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

Annotations cover the safety profile (not read-only, open world, not destructive), and the description adds real value: it discloses the accounting side effect ('changes your books and what your accountant sees'), the confirmation policy, and the required Cotlet API key access. No contradiction with the annotations.

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?

Core purpose is front-loaded, but the first two sentences are largely redundant ('Mark an invoice payment as paid' versus 'Updates the payment of an invoice to paid'), and the confirmation/API-key boilerplate is verbose. Adequate but not tight.

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 mutation tool with no output schema, it covers the essentials: side effects, confirmation behavior, and auth requirements, so an agent can invoke it correctly. Return values are only gestured at ('returns the updated payments'), which is acceptable given no output schema.

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%, so the schema already documents all four parameters, including that invoice_id comes from list_invoices/create_invoice. The description restates the date and payment account at a high level but adds no format or constraint detail beyond the schema, 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?

States a specific verb (mark/update to paid) and resource (invoice payment), plus the key inputs (date, payment account). It is distinguishable from app siblings like get_invoice, list_invoices, and create_invoice without opening the schema.

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?

Provides process context ('confirm each call', 'needs the Cotlet API key') but never states when to use this versus alternative flows or what the prerequisites for the invoice state are. Usage is implied rather than explicitly framed against alternatives.

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