> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mag3nt.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Add funds to an existing card from treasury balance

> Top up an ACTIVE card by allocating additional USDC from your treasury balance. The allocation is atomic: insufficient balance returns 403.




## OpenAPI

````yaml /openapi.yaml post /api/cards/{id}/fund
openapi: 3.1.0
info:
  title: mag3nt API
  version: '2026-06-04'
  description: >
    Payment infrastructure for AI agents. Issue virtual cards, pay for API
    access via x402/AP2/MPP protocols, and settle in USDC on Base.
  contact:
    name: mag3nt
    url: https://mag3nt.com
  license:
    name: Proprietary
    url: https://github.com/mag3nt-com/mag3nt-node/blob/main/LICENSE
servers:
  - url: https://mag3nt.com
    description: Production
security:
  - ApiKeyAuth: []
tags:
  - name: Payments
    description: Universal outbound protocol payments
  - name: Cards
    description: Virtual payment card lifecycle
  - name: Keys
    description: Developer API key management
  - name: Funding
    description: Treasury deposits and balances
  - name: x402
    description: HTTP 402 payment protocol
  - name: AP2
    description: Agent-to-Agent Payment Protocol
  - name: MPP
    description: Micropayment Protocol with streaming
  - name: Pay Links
    description: Shareable payment URLs
  - name: Withdrawals
    description: Withdraw unspent funds back to wallet
  - name: Settlement
    description: On-chain settlement status
  - name: Webhooks
    description: Signed payment.settled notifications for sellers
  - name: Status
    description: System health and configuration
externalDocs:
  description: mag3nt documentation
  url: https://docs.mag3nt.com
paths:
  /api/cards/{id}/fund:
    post:
      tags:
        - Cards
      summary: Add funds to an existing card from treasury balance
      description: >
        Top up an ACTIVE card by allocating additional USDC from your treasury
        balance. The allocation is atomic: insufficient balance returns 403.
      operationId: cardsTopUp
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required:
                - amount
              properties:
                amount:
                  type:
                    - number
                    - string
                  description: Amount in USDC to add
                  example: 25
      responses:
        '200':
          description: Card topped up
          content:
            application/json:
              schema:
                type: object
                properties:
                  success:
                    type: boolean
                  card_id:
                    type: string
                  added:
                    type:
                      - number
                      - string
                  new_limit:
                    type:
                      - number
                      - string
        '403':
          description: Insufficient treasury balance
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/BalanceError'
        '404':
          description: Card not found
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'
components:
  schemas:
    BalanceError:
      type: object
      required:
        - error
        - available
        - requested
      properties:
        error:
          type: string
          example: Insufficient balance
        available:
          type:
            - number
            - string
          example: 40
        requested:
          type:
            - number
            - string
          example: 50
        network:
          type: string
          example: eip155:8453
        asset:
          type: string
          example: USDC
    Error:
      type: object
      required:
        - error
      properties:
        error:
          type: string
          example: Card not found
  securitySchemes:
    ApiKeyAuth:
      type: apiKey
      in: header
      name: Authorization
      description: API key prefixed with 'Bearer sx_live_...'

````