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

Annotations already declare readOnly, idempotent, and non-destructive behavior. The description adds valuable behavioral context beyond annotations: it states the invoice must belong to the caller (restricting valid ids) and that line items are included in the result. No contradiction with annotations.

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 a single, front-loaded sentence that immediately states the action and resource. Every piece of information (caller scope, id parameter, line items) earns its place; no filler or redundant 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 simple one-parameter getter with annotations and an output schema, the description is fully adequate. It communicates the essential selection criteria (caller-scoped, single invoice, line items) and does not need to explain return values because an output schema exists.

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 parameter description ('The invoice id.') is straightforward. The tool description adds semantic nuance by implying that invoice_id must reference one of the caller's invoices, which is not stated in the schema. This qualifies as extra meaning 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?

The description uses a specific verb ('Fetch'), identifies the resource ('one of the caller's invoices'), and adds scope ('by id') and content details ('including its line items'). This clearly distinguishes it from list_invoices, get_deal, and other retrieval tools.

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 phrase 'one of the caller's invoices' clearly conveys the access scope, and 'by id' indicates a prerequisite (having the invoice_id). It does not explicitly name alternatives like list_invoices, but the context is clear enough for an agent to know when to use it: when a specific invoice id is known and line items are needed.

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