> ## Documentation Index > Fetch the complete documentation index at: https://docs.basaltic.sh/llms.txt > Use this file to discover all available pages before exploring further. # Initiate a multipart upload > Stage an object as up to 10000 parts and assemble them in one step. Every part but the last must be at least 5 MiB, and no part may exceed 5 GiB — so an upload's parts can hold far more than any object a caller would send through it. The assembled object is written out in full when the upload is completed, and that write happens inside the completing request: budget for it at roughly 85 MiB/s, and note that the public edge ends any request at one hour. Objects beyond a few hundred GiB need a completion path this API does not offer yet. ## OpenAPI ````yaml /api-reference/specs/storage.yaml post /v1/buckets/{bucket}/multipart-uploads openapi: 3.0.3 info: title: Basaltic Storage API version: 1.0.0 description: | Block volumes with their snapshots and snapshot policies, plus bucket management for object storage. The S3 wire protocol itself is served separately at `objects.{region}.basaltic.cloud`. contact: name: Basaltic Support email: ping@basaltic.sh license: name: Proprietary url: https://basaltic.sh/terms servers: - url: https://storage.{region}.basaltic.sh description: Regional API endpoint variables: region: default: sa-saopaulo-1 description: Region code security: - SignatureAuth: [] paths: /v1/buckets/{bucket}/multipart-uploads: parameters: - name: bucket in: path required: true schema: type: string example: my-app-assets post: tags: - Buckets summary: Initiate a multipart upload description: | Stage an object as up to 10000 parts and assemble them in one step. Every part but the last must be at least 5 MiB, and no part may exceed 5 GiB — so an upload's parts can hold far more than any object a caller would send through it. The assembled object is written out in full when the upload is completed, and that write happens inside the completing request: budget for it at roughly 85 MiB/s, and note that the public edge ends any request at one hour. Objects beyond a few hundred GiB need a completion path this API does not offer yet. operationId: initiateMultipartUpload parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/InitiateMultipartUploadRequest' responses: '201': description: Multipart upload initiated. content: application/json: schema: $ref: '#/components/schemas/MultipartUploadResponse' '400': description: Invalid request body, empty key, or unsupported storage class. '401': $ref: '#/components/responses/Unauthorized' '403': description: Caller lacks the required IAM permission. '404': description: No such bucket. '409': $ref: '#/components/responses/Conflict' '422': $ref: '#/components/responses/UnprocessableEntity' security: - SignatureAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header description: >- Optional client-generated key that makes a create replay-safe. Retrying a request with the same key returns the original outcome verbatim instead of creating a duplicate resource. Reusing a key with a different request body is rejected (422); a request whose key is still being processed returns 409. Records are honored for 24 hours. Use a UUID or similarly unique token. required: false schema: type: string maxLength: 255 example: 550e8400-e29b-41d4-a716-446655440000 schemas: InitiateMultipartUploadRequest: type: object required: - key properties: key: type: string example: videos/clip.mp4 content_type: type: string example: video/mp4 storage_class: type: string example: STANDARD metadata: type: object additionalProperties: type: string MultipartUploadResponse: type: object required: - upload properties: upload: $ref: '#/components/schemas/MultipartUpload' MultipartUpload: type: object required: - upload_id - bucket - key - created_at properties: upload_id: type: string example: 2~tHc6Lc1fQy0Q bucket: type: string example: my-app-assets key: type: string example: videos/clip.mp4 content_type: type: string example: video/mp4 storage_class: type: string example: STANDARD metadata: type: object additionalProperties: type: string created_at: type: string format: date-time example: '2026-01-15T09:30:00Z' Error: type: object required: - error properties: error: type: object required: - code - message - request_id properties: code: type: string description: Error code identifying the type of error example: INVALID_INPUT message: type: string description: Human-readable error message example: Invalid request parameters request_id: type: string format: uuid description: Request ID for debugging example: 550e8400-e29b-41d4-a716-446655440000 responses: Unauthorized: description: Authentication required or token invalid content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: UNAUTHORIZED message: Authentication required request_id: 550e8400-e29b-41d4-a716-446655440000 Conflict: description: Resource conflict (e.g., already exists, invalid state) content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: CONFLICT message: Resource with this name already exists request_id: 550e8400-e29b-41d4-a716-446655440000 UnprocessableEntity: description: | The request is well-formed but cannot be processed as sent. On the operations that accept `Idempotency-Key` this is the key-reuse case: the key was first seen with a different request payload, so replaying the stored outcome would answer a question the caller did not ask. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: code: IDEMPOTENCY_KEY_REUSED message: >- This Idempotency-Key was already used with a different request payload request_id: 550e8400-e29b-41d4-a716-446655440000 securitySchemes: SignatureAuth: type: apiKey in: header name: Authorization description: > Request signing with an access key issued to a service account. An HMAC-SHA256 over a canonical form of the request, close to AWS SigV4. The `basaltic` CLI signs for you. Send `Authorization`, `X-Date` (UTC, `YYYYMMDDTHHMMSSZ`) and `X-Nonce` (random per request); add `X-Content-Sha256` to bind a body, and `X-Amz-Security-Token` when using temporary credentials. ``` Authorization: BASALTIC-HMAC-SHA256 Credential=///basaltic/basaltic_request, SignedHeaders=host;x-date;x-nonce, Signature= ``` `` is the region code you are calling, or `global` for the global services. A signature is valid for 5 minutes from `X-Date`, and mutating requests are replay-guarded on the nonce. **Full signing procedure, including a working implementation:** https://docs.basaltic.sh/authentication ## Rate limits There is no global request budget. A limit applies only where an operation documents a `429`, and that operation says what it counts. Those responses carry `X-RateLimit-Limit`, `X-RateLimit-Remaining`, `X-RateLimit-Reset` and, on a `429`, `Retry-After` — read them rather than hard-coding a number. Retrying before `Retry-After` is refused and extends the window. Everything else is bounded by quota, not by request rate. ````