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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuinely useful context beyond annotations: it discloses the four returned signals including whether the start number remains editable this year, which touches on a mutation boundary the agent otherwise wouldn't know about. Return format/shape is not detailed, keeping it from 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?

A single, front-loaded sentence with zero filler. The parentheticals earn their place by mapping the Spanish billing terminology an agent may encounter in this domain.

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 carries the burden of explaining returns and does so by naming the relevant fields and the changeability flag. It is complete enough to call, though it doesn't state the exact response structure or what happens if no series exists yet.

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?

Zero parameters, so the baseline is 4. The description correctly focuses on return semantics rather than inventing parameters, and the Spanish synonyms ('serie de facturación / numeración', 'prefijo') clarify domain vocabulary.

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 ('Get') and resource ('invoice numbering series') and enumerates what the caller receives: prefix, year, next invoice number, and mutability of the start number. It is clearly distinguishable from get_quote_series and set_invoice_series by the 'invoice'/'current' framing, though it never names those siblings explicitly.

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 intent to read the current series before creating or configuring invoices is implied but never stated, and no alternatives are named (get_invoice_defaults, set_invoice_series). An agent can infer usage but receives no explicit when/when-not guidance.

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