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

The readOnlyHint annotation already establishes the safety profile, so the description only needs to add context. It does disclose the returned data shape (status, due date, totals) and the underlying GET /invoices/{id} call, which is useful, but it says nothing about auth requirements, not-found behavior, or whether credit notes are fetched through the same endpoint.

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 compact sentence front-loads the operation and the key returned fields, with the endpoint reference appended as a useful footnote. No filler, nothing redundant.

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 compensates by naming the fields the agent will receive, and the include hint covers the embedding option. It is nearly complete for a simple read tool; only the credit-note distinction and pagination/embedding caveats 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 description coverage is 100%, so both parameters are already documented in the schema, including the enum list for include. The description's mention of include:invoice_items slightly reinforces the schema but adds no syntax or semantics beyond it, making the baseline 3 appropriate.

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 and resource ("Fetch one invoice or credit note") and enumerates the returned facets (status, due date, totals). The word "one" distinguishes it from the sibling list_invoices, though the sibling is never named outright.

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?

"Fetch one invoice" implies a single-record lookup versus the list tool, and the hint to "include invoice_items for the lines" suggests when to populate the include parameter. However, there is no explicit when-to-use/when-not guidance and no alternative tool named, so usage is only implied.

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