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

Annotations only declare destructiveHint=true, so the description usefully clarifies the mutation semantics: an existing live contact is modified in place, and unspecified fields are left untouched rather than cleared. It also pins the REST operation (PUT /contacts/{contactIdOrNumber}). It does not cover whether listIds replace or append, nor any rate-limit/auth context.

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 tight sentences, front-loaded with the mutation warning and the partial-update rule before the API reference. No filler, nothing repeated from the schema fields.

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, the description carries the return-value burden, but an update tool's response matters less than its side effects, which are stated. The remaining gap is the ambiguous additive-vs-replacement behavior of listIds and customFields, which neither the description nor the schema resolves.

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% and each field is self-documenting, so the baseline is 3. The description goes one step further by resolving the key ambiguity of an update call — that omitted fields are not affected — which the schema alone does not convey. It adds no per-parameter syntax detail beyond that.

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?

Names a specific verb (UPDATES) and resource (a live contact), and enumerates the mutable surfaces: profile fields, list memberships, custom fields. This clearly separates it from the read-side siblings (get_contact, list_contacts) without requiring the schema to be opened.

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 instruction 'Provide only the fields you want to change' establishes partial-update usage, which is real guidance. However, it never states when to reach for this tool versus get_contact/list_contacts, nor any precondition (e.g., the contact must already exist, or whether nonexistent ids create a contact).

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