JSONSchemaType not matching Typescript Type on oneOf with objects

[ldrick]

What version of Ajv are you using? Does the issue happen if you use the latest version?
8.17.1 - currently latest available version

Ajv options object

// not needed, just Typescript Types

JSON Schema

{
    "type": "object",
    "discriminator": {
        "propertyName": "animal"
    },
    "required": ["animal"],
    "oneOf": [
        {
            "title": "Cat",
            "properties": {
                "animal": {
                    "const": "Cat"
                },
                "food": {
                    "enum": ["meat", "grass", "fish"],
                    "type": "string"
                }
            },
            "required": ["food"]
        },
        {
            "title": "Fish",
            "properties": {
                "animal": {
                    "const": "Fish"
                },
                "food": {
                    "enum": ["insect", "worms"],
                    "type": "string"
                },
                "water": {
                    "enum": ["lake", "sea"],
                    "type": "string"
                }
            },
            "required": ["food", "water"]
        }
    ]
}

Sample data

// not needed, just Typescript Types

Your code

type AnimalExampleType =
    | {
          animal: "Cat";
          ears: number; // missing in schema intentionally
          food: "fish" | "grass" | "meat";
      }
    | {
          animal: "Fish";
          food: "insect" | "worms";
          water: "lake" | "sea";
      };

// should report error in tsc, because 'ears' are missing in 'Cat'
const AnimalExampleSchema_OneOf: JSONSchemaType<AnimalExampleType> = {
    type: "object",
    discriminator: {
        propertyName: "animal",
    },
    required: ["animal"],
    oneOf: [
        {
            title: "Cat",
            properties: {
                animal: {
                    const: "Cat",
                },
                food: {
                    enum: ["meat", "grass", "fish"],
                    type: "string",
                },
            },
            required: ["food"],
        },
        {
            title: "Fish",
            properties: {
                animal: {
                    const: "Fish",
                },
                food: {
                    enum: ["insect", "worms"],
                    type: "string",
                },
                water: {
                    enum: ["lake", "sea"],
                    type: "string",
                },
            },
            required: ["food", "water"],
        },
    ],
};

Validation result, data AFTER validation, error messages

// no tsc errors appear, even the 'ears' for 'Cat' are missing

What results did you expect?
tsc to report Type mismatch

[Suhas-098]

I've verified this locally on v8.17.1 (the latest available version) and confirmed it is indeed not working as expected.

In my test setup (TS 5.9.3, strict mode on), I created a union type with required properties. Even when I intentionally left those properties out of the schema's one Of branches, tsc reported 0 errors. However, as a control test, I changed the top-level type from "object" to "string", and TypeScript correctly flagged that as an error. This confirms that the type-checking logic is specifically failing to distribute the strictness across the union branches.