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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds real value on top by enumerating the response contents (seller, buyer, currency, line items with net/gross/VAT, total, paid amount), which is important because no output schema exists. It stops short of noting behavior for a missing/invalid invoice id.

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, zero filler, with the returned payload front-loaded before the endpoint reference. Every clause earns its place by telling the agent what comes back.

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 no output schema, describing the returned fields covers the main gap an agent would face. Only the failure mode (nonexistent invoice id) and the source of valid ids are absent, which is a minor omission.

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 single parameter's description ('Invoice id.') already carries its meaning, so a 3 baseline applies. The description adds no format hints, ID provenance, or constraints beyond what the schema states.

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?

The description names the resource (a single invoice) and enumerates the fields returned, so the agent knows exactly what it retrieves. It implicitly separates itself from sellasist_list_invoices by saying 'One invoice', though it never names the sibling explicitly. The trailing 'GET /invoices/{invoice_id}' pins the underlying operation.

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?

There is no statement of when to use this instead of sellasist_list_invoices, or that an invoice_id must first be obtained from that list tool. Usage is only inferable from the singular phrasing and the required id parameter. No prerequisites, no exclusions.

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