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

Goes beyond the annotations by disclosing real side effects: linked reminders are flipped to paid, a per-tool confirmation prompt is on by default, and the user's API key is required. These are meaningful operational facts the readOnly/destructive hints do not convey.

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?

Front-loaded and mostly purposeful, but it repeats the core action twice ('Book a payment received on an invoice' / 'Books a payment on an invoice') and mixes in tangential notes about confirmation settings and the Cotlet key, diluting the core instruction.

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 mutating, open-world tool the description covers side effects, confirmation behavior, and auth requirements, which is close to complete. Without an output schema it could say more about what a successful booking returns, but nothing critical for invocation is missing.

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%, so the schema already documents the defaults for amount and paid_on. The description restates the amount default and adds the partial-payment use case, which is helpful but largely duplicates structured data; 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 (book a payment) and resource (invoice) with the payment account and date context, and the title 'Mark invoice as paid' clarifies the net effect. It is distinguishable from the sibling sevdesk_mark_voucher_paid via the invoice/voucher resource, though the description never explicitly contrasts them.

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?

Gives useful context (default amount equals the open total, pass a smaller amount for partial payment) but does not state when to prefer this over alternatives such as create_payment_reminder or a manual payment entry. Usage is implied rather than routed.

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