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

Annotations already indicate this is a mutating, non-idempotent operation taking the safety burden down. The description adds behavioral context beyond annotations: the invoice must already exist, one currency group must be handled at a time, and the tool returns what was marked. It does not describe reversal or side effects, but the annotations plus schema cover the main traits.

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 compact sentences front-load the purpose and key prerequisite, then give a terse usage pattern and mention the return value. Every clause contributes meaning without elaboration or redundancy.

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?

The description, combined with the fully described schema, covers the tool's purpose, input modes, precondition, grouping constraints, invoice_number's role, and return value. No critical calling information appears missing even though there is 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 coverage is 100%, so the baseline is 3. The description adds value by explaining the two parameter usage patterns: the ids array from expense_to_invoice versus the project/from/to/currency group, and it explicitly associates currency with a single invoice group. This exceeds what the schema alone conveys.

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 action ('Mark expenses as rebilled') and ties it to a precise precondition ('once the invoice that carries them actually exists'). It also distinguishes its two input modes, which separates it from related tools like expense_to_invoice and expense_update.

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?

The description clearly states when to use the tool and how to select the input mode: pass expense_ids from expense_to_invoice, or pass project/date range/currency. It does not explicitly mention when not to use alternatives, but the relationship to expense_to_invoice and the billing workflow is clear enough.

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