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

Annotations only cover the safety profile (write, non-destructive, closed-world). The description adds three concrete facts beyond them: slug-match dedup returns the existing list with created=false, a per-call cost of $0.05, and an API key requirement. This is rich behavioral disclosure an agent cannot get from annotations.

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?

Three short sentences, zero waste, with the core action front-loaded and the dedup caveat following. Information-dense without padding.

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?

An output schema exists, so return values need not be explained, yet the description still surfaces the meaningful created=false flag. Combined with cost, auth, and dedup behavior, nothing needed to call this tool correctly is missing.

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 name, request_id, and idempotency_key. The description's dedup note relates to the idempotency parameters but adds no syntax or format detail beyond the schema. Baseline 3 applies.

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 (create) and resource (contact list) with the keying mechanism (by name). An agent can immediately distinguish this from list_contact_lists (read) and add_people_to_list (membership mutation) without opening any 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 idempotency note ('a repeat creates nothing') implies correct repeat-call behavior, which is useful context, but there is no explicit when-to-use or when-to-prefer-an-alternative guidance relative to siblings like list_contact_lists. Usage is only implied by the tool's role.

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