mercury_update_customer

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

Annotations only declare destructiveHint=false and openWorldHint=true, so the description carries the real burden and does: it discloses that the write is persistent, that omitted fields retain current values, and that existing invoices are not retroactively modified. Those are precisely the non-obvious mutation semantics an agent needs and go well beyond the annotation set.

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?

Four labeled blocks (statement, USE WHEN, DO NOT USE, SIDE EFFECTS, RETURNS), each one or two sentences, with the core action front-loaded. No sentence is filler.

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 supplies the return shape ({ id, name, email, address, ... }), adds mutation/persistence semantics, and covers sibling routing. For a 4-parameter nested-object update tool, nothing an agent needs to call it correctly is missing.

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 only 50%, so the description must compensate, and it partly does: it names the updatable surface (name, email, billing address) and the critical partial-update rule that only passed fields change. It does not explain the required customerId or the nested address requirements, but the schema's own nested descriptions cover country code and recipient name, so the gap is modest.

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 ('Update an existing Accounts Receivable customer') and immediately scopes it with 'Pass only the fields you want to change.' It also names the sibling it is not (mercury_delete_customer) and rules out the invoice-change interpretation, so an agent can distinguish it from create/delete/update_invoice without opening a 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?

An explicit USE WHEN block names the amendment scenarios (contact details, billing address) and an explicit DO NOT USE block routes deletes to mercury_delete_customer and invoice reassignment to cancel + recreate. When-to-use, when-not, and alternatives are all present.

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