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

Annotations already cover readOnly, idempotent, non-destructive and closed-world, so the description is not obligated to restate safety. It does add real domain context beyond the annotations: the distinction between this invoice's amount and job_total for progress invoices, which tells the agent how to interpret partial billing. Missing error/not-found behavior keeps it from a 5.

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 tight sentences with no filler, and the returned-field list is front-loaded so the agent learns the payload shape immediately. The opening is a bare noun phrase with no verb anchoring it, which is the only structural weakness.

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 one-parameter read tool with no output schema, the description effectively stands in for the missing return definition by naming the fields delivered and clarifying the ambiguous amount/job_total semantics. An agent has everything needed to call it correctly and interpret the result.

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 a single parameter whose description already says invoice_id comes from list_invoices or get_job. The description adds no further syntax, format, or validation detail for invoice_id, so the baseline 3 applies — the schema does the heavy lifting.

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 enumerates exactly what a single invoice record includes (customer, amount, amount due, due terms, sent/paid timestamps), which is a clear resource statement and implicitly contrasts with the plural sibling list_invoices. It never uses an explicit verb like 'retrieve', but the intent is unmistakable.

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: 'One invoice' signals a single-record lookup rather than a listing, and the schema note points at list_invoices/get_job as the id source. There is no explicit when-to-use or when-not statement, and no mention of the sibling send_invoice or record_invoice_payment as follow-up paths.

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