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

Annotations already declare readOnlyHint=false, destructiveHint=true and openWorldHint=true, so the agent knows this is a mutating, potentially destructive write. The description adds the real constraint that only org-defined custom fields are writable, but it does not explain the destructiveHint (e.g. overwriting an existing value) or permission requirements.

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 tightly-worded sentence with the action front-loaded and the helper reference in parentheses. Nothing is wasted and the parenthetical guidance is easy to ignore if not needed.

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 four-parameter mutation with no output schema, the annotations carry the safety profile and the schema covers the key identifier, but the description omits the overwrite/destructive behavior, permission needs, and value-type expectations. Adequate, with clear gaps.

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 50%: 'field' and 'idempotencyKey' are documented in the schema, while 'value' and 'contactId' are bare. The description adds value by specifying that 'field' must be a custom field defined for the organization, but it still leaves 'value' semantics (type handling for numbers/dates) and contactId unaddressed.

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?

The description states a specific verb and resource: 'Write a value into one of the custom fields defined for this organization.' Combined with the tool name, an agent can tell this mutates a field on a contact rather than creating the field definition (create_custom_field). It stops short of explicitly contrasting with update_contact, which also modifies a contact.

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?

It points to list_custom_fields as the way to discover valid fields, which is a useful workflow cue. However, it gives no when-to-use/when-not-to-use guidance relative to update_contact or create_custom_field, so the routing guidance is only implied.

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