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

Annotations declare readOnlyHint=true, so safety is already covered; the description still adds useful behavior: which fields come back (name, email, tax-exempt status) and the backing endpoint GET /tax/customers/{customer_id}. No mention of error behavior when an id is not found, which is the main residual gap for a lookup tool.

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?

Three short sentences, front-loaded with the core action before the id-space nuance. Slightly redundant with the schema on the default-id statement and the trailing HTTP endpoint is marginal, but nothing is wasteful enough to hurt.

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, and the description compensates by naming the returned fields. Input semantics for both id spaces are fully covered. Only omission is failure/not-found behavior, a minor gap for a simple read.

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 both parameters are already documented in the schema, including the enum for id_type and the cus_ prefix. The description restates the same default/override semantics rather than adding new syntax or constraints, so baseline 3 is appropriate.

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 ('Get a customer by id') and immediately clarifies the dual lookup semantics, which no sibling (numeral_create_customer, numeral_get_product, etc.) shares. An agent can distinguish this from other get_* tools 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?

Explains the default path (Numeral cus_ id) and the alternate condition that selects the other path (set id_type to "reference" to use your own reference_customer_id). Clear context for both modes, but no explicit when-not guidance or named alternatives among siblings.

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