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

Annotations already declare readOnlyHint=false, idempotentHint=false and destructiveHint=false, so the safety/write profile is covered. The description reinforces this with "WRITE" and "Adds a new record" and adds the sibling routing. However, it never says whether adding a duplicate email fails, creates a second record, or is silently merged — the single most important behavioral question for a non-idempotent create.

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, front-loaded with the WRITE marker, then the input requirement, then the sibling routing, then the endpoint. Nothing is redundant with the title and no sentence is wasted.

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?

There is no output schema, so the description is the only place a caller could learn what a successful add returns (new contact id? echo of fields?), and it is silent on that. For a 4-parameter write with a nested fields object, the input side is adequately covered but the outcome side is not.

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 only 50%, and the description fills the critical gap by stating that email, phone, or both may be supplied — this explains why neither is required in the schema. It also flags fields as optional custom values and points to sarbacane_list_list_fields for ids. It does not restate email/phone formats, but the patterns already cover those.

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 ("add a contact to a contact list") and even the underlying endpoint POST /v1/lists/{listId}/contacts. It explicitly distinguishes itself from sarbacane_upsert_contact, so an agent can pick between the two without opening a 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?

Names the alternative explicitly ("use sarbacane_upsert_contact to update an existing contact"), which is the main routing decision here. It also states the minimum input requirement (email, phone, or both). It stops short of saying what happens when the contact already exists, which is the remaining ambiguity in when-to-use.

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