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

With only openWorldHint=false and destructiveHint=false in annotations, the description carries meaningful extra burden and delivers: a rejected request returns INVALID_ARGUMENT and writes nothing, plus the rationale for why ssn_last4 cannot be accepted. It does not explain how a given owner is matched for update versus insert, which is the remaining behavioral gap.

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 action is front-loaded in the first sentence and the constraint follows immediately. The final sentence is justificatory but earns its place by explaining the failure model rather than padding.

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?

An output schema exists, so return values need no explanation, and the critical ssn_last4 gotcha is covered. However, for an upsert tool whose owner.id is optional, the description never says what key determines add-versus-update, which is the most important thing an agent needs before calling it.

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?

At 50% schema description coverage across a ~30-field nested owner object, the description compensates for exactly one high-risk field: it tells the agent that ssn_last4 must be omitted. The remaining nested fields (id, kind, verification fields, and how they drive upsert matching) get no added meaning, matching the baseline for partial coverage.

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 states a clear verb+resource: it 'adds or updates one beneficial owner,' and it operationally distinguishes itself by routing SSN-last4 handling to SetOwnerTaxIdentifier. It stops short of differentiating the upsert itself from adjacent sibling operations like banking_set_owner_identity_document_number, so it is clear but not fully sibling-aware.

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 gives one explicit when-not rule with a named alternative: if the owner carries ssn_last4, the call is refused and the agent must use SetOwnerTaxIdentifier instead. That is real routing guidance, though there is no broader statement about when to upsert versus hydrate/refresh flows.

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