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 goes further by disclosing the shape of the response (line items, payments applied, total, total_paid, due), which is genuinely useful given there is no output schema. It does not mention auth needs, rate limits, or error behavior.

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, front-loaded with the action and scope, followed by the returned fields and the underlying endpoint. Every clause carries information; nothing is redundant.

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-record read tool with one fully documented parameter and readOnly annotations, the description is nearly complete, and listing the returned fields compensates for the absent output schema. Only the lack of guidance on failure/not-found behavior keeps it from a 5.

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 there is only one parameter, whose meaning ('The invoice's jnid') is fully documented in the schema. The description repeats the jnid lookup concept without adding syntax, format, or validation detail beyond it, so 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?

The description states a specific verb and resource ('Fetch a single invoice by jnid') and scopes it to one record, which implicitly separates it from the sibling jobnimbus_list_invoices. It never names the sibling explicitly, so differentiation relies on the word 'single'.

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: the agent must have a jnid in hand, which is evident from the required parameter. There is no statement of when to prefer this over jobnimbus_list_invoices or jobnimbus_get_job, and no exclusions or prerequisites are given.

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