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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds practical context on top: the exact shape of the payload returned, that amounts are in the smallest currency unit, and that a Cotlet API key with app access is required. It does not discuss pagination or rate limits, so not 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?

Four short sentences, front-loaded with the resource and return contents, then usage context, then auth. No filler, though the trailing API-key sentence is generic boilerplate that only marginally earns its place.

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?

No output schema exists, yet the description compensates by enumerating the returned fields and their currency-unit convention. Combined with the pre-credit-note usage rule and the auth requirement, an agent has everything needed to invoke this read-only tool 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 description coverage is 100% and the single invoice_id parameter already documents its origin and example (INV-1042). The description adds nothing about the parameter, so the baseline of 3 applies when the schema does all 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?

The description names a specific verb+resource (get one invoice) and enumerates what it returns — line items, payments, credits, amount due, status. It implicitly distinguishes itself from chargebee_list_invoices through 'One invoice', though it never names the sibling for contrast, which keeps it just short of a 5.

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?

It gives a concrete trigger condition: read this before creating a credit note, because the status determines which credit note type is allowed. That is real routing guidance, but no alternative-tool path is named (e.g., when to prefer list_invoices), so it is clear context rather than full when/when-not coverage.

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