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

Annotations already declare destructiveHint=true, idempotentHint=false and a write operation, so the description adds rather than repeats: it discloses that the call may silently UPDATE an existing contact and that the vendor decides which path is taken. That is meaningful behavioral context for a non-idempotent, destructive write. It stops short of stating permission requirements for the API key or any rate limits.

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 compact sentences, front-loaded with the WRITE marker and the core action, then constraints. The trailing endpoint string is mildly redundant for an agent but does not dilute the message.

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 mutation tool with no output schema, the description covers the critical unknowns: that it is a write, that it can mutate an existing contact, that no email is dispatched, and the consent precondition. Missing only return shape (e.g., resulting contact id) and how duplicates/errors surface.

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 baseline is 3. The description reinforces the consent semantics behind the permission flag but adds no syntax or format detail beyond what the schema already documents for the 7 parameters, including the nested custom_fields object.

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+resource ("add one contact to a list") with the WRITE marker and even the underlying endpoint. It also discloses the upsert twist, which helps separate it from pure-create tools, though it never names benchmark_update_contact as the sibling an agent might confuse it with.

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?

Gives a real eligibility condition ("Only add people who agreed to get email") and rules out a side effect ("does not send any email"), but provides no when-to-use vs. benchmark_update_contact/unsubscribe_contact routing guidance. Usage is implied rather than contrasted with alternatives.

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