create_custom_fieldCreate a custom field to store additional contact-specific data in SendGrid. Specify the field name and type—Text, Number, or Date—to extend your contact management.
Create a new custom field for contacts
| Name | Required | Description | Default |
|---|---|---|---|
| name | Yes | Name of the custom field | |
| field_type | Yes | Type of the field |
Changes observed during successful MCP inspections.
v1.1.1Input schema / additionalPropertiesRemoved value: -falsev1.0.0Input schema / $schemaAdded value: +"http://json-schema.org/draft-07/schema#"Input schema / additionalPropertiesAdded value: +falseDoes the description disclose side effects, auth requirements, rate limits, or destructive behavior?
The annotations already declare this is a write operation (readOnlyHint=false), not destructive, and not idempotent. The description adds the 'for contacts' scope but no extra behavioral context such as uniqueness, side effects on existing contacts, or permission requirements. It does not contradict the annotations, so a middle score is appropriate.
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?
The description is a single front-loaded sentence that conveys action, resource, and scope with no redundant wording. Every word earns its place.
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 low-complexity two-parameter create with full schema coverage and annotations describing the write/non-destructive/non-idempotent nature, the description plus structured data is enough for correct invocation. It does not document return values or duplicate handling, but these are not essential for a basic create call.
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%: the schema documents both the required name and field_type, including the allowed enum values Text/Number/Date. The description adds no additional parameter semantics, so the baseline for fully covered schema applies.
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 names a specific action ('Create'), a specific resource ('custom field'), and a scope ('for contacts'). This clearly distinguishes it from sibling tools like update_custom_field, delete_custom_field, and list_custom_fields, and from create_contact, without requiring schema inspection.
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 about when this tool should be chosen over the sibling custom-field tools, nor any note about prerequisites such as whether custom fields can be created once, are optional, or affect contact forms. The context is implied by the action verb, but no explicit when/when-not guidance is given.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.