infer

Call the in-flight model from inside a checkpoint callback.

Inside onCheckpoint, the SDK hands you a function called infer. Calling it runs an inference request against the just-saved checkpoint and returns a raw Response. This is the path that lets you evaluate a half-trained model before the full run finishes.

onCheckpoint: async ({ step, infer }) => {
  const res = await infer({
    messages: [{ role: "user", content: "I can't log in." }],
  });
  console.log(`step=${step}`, await res.text());
}

The default response is an SSE stream (the same shape Studio's Playground consumes). Pass stream: false if you want a single JSON body instead:

const res = await infer({ messages, stream: false });
const data = await res.json();

infer is only available on CheckpointContext. There is no top-level export of it; the callback argument scopes the call to the right job and step automatically.

Common scenarios

• Sanity check. Compare step-50 and step-100 outputs against a fixed prompt.
• Constrain output to a JSON schema. Pass responseFormat: { type: "json_schema", json_schema: { name, schema, strict: true } }; the response body's choices[0].message.content is a JSON string you can JSON.parse into a typed object. See the Structured outputs and function calling recipe.
• Call a function. Pass tools + toolChoice so the model can request a tool call from inside the checkpoint check. Same recipe as above.
• Custom early-stopping. Combine with abortSignal + cancel() to stop a run that has gone off the rails. See the Early stopping recipe.
• Live preview. Forward the checkpoint output to Slack or your own review queue.

Reference

For the full InferArgs shape, the streaming-vs-JSON tradeoffs, the SSE frame format, the constraints on retargeting, and pointers for decoding the SSE delta stream, see the infer reference.