned report), which keeps it short of 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 sentences, no filler: the capability and supported formats come first, then cost/auth constraints. Every clause carries information an agent needs before calling.

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 nested-object tool with no output schema, the definition covers input shape, derivation behavior (totals/VAT), validation, cost and auth. The only gap is return shape — whether the agent gets the XML string, a link, or a validation report — which matters because no output schema exists.

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% and the InvoiceData shape is documented inline plus a pointer to /openapi.json, so the schema does the heavy lifting. The description does add a little meaning the enum lacks — mapping 'cii' to Factur-X/ZUGFeRD and noting VAT/totals are derived rather than supplied — but it adds no detail on the format parameter's selection trade-offs.

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 (create/generate) and a precise resource (Peppol BIS 3 UBL, XRechnung UBL/CII, Factur-X/ZUGFeRD CII XML invoice) from structured invoice data. The enumeration of supported formats plus 'totals and VAT computed and the result validated' clearly separates it from siblings like convert_invoice, validate_invoice and parse_invoice.

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?

Implied usage is clear — you have structured invoice data and want standards-compliant XML — and the note about one metered call plus an account requirement gives cost/access context. However, it never explicitly states when to choose convert_invoice (format-to-format) or validate_invoice (validate only) instead, so the agent must infer the boundary.

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