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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, covering the safety profile. The description adds useful context: it scopes to the caller's own invoices and promises line items in the response. It does not mention error handling or auth specifics, but with annotations present, the added detail is sufficient.

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, front-loaded sentence that states the verb, resource, and key details (caller scope, line items) with zero waste. It is immediately scannable and gives the agent exactly what it needs to decide.

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 single-parameter read tool with annotations covering safety and no output schema, the description fully covers what the tool does, what it returns, and its scope. There is no missing information that would prevent an agent from calling 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 coverage is 100%, with the schema describing invoice_id as 'The invoice id.' The description adds no extra meaning about the parameter format, validation, or usage beyond 'by id'. Since the schema already documents the parameter adequately, the baseline of 3 applies.

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 ('invoices by id'), and notes it includes line items. This clearly distinguishes it from list_invoices (which returns multiple invoices) and get_invoice_aging (which focuses on aging data). The scope 'caller's invoices' further clarifies what is returned.

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 implies usage for a single invoice retrieval when an id is available, and the mention of line items suggests a need for detailed data. However, it does not explicitly contrast with sibling tools like list_invoices or state when not to use this tool. There is no explicit routing or alternative guidance, leaving the agent to infer based on context.

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