Customer Error Message: `"errorMessage"` does not work with `if` / `then` / `else` blocks

[AHOCHWE]

🐛 Bug Report / Feature Request

The errorMessage keyword does not appear to support if / then / else blocks.

---

What I'm trying to do

I want to define custom error messages for conditional schemas using if / then logic, like this:

{
  "if": {
    "properties": {
      "type": { "const": "business" }
    }
  },
  "then": {
    "required": ["businessId"]
  },
  "errorMessage": {
    "if": "Type condition failed",
    "then": "Missing businessId when type is 'business'",
    "else": "Missing required fields for other types"
  }
}

I also tried:

"errorMessage": "Custom message for conditional failure"

and

"then": {
  "required": ["someField"],
  "errorMessage": {
    "required": "someField must be provided"
  }
}

Actual Behavior

• The schema validates correctly
• But custom messages in errorMessage for if, then, or else are ignored
• Instead, it shows the default errors

---

Expected Behavior

I expected that:

"errorMessage": {
  "then": "Missing businessId when type is 'business'"
}

…would override the default message when the then clause fails (i.e., businessId is missing).

Minimal Reproducible Schema

{
  "$schema": "https://json-schema.org/draft-07/schema",
  "type": "object",
  "properties": {
    "type": { "type": "string" },
    "businessId": { "type": "string" }
  },
  "if": {
    "properties": {
      "type": { "const": "business" }
    },
    "required": ["type"]
  },
  "then": {
    "required": ["businessId"]
  },
  "errorMessage": {
    "then": "Missing 'businessId' when 'type' is 'business'"
  }
}

---

Expected Result

Error message returned should be:

Missing 'businessId' when 'type' is 'business'

---

Questions

• Is errorMessage for if/then/else unsupported? Or am I doing something wrong?
• If not supported, would you consider this a useful feature to add?

[justin-tay]

if/then/else do not generate error messages and you should not expect custom error message handling to apply.

The following schema does what you want.

{
  "$schema": "https://json-schema.org/draft-07/schema",
  "type": "object",
  "properties": {
    "type": { "type": "string" },
    "businessId": { "type": "string" }
  },
  "if": {
    "properties": {
      "type": { "const": "business" }
    },
    "required": ["type"]
  },
  "then": {
    "required": ["businessId"]
  },
  "errorMessage": {
    "required": "Missing 'businessId' when 'type' is 'business'"
  }
}

package com.networknt.schema;

import java.util.Set;

import org.junit.jupiter.api.Test;

import com.networknt.schema.SpecVersion.VersionFlag;

class ErrorMessageTest {

    @Test
    void testErrorMessage() {
        String schemaData = "{\r\n"
        		+ "  \"$schema\": \"https://json-schema.org/draft-07/schema\",\r\n"
        		+ "  \"type\": \"object\",\r\n"
        		+ "  \"properties\": {\r\n"
        		+ "    \"type\": { \"type\": \"string\" },\r\n"
        		+ "    \"businessId\": { \"type\": \"string\" }\r\n"
        		+ "  },\r\n"
        		+ "  \"if\": {\r\n"
        		+ "    \"properties\": {\r\n"
        		+ "      \"type\": { \"const\": \"business\" }\r\n"
        		+ "    },\r\n"
        		+ "    \"required\": [\"type\"]\r\n"
        		+ "  },\r\n"
        		+ "  \"then\": {\r\n"
        		+ "    \"required\": [\"businessId\"]\r\n"
        		+ "  },\r\n"
        		+ "  \"errorMessage\": {\r\n"
        		+ "    \"required\": \"Missing 'businessId' when 'type' is 'business'\"\r\n"
        		+ "  }\r\n"
        		+ "}";
        JsonSchemaFactory factory = JsonSchemaFactory.getInstance(VersionFlag.V202012);
        JsonSchema schema = factory.getSchema(schemaData, SchemaValidatorsConfig.builder().errorMessageKeyword("errorMessage").build());
        Set<ValidationMessage> result = schema.validate("{\"type\":\"business\"}", InputFormat.JSON);
        System.out.println(result);
    }
}