What causes "[{}] is not a boolean value, cannot convert to boolean" in elastic/elasticsearch?

Normalize the field with a preceding 'gsub' or 'script' processor so the value is exactly 'true' or 'false' before the convert step.

elastic/elasticsearch · error · IllegalArgumentException

[{}] is not a boolean value, cannot convert to boolean

Error message

[{}] is not a boolean value, cannot convert to boolean

What it means

Thrown by the ConvertProcessor BOOLEAN constant when the field value's string form is not case-insensitively equal to 'true' or 'false'. Unlike numeric conversions there is no underlying parse exception; the processor explicitly checks only those two literals and rejects everything else (including '1'/'0', 'yes'/'no', 'Y'/'N'). It fires during pipeline execution while converting a field to boolean.

Source

Thrown at modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/ConvertProcessor.java:90

        FLOAT {
            @Override
            public Object convert(Object value) {
                try {
                    return Float.parseFloat(value.toString());
                } catch (NumberFormatException e) {
                    throw new IllegalArgumentException("unable to convert [" + value + "] to float", e);
                }
            }
        },
        BOOLEAN {
            @Override
            public Object convert(Object value) {
                if (value.toString().equalsIgnoreCase("true")) {
                    return true;
                } else if (value.toString().equalsIgnoreCase("false")) {
                    return false;
                } else {
                    throw new IllegalArgumentException("[" + value + "] is not a boolean value, cannot convert to boolean");
                }
            }
        },
        IP {
            @Override
            public Object convert(Object value) {
                // IllegalArgumentException is thrown if unable to convert
                InetAddresses.forString((String) value);
                return value;
            }
        },
        STRING {
            @Override
            public Object convert(Object value) {
                return value.toString();
            }
        },
        AUTO {

View on GitHub (pinned to db6a809a66)

Solutions

Normalize the field with a preceding 'gsub' or 'script' processor so the value is exactly 'true' or 'false' before the convert step.
Use a 'script' processor with painless logic (e.g. ctx.flag = ['1','yes','y','true'].contains(ctx.flag?.toLowerCase()) ) as a replacement for convert.
Correct the producing application to emit JSON booleans (true/false) rather than strings.

Example fix

// before - field "active" = "yes"
{"convert": {"field": "active", "type": "boolean"}}
// after - map common truthy strings first
{"script": {"source": "def v = ctx.active?.toString()?.toLowerCase(); ctx.active = (v == '1' || v == 'yes' || v == 'y' || v == 'true') ? 'true' : 'false'"}},
{"convert": {"field": "active", "type": "boolean"}}

Defensive patterns

Strategy: validation

Validate before calling

// Pre-map common truthy/falsy strings before convert to boolean:
{"script": {"source": "def v = ctx.active?.toString()?.toLowerCase(); if (v != null) { ctx.active = (['1','yes','y','true','on'].contains(v)) ? 'true' : 'false'; }"}}

Type guard

// painless guard
boolean isBooleanLiteral(def v) { def s = v?.toString()?.toLowerCase(); return 'true'.equals(s) || 'false'.equals(s); }

Try / catch

// Use pipeline on_failure to quarantine non-boolean values:
{"on_failure": [{"set": {"field": "error", "value": "boolean-convert"}}, {"index": {"index": "ingest-dlq"}}]}

Prevention

Normalize Y/N, 1/0, yes/no upstream to literal 'true'/'false'.
Prefer emitting JSON booleans from the producer when possible.
Document the strict 'true'/'false' rule for pipeline authors.

When it happens

Trigger: A 'convert' processor with 'type: boolean' is run on a document whose target field is '1', '0', 'yes', 'no', 'Y', 't', 'enabled', or any value other than the literal strings 'true'/'false'.

Common situations: Migrating from a system that serializes booleans as 0/1 integers; CSV/log ingest where flags are Y/N; JSON producers that emit 'Yes'; legacy fields using 'ENABLED'/'DISABLED'.

Related errors

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/98f2a1580ee4fc3c. Report an issue: GitHub.