---
title: "Queue a forward pass"
method: POST
path: "/v1/training/jobs/{job_id}/forward"
tags: ["training"]
---

# Queue a forward pass

`POST /v1/training/jobs/{job_id}/forward`

## Request

- Base URL: `https://api.fastino.ai`
- URL: `https://api.fastino.ai/v1/training/jobs/{job_id}/forward`
- Auth: one of:
  - API key in header `X-API-Key`
  - HTTP bearer

Compute the loss of a batch of token-level datums without gradients (Tinker's `forward`). Read the result at GET /v1/training/jobs/{job_id}/futures/{future_id}.

## Path parameters

- `job_id` string, required

## Headers

- `Idempotency-Key` string, nullable: Client-generated key (a UUID is ideal; it must be URL-safe to look it up) that makes the call safe to send again after a lost response. A repeat with the same key and body from the same caller returns the first call's answer (the same job and future) without running or billing it again, or a 409 while the first call still runs; the same key with another route, job, body or caller is a 422. Look a key up with GET /v1/training/operations/{idempotency_key}.

## Request body

- ForwardBody: ``POST .../forward`` and ``.../forward-backward``: one batch of token-level datums, as in Tinker.
  - `datums` Datum[], required
    - `loss_fn_inputs` object
    - `model_input` ModelInput, required: A token sequence made of text chunks.
      - `chunks` EncodedTextChunk[]
        - `tokens` integer[], required
        - `type` 'encoded_text'
  - `loss_fn` string: cross_entropy, importance_sampling, ppo, cispo or dro.
  - `loss_fn_config` object, nullable

## Response `200`

Successful Response

- FutureAccepted: A queued call: read its result at ``GET /v1/training/jobs/{job_id}/futures/{future_id}``.
  - `future_id` string, required
  - `job_id` string, required
  - `type` string, required: The call: create, load, forward, forward_backward, optim_step, save, sample.

## Other responses

- `422`: Validation Error

## Changes

- **2026-10-17** `81465e18c1d2`: 1 info
  - added the new optional `header` request parameter `Idempotency-Key`
- **2026-10-15** `eef86c1497c1`: 1 info
  - endpoint added

[Operation history](https://skmtc.dev/fastino-ai/apis/pioneer-inference-api/changes/v1/training/jobs/:job_id/forward/post.md)

---

[API](https://skmtc.dev/fastino-ai/apis/pioneer-inference-api.md) - [All operations](https://skmtc.dev/fastino-ai/apis/pioneer-inference-api/llms.txt) - [OpenAPI document](https://skmtc.dev/fastino-ai/apis/pioneer-inference-api/revisions/81465e18c1d2?raw)