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

With readOnlyHint=true already declaring this is a safe read, the description's added value is limited to the returned field list and the REST endpoint. It says nothing about not-found behavior, rate limits, or auth requirements, so 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?

Two short sentences, front-loaded with the purpose then the lookup mechanics; no filler and every clause carries information.

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 a read-only single-parameter getter with no output schema, the description usefully enumerates the returned fields and names the endpoint. Minor gap: no mention of error/not-found behavior, but nothing essential to calling it 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 100% and the single 'id' parameter is fully documented in the schema, including that it accepts 'cus_...' or an email. The description merely repeats that, adding no syntax or format detail beyond the schema.

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+resource ('Get a customer's details') and enumerates the fields returned (email, name, phone, saved instruments), plus the lookup key formats. It is clearly distinguishable from the sibling checkout_create_customer.

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?

Explains the two ways to identify the customer ('cus_...' id or email) and cites the underlying endpoint, giving clear context for invocation. It does not state any when-not conditions or explicitly name an alternative tool, so it stops short of a 5.

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