windmill-labs/windmill · error

`oneOf` needs to be an array

Error message

`oneOf` needs to be an array

What it means

`SchemaValidationRule::from_primitive` encountered `oneOf` on an object-typed schema where the `oneOf` value is not a JSON array. Windmill builds a map of labeled variants from the array, so a non-array `oneOf` is rejected.

Source

Thrown at backend/windmill-common/src/schema.rs:87

                schema_rules.push(SchemaValidationRule::IsInteger);
            }
            JsonPrimitiveType::Object => {
                let mut obj_rules = vec![];

                if let Some(properties) = val.get("properties") {
                    let properties = properties
                        .as_object()
                        .ok_or(anyhow!("Field properties should be an object"))?;

                    for (key, v) in properties {
                        obj_rules.push((key.clone(), SchemaValidationRule::from_value(v)?))
                    }

                    schema_rules.push(SchemaValidationRule::IsObject(obj_rules));
                } else if let Some(one_of) = val.get("oneOf") {
                    let one_of = one_of
                        .as_array()
                        .ok_or(anyhow!("`oneOf` needs to be an array"))?;
                    let mut rules_map: HashMap<String, Vec<SchemaValidationRule>> = HashMap::new();

                    for variant in one_of {
                        let variant_label = variant
                            .get("title")
                            .ok_or(anyhow!(
                                "oneOf variant definition should have a `title` field"
                            ))?
                            .as_str()
                            .ok_or(anyhow!(
                                "oneOf variant definition `title` field should be a string"
                            ))?;
                        if !rules_map.contains_key(variant_label) {
                            rules_map.insert(
                                variant_label.to_string(),
                                SchemaValidationRule::from_value(variant)?,
                            );
                        } else {

View on GitHub (pinned to e474e8803c)

Solutions

1. Make `oneOf` an array of variant schemas, each with a string `title`.
2. If you intended free-form object properties, use `properties` instead of `oneOf`.
3. Run the schema through a JSON Schema validator before deploying.

Example fix

// before
{ "type": "object", "oneOf": { "title": "A" } }
// after
{ "type": "object", "oneOf": [ { "title": "A", "type": "object", "properties": {} } ] }

const s = JSON.parse(schemaRaw);
if ("oneOf" in s && !Array.isArray(s.oneOf)) {
  throw new Error("`oneOf` must be an array of variant schemas");
}

function hasValidOneOf(s) {
  return !("oneOf" in s) || Array.isArray(s.oneOf);
}

try {
  await wm.deploy(script);
} catch (e) {
  if (String(e.message).includes("oneOf` needs to be an array")) {
    throw new Error("Wrap your oneOf value in an array: oneOf: [variant, ...]");
  } else throw e;
}

When it happens

Trigger: A schema with `"type": "object"` and `"oneOf": {...}` (object/string/number instead of an array of variant schemas).

Common situations: Hand-written schemas confusing `oneOf` with `anyOf` or `allOf` semantics, malformed tool-generated output, or editing the schema in the Windmill UI/raw JSON and dropping the array brackets.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• oneOf variant definition should have a `title` field
• oneOf variant definition `title` field should be a string
• Field properties should be an object
• oneOf definition has a duplicate variant `{variant_label}`
• Object type should have a `properties` or `anyOf` field, or

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/ecbf5f640977d408. Report an issue: GitHub.