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

readOnlyHint=true already declares the safety profile, so the bar is lower. The description adds the meaningful union semantics ('own any of the given phone numbers'), but says nothing about pagination, result limits, or what happens when no match exists; the endpoint string is implementation detail, not behavior.

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?

Single front-loaded sentence stating purpose plus a short scoping clause, then the backend endpoint. It is tight, though the 'Shopmonkey: POST /v3/...' tail is implementation leakage that adds little for an agent.

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?

Only two parameters, 100% schema coverage, no output schema, and a read-only annotation – so the description need not explain returns. It supplies the key semantic ('any of the numbers') and enough context to call correctly; only alternative-routing guidance 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 description coverage is 100% and both parameters (phoneNumbers, excludeCustomerId) are documented in the schema, so the schema does the heavy lifting. The description adds no format or matching semantics beyond what the schema already states, which is the baseline 3 case.

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 (Find) plus resource (customers) and the discriminator (by phone number 'that own any of the given phone numbers'). It is clear what the tool returns, but it never names or contrasts against close siblings like find_customers_by_email or search_customers, so an agent still infers the boundary from the tool name.

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 phrase 'the usual way to identify a caller' implies a caller-lookup scenario, but there is no explicit when-to-use/when-not, no prerequisite, and no routing to the email or general-search alternatives present in the sibling list. Usage is implied rather than guided.

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