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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description still adds real value by specifying the breadth of the returned payload, including linked invoices, which tells the agent this is a heavyweight read rather than a thin summary. It stops short of noting any auth, rate-limit, or error behavior (e.g., what happens with an unknown ID).

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 sentences, front-loaded with the core action and scope, followed by return contents and usage triggers. No filler, no restatement of the title, and every clause carries information an agent can act on.

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 one-parameter, read-only tool with full annotation coverage and no output schema, the description supplies everything needed: what it returns, when to call it, and the read-before-update pattern. Nothing an agent needs in order to invoke 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 100% and the single parameter already documents itself as "Customer ID (from list_customers)", including provenance. The description's phrase "by ID" adds no format, validation, or fallback information beyond the schema, so the baseline of 3 applies.

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 first sentence gives a specific verb and resource ("Fetch full details of a single customer by ID") and immediately scopes it to one record rather than a list, which separates it from list-style siblings. It also enumerates the returned fields (name, email, phone, address, tax ID, website, linked invoices), so an agent knows exactly what it gets back without opening any 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?

It states two concrete triggers: as a read-before-write step prior to updating a customer, and when the user asks about a specific customer's details. That is clear positive guidance, but there is no explicit exclusion (e.g., when to prefer a list or search tool for partial/unknown IDs).

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