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

Annotations already establish readOnlyHint=true and destructiveHint=false, and the description builds on that by detailing the returned state (open balance, sent method, registered payments, credit notes) plus a concrete auth prerequisite (Cotlet API key with access to this app). Nothing contradicts 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?

Front-loaded with the core purpose and the routing advice, but the second sentence's long enumeration of return fields is somewhat run-on and largely restates the first sentence's summary.

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, so the description must carry the return contract — and it does, listing lines, totals, open balance, sent state, payments and credit notes. Combined with the auth note and sibling routing, an agent has everything needed to call it 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?

With one parameter at 100% schema coverage, the schema already documents invoice_id and its source ('from list_invoices or create_invoice'). The description references 'one invoice' but adds no format, range, or lookup detail beyond the schema, so the 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+resource ('Get invoice' / 'Returns one invoice') and enumerates exactly what that invoice contains (lines, net, VAT, gross, open balance, sent state, payments, credit notes). An agent can tell it apart from fiken_list_invoices and fiken_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?

Gives explicit when-to-use with named siblings and the condition that selects them: read before register_invoice_payment so a short payment is registered as partial, and before create_reminder so a paid invoice isn't chased. This is exactly the routing guidance the dimension asks for.

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