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

Annotations only carry destructiveHint=true, which already tells the agent this is a mutating operation; the description reinforces it with 'WRITE' and adds the endpoint POST /invoice/{invoiceId}. It also usefully clarifies 'all fields optional', implying a partial merge. It does not disclose what happens to omitted fields, whether status transitions are validated, or permission requirements — meaningful gaps for a 15-parameter mutation.

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 tight sentences with the WRITE marker front-loaded and the endpoint appended, so the agent's eye lands on the operation type first. Slightly terse for a 15-parameter tool but no wasted words.

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?

With 15 parameters, one required, nested objects, and no output schema, the description is minimal but workable because the schema covers all fields at 100%. It could say more about merge semantics and status-transition rules given there are no enums to constrain status values.

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 100%, so every parameter is already documented in the schema; the baseline is 3. The description adds only the '(all fields optional)' note and example status values, which marginally overlaps the schema's own status description rather than extending it.

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 and resource ('updates an existing invoice/bill') and frames it as a WRITE, which cleanly separates it from mercoa_get_invoice and mercoa_find_invoices. It does not explicitly name a sibling to route against or note that it differs from mercoa_create_invoice, so it stops short of a 5.

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?

'Commonly used to advance status (e.g. DRAFT→NEW→SCHEDULED) or change amounts, dates, or payment methods' gives a concrete primary scenario, which is stronger than nothing. However, there are no when-not conditions and no alternatives named (e.g. use create_invoice for new records), leaving the agent to infer selection.

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