> ## Documentation Index > Fetch the complete documentation index at: https://docs.together.ai/llms.txt > Use this file to discover all available pages before exploring further. # Custom forward-backward pass > Submits a forward-backward pass driven by externally computed gradients of the loss with respect to per-token log-probabilities. ## OpenAPI ````yaml openapi.yaml POST /rl/training-sessions/{session_id}/operations/custom-forward-backward openapi: 3.1.0 info: title: Together APIs description: The Together REST API. See https://docs.together.ai for more details. version: 2.0.0 termsOfService: https://www.together.ai/terms-of-service contact: name: Together Support url: https://www.together.ai/contact license: name: MIT url: https://github.com/togethercomputer/openapi/blob/main/LICENSE servers: - url: https://api.together.ai/v1 description: Default environment for APIs - url: https://api-inference.together.ai/v2 description: Optimized environment for inference security: - bearerAuth: [] paths: /rl/training-sessions/{session_id}/operations/custom-forward-backward: post: tags: - RL summary: Custom forward-backward pass description: >- Submits a forward-backward pass driven by externally computed gradients of the loss with respect to per-token log-probabilities. operationId: customForwardBackward parameters: - name: session_id in: path required: true schema: description: Training session ID type: string - name: Idempotency-Key in: header required: true schema: description: >- Required key that makes retries return the original operation; use a new key for changed request bodies. type: string requestBody: content: application/json: schema: $ref: '#/components/schemas/RL.CustomForwardBackwardBody' required: true responses: '200': description: '' content: application/json: schema: $ref: '#/components/schemas/RL.CustomForwardBackwardOperation' default: description: An unexpected error response. content: application/json: schema: $ref: '#/components/schemas/ErrorData' components: schemas: RL.CustomForwardBackwardBody: type: object description: Request body for a custom forward-backward pass. required: - samples - gradients properties: samples: type: array description: Batch of training samples items: $ref: '#/components/schemas/RL.TrainingSample' gradients: type: array description: >- Per-sample per-token gradients of the loss with respect to log-probabilities items: $ref: '#/components/schemas/RL.TargetLogprobGradients' RL.CustomForwardBackwardOperation: type: object description: Async custom forward-backward pass operation required: - id - status properties: id: type: string example: 550e8400-e29b-41d4-a716-446655440000 description: Operation ID status: $ref: '#/components/schemas/RL.TrainingOperationStatus' example: TRAINING_OPERATION_STATUS_PENDING description: Operation status output: $ref: '#/components/schemas/RL.CustomForwardBackwardResult' description: Result on success error: $ref: '#/components/schemas/RL.TrainingOperationError' description: Error details on failure ErrorData: type: object required: - error properties: error: type: object properties: message: type: string nullable: false type: type: string nullable: false param: type: string nullable: true default: null code: type: string nullable: true default: null required: - type - message RL.TrainingSample: type: object required: - model_input - loss_fn_inputs properties: model_input: $ref: '#/components/schemas/RL.ModelInput' description: Model input loss_fn_inputs: type: object description: >- Per-token loss tensors keyed by name. Include `target_tokens` and the inputs required by the selected loss. Each tensor must declare `int64` or `float32`, be one-dimensional, and have the same length. additionalProperties: $ref: '#/components/schemas/RL.TensorData' routed_experts_key: type: string description: >- Opaque key returned with a sampled sequence. Pass it unchanged with the corresponding training sample to reuse the same expert selections. The selections must cover the entire training sample or all but its final token. Training fails if the key is no longer available. minLength: 1 maxLength: 256 pattern: ^routing/[A-Za-z0-9._-]+/[0-9a-f]{32}\.[1-9][0-9]*$ RL.TargetLogprobGradients: type: object description: Per-token gradients of the loss with respect to target log-probabilities required: - data properties: data: type: array description: Float array of per-token gradients (d loss / d log p) example: - -0.1 - 0.05 - -0.08 - 0.12 - -0.03 items: type: number dtype: $ref: '#/components/schemas/RL.DType' example: D_TYPE_FLOAT32 description: Data type of the float array RL.TrainingOperationStatus: type: string enum: - TRAINING_OPERATION_STATUS_UNSPECIFIED - TRAINING_OPERATION_STATUS_PENDING - TRAINING_OPERATION_STATUS_RUNNING - TRAINING_OPERATION_STATUS_COMPLETED - TRAINING_OPERATION_STATUS_FAILED default: TRAINING_OPERATION_STATUS_UNSPECIFIED RL.CustomForwardBackwardResult: type: object description: Result of a custom forward-backward pass operation properties: {} additionalProperties: false x-stainless-empty-object: true RL.TrainingOperationError: type: object description: Error details for a failed training operation properties: code: $ref: '#/components/schemas/RL.TrainingOperationErrorCode' description: Application error code example: TRAINING_OPERATION_ERROR_CODE_TIMEOUT message: description: Human-readable error message type: string example: Operation timed out RL.ModelInput: type: object required: - chunks properties: chunks: description: Input chunks for the model type: array items: $ref: '#/components/schemas/RL.ModelInputChunk' type: object RL.TensorData: type: object description: A tensor encoded as flattened row-major values, with an optional shape. required: - data - dtype properties: data: type: array description: Flattened one-dimensional values encoded as JSON numbers. example: - 1 - 2 - 3 items: type: number format: double dtype: type: string enum: - int64 - float32 description: Tensor element type, either `int64` or `float32`. example: int64 shape: type: array description: >- Optional tensor shape; training operations accept one-dimensional tensors only, and the dimension must match the data length. example: - 3 items: type: integer sparse_crow_indices: type: array description: Unsupported for training operations. Omit this field. example: - 0 - 2 items: type: integer sparse_col_indices: type: array description: Unsupported for training operations. Omit this field. example: - 0 - 2 items: type: integer RL.DType: type: string enum: - D_TYPE_UNSPECIFIED - D_TYPE_INT64 - D_TYPE_FLOAT32 - D_TYPE_BFLOAT16 default: D_TYPE_UNSPECIFIED RL.TrainingOperationErrorCode: type: string description: Application error code for a failed training operation enum: - TRAINING_OPERATION_ERROR_CODE_UNSPECIFIED - TRAINING_OPERATION_ERROR_CODE_RESOURCE_EXHAUSTED - TRAINING_OPERATION_ERROR_CODE_TIMEOUT - TRAINING_OPERATION_ERROR_CODE_INTERNAL_ERROR - TRAINING_OPERATION_ERROR_CODE_SESSION_NOT_ACTIVE - TRAINING_OPERATION_ERROR_CODE_INVALID_INPUT - TRAINING_OPERATION_ERROR_CODE_NON_FINITE_LOSS default: TRAINING_OPERATION_ERROR_CODE_UNSPECIFIED RL.ModelInputChunk: type: object description: A single chunk of model input content. required: - encoded_text properties: encoded_text: $ref: '#/components/schemas/RL.EncodedTextChunk' description: Pre-tokenized text content for this input chunk. RL.EncodedTextChunk: type: object description: Pre-tokenized text content for a model input chunk. required: - tokens properties: tokens: description: Pre-tokenized text input type: array example: - 123 - 456 - 789 items: oneOf: - type: string - type: integer securitySchemes: bearerAuth: type: http scheme: bearer x-bearer-format: bearer x-default: default ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.