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

Annotations already declare readOnlyHint=false, idempotentHint=true, and destructiveHint=false, so the safety profile is largely covered structurally. The description adds the useful scope detail that subscription state is mutable here (an overlap with suppression/audience tools), but says nothing about partial-update behavior, permission needs, or side effects on existing fields.

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?

A single front-loaded sentence with no filler; the editable scope is stated immediately and nothing redundant is included.

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 and a nested customFields object, the description covers the field families but omits merge/partial-update semantics and the shape of customFields. Given annotations carry the safety profile, this is adequate-but-incomplete rather than inadequate.

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 only 33% (contactId and apiKey), leaving firstName, lastName, subscribed, and customFields undescribed in the schema. The description names the field categories generically — profile fields, custom fields, subscription state — which hints at the mappings, but does not clarify the customFields key/value contract or that omitted fields are left unchanged. It only partially compensates for the coverage gap.

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?

Clear specific verb+resource ('Update a contact') with an enumeration of the three editable field families (profile, custom, subscription state). It is readily distinguishable from create_contact and delete_contact by name and intent, though it does not explicitly name a sibling. No tautology.

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?

There is no guidance on when to use this tool versus alternatives such as add_contacts_to_audience, add_suppression, or remove_contact_from_audience, and no prerequisites or preconditions are stated. The agent must infer everything from the tool name.

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