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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds real value beyond that by disclosing the response content — the invoice arrives with its customer and line items embedded — which matters given there is no output schema. It omits error behavior (e.g. what happens for an unknown invoice number) and any auth/prerequisite notes.

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 padding. The purpose and payload are front-loaded before the endpoint detail, which is the right order for a reader scanning for relevance.

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 annotations covering safety, the definition covers purpose, lookup key and returned shape. A note on not-found behavior or on whether payments are included would close the remaining small gap.

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% and the single required parameter is documented with an example ('R-002'), so the description does not need to restate it. The API path template reinforces that invoice_number is the lookup key, but adds no syntax or format detail beyond what the schema already gives. Baseline 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?

States a specific verb and resource ('Get one customer invoice') and names the embedded data ('customer and line items'). The word 'one' plus 'customer' implicitly separates it from runmyaccounts_list_invoices and runmyaccounts_get_vendor_invoice, though neither sibling is called out by name.

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?

Usage is only implied by the singular 'one customer invoice' phrasing. There is no explicit statement of when to use this lookup versus list_invoices (for enumerating) or get_vendor_invoice (for the payable side), leaving the agent to infer the routing.

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