Search REST API needs odata-version: 3.0

Symptom

Calls to /_api/search/query fail with an unhelpful 500 Internal Server Error — typically from SPFx (SPHttpClient) or another modern HTTP client, while the very same query works when pasted into the browser's address bar.

Cause

The search endpoint still speaks OData 3.0. Modern clients — SPFx SPHttpClient in particular — send odata-version: 4.0 by default, and the search service fails hard on it instead of degrading gracefully.

Fix

Send the header explicitly on every /_api/search/* call:

const res = await this.context.spHttpClient.get(
  `${webUrl}/_api/search/query?querytext='${encodeURIComponent(query)}'`,
  SPHttpClient.configurations.v1,
  {
    headers: {
      'odata-version': '3.0',
      'Accept': 'application/json;odata=nometadata'
    }
  }
);

Plain fetch or any other client: same idea — add odata-version: 3.0.

Notes

• POST /_api/search/postquery needs the same header.
• Diagnostic shortcut: if search calls 500 while the rest of /_api works fine, check this header first — it is almost always the answer.