Unknown key for a – How to solve this OpenSearch exception

Opster Team

Aug-23, Version: 1-1.1

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Briefly, this error occurs when OpenSearch encounters an unrecognized key in a configuration file or a query. This could be due to a typo, incorrect syntax, or using a key that doesn’t exist in the current version of OpenSearch. To resolve this issue, you should first verify the key in question against the OpenSearch documentation. Ensure that the key is spelled correctly, is in the correct format, and is appropriate for the version of OpenSearch you are using. If the key is deprecated, replace it with the current equivalent.

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This guide will help you check for common problems that cause the log ” Unknown key for a ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: search.

Log Context

Log “Unknown key for a” class name is FetchSourceContext.java. We extracted the following from OpenSearch source code for those seeking an in-depth context :

 List includesList = new ArrayList<>();
 while ((token = parser.nextToken()) != XContentParser.Token.END_ARRAY) {
 if (token == XContentParser.Token.VALUE_STRING) {
 } else {
 throw new ParsingException(parser.getTokenLocation(); "Unknown key for a " + token
 + " in [" + currentFieldName + "]."; parser.getTokenLocation());
 includes = includesList.toArray(new String[includesList.size()]);
 } else if (EXCLUDES_FIELD.match(currentFieldName; parser.getDeprecationHandler())) {


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