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

Annotations already declare readOnlyHint=true and openWorldHint=true, and the description reinforces this with 'Read-only; nothing is created, sent or changed.' It adds valuable behavioral context beyond the annotations: each invoice carries its own currency, results are paginated at 100 per page, and the response echoes appliedFilters. It does not contradict the annotations. A small gap is that it doesn't describe the exact response shape, but the description carries substantial behavioral context.

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?

The description is dense but well-organized: it front-loads the core purpose, then filters, then read-only guarantee, then currency/pagination/appliedFilters caveats, then the sibling pointer. Every sentence earns its place, though it is slightly long and could be tightened without losing meaning.

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 list tool with 8 optional parameters, no required parameters, and no output schema, the description covers the key operational caveats: pagination, currency independence, appliedFilters echo, and the sibling for detail. It doesn't describe the exact return fields, but the absence of an output schema and the presence of a sibling detail tool make this acceptable. The description is complete enough for an agent to call it correctly.

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 the schema already documents all 8 parameters. The description adds meaning beyond the schema by clarifying the reference/referenceContains mutual exclusivity ('use exactly one, never both'), the inclusive date range semantics, and the pageCount caveat. It doesn't restate every parameter, but it adds the cross-parameter constraints that the schema alone doesn't express.

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 states a specific verb ('List') and resource ('invoices from Xero for the current organisation'), and immediately distinguishes the two invoice types (ACCREC sales vs ACCPAY bills). It also names the sibling get_xero_invoice for full line-item detail, so an agent can tell this list tool apart from the detail tool without opening either schema.

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 description gives explicit when-to-use guidance: use get_xero_invoice for full line-item detail on one invoice, and it explains the optional filters (status, date range, reference) with clear semantics. It also warns about pagination ('if pageCount shows more pages than were returned, say so') and about relaying appliedFilters, which is actionable usage guidance beyond the schema.

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