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

readOnlyHint=true already establishes this is a safe, non-mutating read. The description adds genuine value beyond that by enumerating the returned content (contact details, emails, phone numbers, vehicle/order/appointment counts), which matters since there is no output schema. It omits error behavior for an unknown id, keeping it out of 5 territory.

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?

One front-loaded sentence carrying the key, the returned payload, and the underlying endpoint — every clause earns its place with no filler.

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 single-parameter read tool with no output schema, naming the fields it returns is what the agent needs and the annotations already cover safety. It lacks any note on not-found behavior, which is a minor gap rather than a blocking one.

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% for the single customerId parameter, and the description only restates it as 'by id' with no format, prefix, or lookup semantics. Baseline 3 applies when the schema fully documents the input.

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 (fetch) and resource (a single customer) plus the lookup key (id) and the endpoint. 'By id' distinguishes it from the sibling search_customers, find_customers_by_email, and find_customers_by_phone, so an agent can route correctly 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?

The 'by id' framing makes the selection condition clear: use this when you already have a customer id, versus the search/find siblings when you don't. It never explicitly names those alternatives or states an exclusion, 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.