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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuine value by disclosing exactly what is returned (line items, VAT, payments, document URL), which matters because there is no output schema; it stops short of noting auth or error behavior.

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 short sentences, zero filler, and the return-content detail is front-loaded before the terse API hint. Nothing is wasted.

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, the description covers the return shape well enough for an agent to know what it will get. Minor gaps: no note on behavior for missing/invalid ids and no signal on nesting or size of the response.

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?

With a single required parameter and 100% schema description coverage, the schema already documents invoice_id fully. The description reinforces that the id is a FastBill invoice id (not an external/number reference), which is a small but real clarification beyond the schema.

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 ('One invoice by FastBill invoice id') and even enumerates the payload the caller gets back (line items, VAT, payments, document URL). It implicitly contrasts with the list_* siblings by emphasizing a single record, though it does not name an alternative 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?

'One invoice by ... id' implies the fetch-by-identifier use case, and 'invoice.get with INVOICE_ID' hints at the underlying call. But there is no explicit guidance on when to prefer this over fastbill_list_invoices or the other invoice tools, and no mention of what happens for an unknown id.

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