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

readOnlyHint=true already tells the agent this is a safe read, so the description's job is lighter. It adds useful context by disclosing the shape of the returned object (items, taxes, payments, permalink, pdf) and the underlying endpoint, but says nothing about not-found behavior or error semantics.

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 sentences with zero filler, and the core action plus scope is front-loaded ahead of the return-payload and endpoint detail.

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 one-parameter read tool with annotations covering safety, this is nearly complete: the return contents are enumerated even though no output schema exists. Only retrieval failure behavior is left unstated, which is a minor gap at this complexity.

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?

The schema documents the single parameter at 100% coverage ('The resource id.'), so the baseline is 3. The description's 'by id' confirms it is the invoice id rather than any generic resource id, but adds no format, prefix, or sourcing detail beyond that.

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 (Fetch) and resource (a single invoice by id), and the word 'single' separates it from the sibling quaderno_list_invoices without needing to name it. It also enumerates what the returned object contains, so the agent knows exactly what it gets back.

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 id' signals a retrieval-after-you-know-the-id pattern, and 'full object' hints it is the place to get details. It never states when to prefer this over quaderno_list_invoices or quaderno_request, and gives no preconditions or exclusions.

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