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

The description adds meaningful behavioral context beyond the annotations: partial-update semantics, amount unit convention ('MAJOR units'), and the rebilled-expense restriction tied to unlink_rebill. These are non-obvious behaviors that an agent needs to know. No contradiction with the readOnlyHint=false annotation exists.

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?

The description is compact and front-loaded: the core operation is stated first, followed by the two most important caveats. Every sentence adds value, and there is no redundant or filler content.

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?

Given 12 parameters and no output schema, the description covers the most critical operational details but leaves gaps. It does not explain the response format, error behavior, or several parameter formats. The rebilled logic is well covered, but the overall context is not fully complete for a complex update tool.

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 only 25%, so the description carries most of the burden for parameter meaning. It clarifies amount units and explains the relationship between rebilled, unlink_rebill, and editable fields, but it does not clarify date format, note semantics, project, category, merchant, currency, billable, or vat_rate meaning beyond schema types.

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 clearly states the operation: 'Change one expense by id', identifying the exact resource and scope. It also distinguishes this from sibling tools by emphasizing partial updates ('only the fields you pass move'), which separates it from expense_add, expense_delete, and expense_list.

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 implies when to use it: when an existing expense needs to be changed. It also provides a specific conditional guideline about rebilled expenses, explaining when amount, currency, or vat_rate edits are refused. However, it does not explicitly contrast this tool with alternatives like expense_add or expense_delete, so usage guidance is only partially explicit.

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