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

The readOnlyHint annotation already establishes that this is a safe read operation, so the description does not need to re-justify safety. It adds the API endpoint and clarifies that messages mean deliveries, but it does not describe response format, pagination behavior, or any other behavioral nuance beyond the schema.

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?

The description is two short sentences with no filler. The core purpose comes first, and the API endpoint provides precise reference without redundancy.

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 low-complexity read-only tool with fully documented parameters, the description is nearly sufficient. It could mention the shape of the returned list, but the absence of an output schema and the clear endpoint make this 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%, so the input schema fully documents customer_id, limit, and start. The description adds no parameter-level meaning beyond what the schema already provides, which meets the baseline for full schema coverage.

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 description uses a specific verb ('List') and a clear resource ('messages (deliveries) sent to a single person'), which immediately distinguishes it from list-all sibling tools. The endpoint string reinforces the exact scope. This is unambiguous.

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 'sent to a single person' implies the intended use case, and the customer_id requirement reinforces that. However, it does not explicitly contrast with customerio_list_messages or state when to prefer one over the other, so the guidance is implied rather than explicit.

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