AddValidateUpdateItemUsingPath answers 200 when it creates nothing — while a write to a missing column is a loud 400

Symptom

Two traps that point in opposite directions, both measured live (SharePoint Online, 2026-09-24, throwaway lists, cleaned up afterwards):

1. The quiet one. An import creates items through AddValidateUpdateItemUsingPath — the endpoint that applies the same validation as SharePoint's forms. One row carries "abc" for a Number column. The call returns HTTP 200; the code counts it as created. The item does not exist.
2. The loud one. A seed runs right after a release that adds a new column, before provisioning has created it. You may have read that SharePoint silently drops unknown Text properties. It does not: every write fails.

Cause

AddValidateUpdateItemUsingPath and ValidateUpdateListItem validate each form value the way a SharePoint form does and return one result per field. A value the field rejects is a validation result, not a request failure, so the HTTP status stays 200 — even when the rejection means the item was never written. A property the list's entity type does not know is different: the request body cannot be bound at all, and that is a 400 before any validation happens.

Fix

Check every field result of a validate call:

const res = await spHttpClient.post(
  `${webUrl}/_api/web/GetList('${listUrl}')/AddValidateUpdateItemUsingPath`,
  SPHttpClient.configurations.v1,
  {
    headers: { 'Accept': 'application/json', 'Content-Type': 'application/json' },
    body: JSON.stringify({
      listItemCreateInfo: { FolderPath: { DecodedUrl: listUrl }, UnderlyingObjectType: 0 },
      formValues: [{ FieldName: 'Amount', FieldValue: value }],
      bNewDocumentUpdate: false
    })
  }
);
if (!res.ok) throw new Error(`HTTP ${res.status}: ${await res.text()}`);
const results: Array<{ FieldName: string; HasException: boolean; ErrorMessage?: string }> = (await res.json()).value || [];
const failed = results.filter(r => r.HasException);
if (failed.length) throw new Error(failed.map(r => `${r.FieldName}: ${r.ErrorMessage}`).join('; '));   // nothing was written

And for plain POST/MERGE writes: never swallow the 400. If a seed reports success while a new column stays empty on every item, a 400 was caught and ignored somewhere — the platform did not drop the value for you.

Notes

• After deploying a schema change, wait for provisioning to finish before seeding, or have the seed re-check the column (GET …/fields/getbyinternalnameortitle('<name>') — a missing field is a 400 there too, not a 404: getbyinternalnameortitle returns 400).
• The same "200 is not success" rule holds for ValidateUpdateListItem on existing items: Created/Modified on a document library.
• Validation does not cover Choice vocabularies — a value outside Choices is stored, 200/201 all the way: Choice fields accept any value.