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

Annotations already flag destructiveHint=true, idempotentHint=false, and openWorldHint=true, but the description adds behavior those hints cannot convey: implicit contact creation, a confirmation-email gate for lists that require it, membership only after confirmation, and a forced confirmation email for previously unsubscribed people. It stops short of saying what happens when the person is already on the list, which matters given idempotentHint=false.

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?

Four short sentences, front-loaded with the core action and followed by the conditional behaviors, with no filler. The final consent sentence is arguably policy rather than mechanics, but it is brief and earns its place.

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?

With no output schema and only partial parameter documentation, the description covers the important behavioral branches (creation, confirmation, unsubscribed contacts) but leaves gaps an agent needs: the response shape, and the effect of adding someone already on the list, which is a live concern given idempotentHint=false.

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 50%: list_id and email_address are documented, while first_name and last_name have no description anywhere. The description adds essentially no parameter meaning beyond the schema — it hints that contact creation may use the person's identity but never explains how the optional name fields are applied.

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 opening sentence states a specific verb (adds), resource (one person / a list), and a key side effect (creating the contact if needed). That combination clearly separates it from contact-oriented siblings such as tag_contact, update_contact, or unsubscribe_contact, which operate on different resources or attributes.

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 description supplies a consent rule ("Only add people who have agreed to hear from the account") and explains the double opt-in consequence, which is real usage guidance. However, it never names an alternative tool or states when to prefer a different path (e.g., tagging an existing contact instead of adding them to a list), so the when-not guidance is only implied.

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