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

Annotations already declare destructiveHint=true, idempotentHint=false, and readOnlyHint=false, so the safety profile is largely carried by structured data. The description adds the partial-update constraint, which is real behavioral value, but says nothing about what destruction means here (overwriting prior values) or that repeated calls may not be idempotent. Given annotation coverage, a 3 is appropriate.

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?

Three short sentences with the action front-loaded, then the ID lookup, then the update rule. Every sentence carries distinct information and none restates the title.

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 an 11-parameter destructive mutation with no output schema, the description covers the essential invocation path (find ID, send only changed fields) but omits the consequences of the write — value overwriting, null-clearing of linked fields — that an agent would want before calling a tool flagged destructive. Adequate but with clear gaps.

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 earns an extra point by stating PATCH-style semantics — only changed fields are passed and the rest are left untouched — which the schema's required-only-id shape implies but never states. It does not, however, call out the null-to-clear convention for notes, vendor, invoiceId, customerId, and paymentMethod.

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 opens with a specific verb+resource ('Update an existing expense'), which cleanly separates it from create_expense, delete_expense, list_expenses, and fetch_expense. It does not explicitly name those siblings as alternatives, but the purpose is unambiguous 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?

It supplies both a prerequisite workflow ('Use list_expenses to find the expense ID first') and a mutation rule ('Pass only the fields that need to change'). What it lacks is any statement of when not to use it (e.g., for full replacement) or permission/implication guidance for a destructive write.

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