windmill-labs/windmill · error

oneOf definition has a duplicate variant `{variant_label}`

Error message

oneOf definition has a duplicate variant `{variant_label}`

What it means

Windmill keys `oneOf` variants by their `title`; when two variants in the same `oneOf` array share the same title, the second would silently overwrite the first in the rules map. Instead, parsing aborts with this error naming the duplicated variant.

Source

Thrown at backend/windmill-common/src/schema.rs:106

                    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 {
                            return Err(anyhow!(
                                "oneOf definition has a duplicate variant `{variant_label}`"
                            ));
                        }
                    }

                    schema_rules.push(SchemaValidationRule::IsOneOf(rules_map))
                } else {
                    let is_resource = val
                        .get("format")
                        .and_then(|f| f.as_str())
                        .map(|f| f.starts_with("resource"))
                        .unwrap_or(false);
                    if !is_resource {
                        return Err(anyhow!(
                        "Object type should have a `properties` or `anyOf` field, or be a resource"
                        ));
                    }
                }

View on GitHub (pinned to e474e8803c)

Solutions

1. Rename each variant's `title` to a unique string.
2. Deduplicate variants that are truly identical (keep one).
3. Add a deployment-time JSON Schema check to catch duplicate titles early.

Example fix

// before
{ "oneOf": [ { "title": "A", "type": "string" }, { "title": "A", "type": "number" } ] }
// after
{ "oneOf": [ { "title": "A (string)", "type": "string" }, { "title": "A (number)", "type": "number" } ] }

const s = JSON.parse(schemaRaw);
if (Array.isArray(s.oneOf)) {
  const titles = s.oneOf.map(v => v?.title);
  const dupes = titles.filter((t, i) => titles.indexOf(t) !== i);
  if (dupes.length) throw new Error(`Duplicate oneOf titles: ${[...new Set(dupes)]}`);
}

function hasUniqueVariantTitles(s) {
  if (!Array.isArray(s.oneOf)) return true;
  const titles = s.oneOf.map(v => v?.title);
  return new Set(titles).size === titles.length;
}

try {
  await deploy(schema);
} catch (e) {
  const m = String(e.message).match(/duplicate variant `(.*?)`/);
  if (m) throw new Error(`Rename the duplicated variant title "${m[1]}"`);
  throw e;
}

When it happens

Trigger: A `oneOf` array containing two or more variant schemas whose `title` strings are identical, encountered while parsing the object schema.

Common situations: Duplicating a variant block and forgetting to rename its title; generator templates emitting the same default title; merging schemas from multiple sources.

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` needs to be an array
• oneOf variant definition should have a `title` field
• oneOf variant definition `title` field should be a string
• Variant name already exists
• Field properties should be an object

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/995c4d9a20b22a13. Report an issue: GitHub.