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

Annotations already declare destructiveHint=true and idempotentHint=true, and the description adds valuable context beyond them: it marks the operation as WRITE, identifies the matching key, and warns that custom field values are overwritten. It still omits details such as auth requirements or all mutation side effects, so it is not fully exhaustive.

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 a single front-loaded sentence that begins with 'WRITE' and includes the key behavior, overwrite warning, and endpoint. Every element 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?

The definition covers the core upsert semantics and destructive overwrite behavior, which is important for a mutation tool with rich annotations. However, it leaves a significant gap: the schema requires only list_id, and the description does not state that at least one of email or phone is needed to identify or create the contact.

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%, and the description adds meaning for the least-documented parameters: email/phone act as the existence-matching pair, and custom field values are overwritten. It does not fully clarify whether at least one identifier is required, but it meaningfully supplements the schema.

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 states a specific verb/resource pair ('add or update a contact') and scope ('to a list'), plus the upsert condition. It is clear operationally, but it does not explicitly distinguish this from the sibling sarbacane_create_contact tool.

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 explains the upsert condition: update when the same email/phone pair already exists. However, it gives no explicit guidance on when to choose this over sarbacane_create_contact or other contact tools, and no exclusions or prerequisites are stated.

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