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Bulk update subscriber custom field values

bulk_update_subscriber_custom_field_values
Destructive

Update custom field values for multiple subscribers in one call. Set confirm=true to apply, since changes can affect delivery or audience membership.

Instructions

Bulk update subscriber custom field values. May affect delivery, audience membership, published data or irreversible state. Requires confirm=true. Requires OAuth; API keys are not supported for this endpoint.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
accountNoNamed private account from KIT_ACCOUNTS. Defaults to KIT_DEFAULT_ACCOUNT or the first configured account.
confirmNoSet true only when the user asked for exactly this action.
payloadNoComplete JSON body instead of individual body flags. Supports nullable fields and nested bulk structures. Cannot be combined with body flags or payload_file.
callback_urlNo
payload_fileNoLocal JSON request body file, at most 5 MB. Contents are validated before the API call and are never logged.
custom_field_valuesNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed8 schema fields changedv3.0.0
    • addedInput schema / $defs
      Added value: +{
+  "custom_field_values": {
+    "items": {
+      "additionalProperties": false,
+      "properties": {
+        "subscriber_custom_field_id": {
+          "type": "integer"
+        },
+        "subscriber_id": {
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "value": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "subscriber_id",
+        "subscriber_custom_field_id",
+        "value"
+      ],
+      "type": "object"
+    },
+    "type": "array"
+  }
+}
    • changedInput schema / properties / confirm / description
      Previous value: -"Must be true for audience/delivery changes, publishing, destructive operations and signing-secret changes. Use only for an action requested by the user."New value: +"Set true only when the user asked for exactly this action."
    • addedInput schema / properties / custom_field_values / $ref
      Added value: +"#/$defs/custom_field_values"
    • removedInput schema / properties / custom_field_values / items
      Removed value: -{
-  "additionalProperties": false,
-  "properties": {
-    "subscriber_custom_field_id": {
-      "type": "integer"
-    },
-    "subscriber_id": {
-      "type": [
-        "integer",
-        "null"
-      ]
-    },
-    "value": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "subscriber_id",
-    "subscriber_custom_field_id",
-    "value"
-  ],
-  "type": "object"
-}
    • removedInput schema / properties / custom_field_values / type
      Removed value: -"array"
    • addedInput schema / properties / payload / properties / custom_field_values / $ref
      Added value: +"#/$defs/custom_field_values"
    • removedInput schema / properties / payload / properties / custom_field_values / items
      Removed value: -{
-  "additionalProperties": false,
-  "properties": {
-    "subscriber_custom_field_id": {
-      "type": "integer"
-    },
-    "subscriber_id": {
-      "type": [
-        "integer",
-        "null"
-      ]
-    },
-    "value": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "subscriber_id",
-    "subscriber_custom_field_id",
-    "value"
-  ],
-  "type": "object"
-}
    • removedInput schema / properties / payload / properties / custom_field_values / type
      Removed value: -"array"
  2. First observedv2.0.1

TDQS

A3.7/5.0
Behavior4/5

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

Annotations already declare destructiveHint=true and idempotentHint=false, but the description adds genuinely new context: effects on delivery, audience membership and published data, the confirm=true guard, and the OAuth/API-key auth constraint. This is meaningful disclosure beyond the structured fields.

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?

Four short sentences, front-loaded with the operation and immediately following with the risky-effects warning and hard prerequisites. No filler or repetition.

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

Completeness4/5

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

For a nested bulk mutation with no output schema, the description covers the critical unknowns: required confirmation, auth restriction, and potential side effects. Remaining gaps (pagination/return shape, callback_url behavior) are minor since no output schema exists.

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 coverage is 67%, so most parameters are documented in the schema. The description restates the confirm requirement but adds no detail about how payload, payload_file and body flags interact, nor about callback_url semantics. 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 (bulk update) and resource (subscriber custom field values), which cleanly separates it from update_custom_field and bulk_create_custom_fields. However, it does not explicitly name the sibling it is not, so differentiation is inferred rather than stated.

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

Usage Guidelines3/5

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

The description supplies strong prerequisites (confirm=true, OAuth-only, possible irreversible effects), which imply this is a high-friction write operation. It never states when to use this versus alternatives like update_subscriber or a single-field update, so the when-to-use guidance remains implicit.

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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