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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered, but the description adds real value by enumerating the returned line-item fields and, more importantly, the fields deliberately omitted (descriptions, payment history, billing contact). That omission note is genuine behavioral context the annotations do not provide.

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, front-loaded with the return payload before the exclusions, with the endpoint path tacked on last. Every clause carries information.

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 single-parameter read tool with full schema coverage and clear annotations, the description covers what an agent needs, and with no output schema present the run-down of returned and omitted fields is genuinely useful. Only minor gaps remain, such as behavior on an unknown invoice id.

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?

One required parameter (invoice_id) with 100% schema description coverage, so the schema already documents its meaning. The description only implies the id selects a single invoice and adds no format or sourcing detail beyond that, which is the expected baseline when the schema does the work.

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 id'), the fields it carries (line items with date, quantity, amount, tax, plus totals), and the endpoint path. An agent can cleanly separate it from the sibling list_invoices, which returns many.

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 invoice by id' implicitly signals the single-record counterpart to list_invoices, and the explicit exclusion of descriptions, payment history and billing contact tells the agent when this tool will not satisfy a need. It never names list_invoices as the alternative, so the routing is left partly to inference.

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