> ## Documentation Index > Fetch the complete documentation index at: https://docs.0xmonaco.com/llms.txt > Use this file to discover all available pages before exploring further. # Post apiv1marginsub accounts > Open a sub-account: another margin account of the caller, keyed by `subaccount_key`, with its own collateral and its own risk buckets. The default margin account (key `default`) is the only one deposits and withdrawals touch; fund a sub-account with `TransferCollateral`. Re-creating the key of a closed sub-account re-opens the same account. Keys starting with `copy:` are reserved. An owner may hold a limited number of open sub-accounts, set by their weighted 14-day volume tier (10 at the first tier, up to 50) or an admin override; a create past it is refused. Copy-trading sub-accounts do not count. ## OpenAPI ````yaml /proto-openapi/api/openapi.yaml post /api/v1/margin/sub-accounts openapi: 3.0.3 info: title: Monaco Protocol API description: REST API for the Monaco Protocol hybrid CLOB exchange. version: 1.0.0 servers: - url: https://staging.apimonaco.xyz description: Staging server (Testnet) security: [] tags: - name: AccountsService - name: ApplicationsService - name: AuthService - name: BuildercodeRewardsService - name: CopyTradingService - name: DelegatedAgentsService - name: FaucetService - name: FeesService - name: HealthService - name: ManagedMarketsService description: |- Wallet-authenticated maker assignments. The signed session must match both configured maker user and application. Owner IDs are process fences, never auth. Only wallet sessions are supported; delegated-agent sessions receive 403. - name: MarginAccountsService description: |- Current public isolated-margin semantics: - a user has one parent margin account per application scope - opening orders create or reuse isolated position buckets under that parent - parent account creation is handled internally by margin workflows - name: MarketService - name: OrderbookService - name: OrdersService - name: PositionsService description: |- Current public isolated-margin semantics: - positions link to a parent margin account and, when applicable, a bucket id - opening orders create or reuse isolated position buckets under the parent - users can open another isolated position by reusing the parent account with a different market bucket - name: PulseService - name: SweeperService - name: TraderCodeService - name: TradesService - name: WhitelistService - name: WithdrawalsService paths: /api/v1/margin/sub-accounts: post: tags: - MarginAccountsService description: |- Open a sub-account: another margin account of the caller, keyed by `subaccount_key`, with its own collateral and its own risk buckets. The default margin account (key `default`) is the only one deposits and withdrawals touch; fund a sub-account with `TransferCollateral`. Re-creating the key of a closed sub-account re-opens the same account. Keys starting with `copy:` are reserved. An owner may hold a limited number of open sub-accounts, set by their weighted 14-day volume tier (10 at the first tier, up to 50) or an admin override; a create past it is refused. Copy-trading sub-accounts do not count. operationId: create_sub_account requestBody: content: application/json: schema: $ref: '#/components/schemas/CreateSubAccountRequest' required: true responses: '200': description: OK content: application/json: schema: $ref: '#/components/schemas/CreateSubAccountResponse' '429': description: >- Movement rate limit exceeded — withdrawals and collateral transfers share a per-user budget; retry after the interval in details.retryAfter security: - monacoSignature: [] - monacoHttpSignature: [] components: schemas: CreateSubAccountRequest: type: object properties: subaccountKey: type: string description: >- Sub-account key: up to 64 letters, digits, `_`, `-`, `.` or `:`. Keys starting with `copy:` are reserved, and `default` names the default margin account. nullable: true label: type: string description: Optional display label. nullable: true CreateSubAccountResponse: type: object properties: marginAccountId: type: string nullable: true subaccountKey: type: string nullable: true marginAccountState: type: string description: 'Values: ACTIVE.' nullable: true label: type: string nullable: true securitySchemes: monacoSignature: type: apiKey description: >- Ed25519 session-key request signing. Every authenticated request carries three headers: `X-Monaco-PublicKey` (64-char lowercase-hex session public key), `X-Monaco-Timestamp` (Unix milliseconds, within 30s of server time), and `X-Monaco-Signature` (hex ed25519 signature). The signature is over `METHOD\npath?query\ntimestamp_ms\nSHA256_hex(body)`, where the body hash is the SHA-256 of the empty byte string when there is no body. Obtain the session keypair from `POST /api/v1/auth/challenge` followed by `POST /api/v1/auth/verify`. name: X-Monaco-Signature in: header monacoHttpSignature: type: apiKey description: >- RFC 9421 Ed25519 session-key request signatures. Send Signature-Input, Signature, and Content-Digest together; never combine them with X-Monaco-* credentials. The monaco signature covers @method, @path, @query, and content-digest in that order, with created (Unix seconds within 30s of server time), keyid (registered lowercase-hex session public key), and alg=ed25519 parameters. Content-Digest uses RFC 9530 sha-256 over the exact body bytes, including the empty body. Query coverage retains ordering and percent encoding. Timestamp freshness does not reject repeated identical requests; use endpoint idempotency where supported. Existing legacy signing remains supported during server-first migration; SDK RFC 9421 signing is opt-in. See https://docs.0xmonaco.com/reference/http-message-signatures for the complete profile and trust model. name: Signature in: header ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.