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

The readOnlyHint annotation already establishes this is a safe, non-mutating read. The description adds useful framing that 'messages' means delivery records and pins the exact REST endpoint, but says nothing about pagination behavior, result ordering, or rate limits. With annotations carrying the safety profile, this is adequate but not rich.

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, zero filler, with the resource and scope front-loaded and the endpoint reference trailing. Nothing could be removed without losing 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 simple read-only list tool with three fully documented parameters and no output schema, the description covers purpose, scope, and endpoint. The only gap is return-shape and pagination semantics, which are partly handled by the 'start' cursor description in the schema.

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 schema already documents customer_id, limit, and start with their meaning. The description adds no parameter-level detail beyond what is in the schema, which is the expected baseline when the schema does the work.

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 ('List messages (deliveries)') plus a clear scope qualifier ('sent to a single person') and even names the underlying API endpoint. The scope qualifier cleanly separates it from the sibling customerio_list_messages, which lacks the single-person constraint.

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?

Usage is implied by the phrase 'sent to a single person', which tells the agent this requires a customer_id and returns only that person's history. However, it never explicitly states when to prefer this over customerio_list_messages or what happens with pagination across calls.

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