elastic/elasticsearch · error · IllegalArgumentException

No encoder found for media type [{}]

Error message

No encoder found for media type [{}]

What it means

Thrown by CustomMustacheFactory.createEncoder() when the requested media type (content type) is not in the ENCODERS map. The factory supports exactly five media types: 'application/json; charset=UTF-8', 'application/json;charset=utf-8', 'application/json', 'text/plain', and 'application/x-www-form-urlencoded'. Any other value passed via the content_type script option triggers this exception.

Source

Thrown at modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/CustomMustacheFactory.java:106

    private CustomMustacheFactory(String mediaType, boolean detectMissingParams) {
        super(resourceName -> null); // we do not resolve templates via files or the classpath, etc.
        setObjectHandler(new CustomReflectionObjectHandler(detectMissingParams));
        this.encoder = createEncoder(mediaType);
    }

    @Override
    public void encode(String value, Writer writer) {
        try {
            encoder.encode(value, writer);
        } catch (IOException e) {
            throw new MustacheException("Unable to encode value", e);
        }
    }

    static Encoder createEncoder(String mediaType) {
        final Supplier<Encoder> supplier = ENCODERS.get(mediaType);
        if (supplier == null) {
            throw new IllegalArgumentException("No encoder found for media type [" + mediaType + "]");
        }
        return supplier.get();
    }

    @Override
    public MustacheVisitor createMustacheVisitor() {
        return new CustomMustacheVisitor(this);
    }

    public static Builder builder() {
        return new Builder();
    }

    private static class CustomMustacheVisitor extends DefaultMustacheVisitor {

        CustomMustacheVisitor(DefaultMustacheFactory df) {
            super(df);
        }

View on GitHub (pinned to db6a809a66)

Solutions

1. Use one of the five supported content types: 'application/json', 'application/json; charset=UTF-8', 'application/json;charset=utf-8', 'text/plain', or 'application/x-www-form-urlencoded'
2. If you don't need a specific content type, omit the content_type option entirely — it defaults to 'application/json'
3. For URL-encoded query parameter output, use 'application/x-www-form-urlencoded'
4. For plain text passthrough without JSON escaping, use 'text/plain'

Example fix

// before — search template request with unsupported content type:
POST /_search/template
{
  "source": { "query": { "match": { "title": "{{q}}" } } },
  "params": { "q": "test" },
  "content_type": "application/xml"
}

// after — use a supported content type or omit it:
POST /_search/template
{
  "source": { "query": { "match": { "title": "{{q}}" } } },
  "params": { "q": "test" },
  "content_type": "text/plain"
}

// Validate content_type before submitting search template
Set<String> VALID_CONTENT_TYPES = Set.of(
    "application/json",
    "application/json; charset=UTF-8",
    "application/json;charset=utf-8",
    "text/plain",
    "application/x-www-form-urlencoded"
);
if (contentType != null && !VALID_CONTENT_TYPES.contains(contentType)) {
    throw new IllegalArgumentException("Unsupported content type: " + contentType);
}

When it happens

Trigger: Executing a search template with the 'content_type' option set to an unsupported value. For example: {"source": {...}, "params": {...}} with script options specifying a content type like 'application/xml' or 'text/csv'. The option is read from Script.CONTENT_TYPE_OPTION in MustacheScriptEngine.createMustacheFactory() and passed to the builder.

Common situations: Passing a custom or wrong MIME type in the content_type option. Using 'application/xml' or 'application/yaml' which are not supported. Typing the content type incorrectly (e.g., 'application/jason'). Copying a content type from a different API that supports more formats.

Related errors

• Mustache script result size limit exceeded
• Unable to encode value
• Cannot expand '%s' because partial templates are not support
• Cannot expand '%s' because dynamic partial templates are not
• Mustache function [{}] must contain one and only one identif

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/e340d4fa5885cc5a. Report an issue: GitHub.