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Create a custom field

create_custom_field

Create a custom field in a Kit account to track subscriber data beyond default fields. Changes account configuration and supports bulk JSON payloads.

Instructions

Create a custom field. Changes account configuration.

Input Schema

NameRequiredDescriptionDefault
labelNo
accountNoNamed private account from KIT_ACCOUNTS. Defaults to KIT_DEFAULT_ACCOUNT or the first configured account.
payloadNoComplete JSON body instead of individual body flags. Supports nullable fields and nested bulk structures. Cannot be combined with body flags or payload_file.
payload_fileNoLocal JSON request body file, at most 5 MB. Contents are validated before the API call and are never logged.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv3.0.0
    • removedInput schema / properties / confirm
      Removed value: -{
-  "description": "Must be true for audience/delivery changes, publishing, destructive operations and signing-secret changes. Use only for an action requested by the user.",
-  "type": "boolean"
-}
  2. First observedv2.0.1

TDQS

B3.1/5.0
Behavior3/5

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

Annotations already declare readOnlyHint=false, openWorldHint=true, idempotentHint=false and destructiveHint=false, so the safety profile is covered. "Changes account configuration" adds a modest hint that this mutates account-wide settings, but says nothing about permissions, validation failures, or what happens on duplicate labels.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two short, front-loaded sentences with no padding. The second sentence is thin and largely restates the non-read-only nature already implied by annotations, but nothing is verbose.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a creation tool with no output schema and a nested payload parameter, the description omits what the call returns, whether the label must be unique, and any account-scoping caveats. It is under-specified rather than merely concise.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 75% and the nested payload object carries its own detailed description (mutual exclusivity with body flags and payload_file). The description itself contributes zero parameter semantics, but the schema does the heavy lifting, so a baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ("Create a custom field"), so the core action is unambiguous. However, it does nothing to distinguish itself from the close siblings bulk_create_custom_fields, update_custom_field, and delete_custom_field, and the trailing sentence adds no purpose information.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

No when-to-use guidance, no prerequisites, and no mention of the obvious alternative bulk_create_custom_fields for creating multiple fields at once. The agent must infer the choice entirely from the tool names.

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

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