Backend Contract

The default JSON transport sends model values keyed by resolved mappedTo targets:

{
  "inputs": {
    "feature_key": "serialized value"
  }
}

It expects any parseable response. The recommended response is:

{
  "reports": {
    "report_key": {
      "value": "model output"
    }
  },
  "meta": {
    "model": "version"
  }
}

Use request.displayValues for review/export data keyed by displayKey; fields without displayKey are omitted. Use request.serializedValues or request.modelValues for backend/model data keyed by mappedTo. Field id remains a runtime handle for UI state.

Use createSubmissionSnapshot(form, options) when an app needs the same records for review, persistence, or export before submit.

Use createMultiBackendSubmissionSnapshot(form, { backends }) when one visible form feeds several models with different mappedTo keys. Use executeMultiBackendPipeline({ form, backends }) when one user action should submit each backend and keep per-backend results, report fetch outputs, errors, skipped reports, and report contexts. createFanoutTransport is still transport-level fanout; it does not resolve schema mappings per backend.

Use createJsonTransport({ body }) when the backend needs a different request shape:

const transport = createJsonTransport({
  endpoint: "/api/predict",
  body(request) {
    return JSON.stringify({
      values: request.modelValues,
      reports: request.reports.map((report) => report.mappedTo),
    });
  },
});

Use createJsonTransport({ parse }) when the backend returns text, nested JSON, or a streaming gateway result.

Legacy outputs responses are compatibility behavior only. New backends should return reports.