> ## Documentation Index
> Fetch the complete documentation index at: https://developers.benchmarkemail.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Apply a delta mutation to a contact.

> Delta mutation for one existing contact. Accepts any combination of `addTags`, `removeTags`, `addLists`, `removeLists`, and `updateFields`; omitted operations leave the corresponding facet unchanged. Status changes are handled by `PATCH /api/contact/:contactId`, not this endpoint. Operations are atomic — all-or-nothing in a single save. Idempotent: adding a tag already present, removing a tag not present, or setting a field to its current value is a no-op and is rejected with 400 only when *every* operation in the request resolves to no change. `__v` is optional: when supplied, optimistic-concurrency control is enforced (stale version → 400 ConcurrencyError); when omitted, the request runs last-write-wins.



## OpenAPI

````yaml /openapi.json put /api/contact/{contactId}/mutate
openapi: 3.0.0
info:
  description: >-
    Public REST API for Benchmark Email.


    Authenticate every request by including your API key in the `X-API-Key`
    header.

    Each key is issued with one or more scopes that control which endpoints it
    may call.


    Rate limits: 60 requests per minute per account.

    Monthly quota: determined by your subscription plan.
  title: Benchmark Email API
  version: 1.0.0
servers:
  - description: >-
      Your account's API base URL. Copy the full URL shown on Settings > API
      Keys in your Benchmark Email account and paste it here.
    url: '{apiBaseUrl}'
    variables:
      apiBaseUrl:
        default: ''
        description: >-
          Your account's API base URL. Copy it from Settings > API Keys in your
          Benchmark Email account.
security:
  - apiKeyAuth: []
tags:
  - description: Account related endpoints
    name: account
  - description: Contact related endpoints
    name: contact
  - description: Contact structure related endpoints
    name: contact-structure
  - description: List related endpoints
    name: lists
  - description: Email domain related endpoints
    name: email-domain
  - description: Email campaign related endpoints
    name: email-campaign
  - description: Email template related endpoints
    name: email-template
  - description: Report related endpoints
    name: reports
paths:
  /api/contact/{contactId}/mutate:
    put:
      tags:
        - contact
      summary: Apply a delta mutation to a contact.
      description: >-
        Delta mutation for one existing contact. Accepts any combination of
        `addTags`, `removeTags`, `addLists`, `removeLists`, and `updateFields`;
        omitted operations leave the corresponding facet unchanged. Status
        changes are handled by `PATCH /api/contact/:contactId`, not this
        endpoint. Operations are atomic — all-or-nothing in a single save.
        Idempotent: adding a tag already present, removing a tag not present, or
        setting a field to its current value is a no-op and is rejected with 400
        only when *every* operation in the request resolves to no change. `__v`
        is optional: when supplied, optimistic-concurrency control is enforced
        (stale version → 400 ConcurrencyError); when omitted, the request runs
        last-write-wins.
      operationId: put_api_contact_by_contactId_mutate
      parameters:
        - in: path
          name: contactId
          required: true
          schema:
            type: string
      requestBody:
        content:
          application/json: {}
      responses:
        '200':
          description: Successfully mutated
        '400':
          description: >-
            Validation error, no-op rejection, or concurrency conflict (stale
            __v)
        '401':
          description: Missing or invalid bearer token or API key
        '403':
          description: API key lacks the contacts:write scope
        '404':
          description: Contact not found in caller's account
      security:
        - apiKeyAuth: []
components:
  securitySchemes:
    apiKeyAuth:
      description: API key authentication
      in: header
      name: X-API-Key
      type: apiKey

````

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