Object type should have a `properties` or `anyOf` field, or

Error message

Object type should have a `properties` or `anyOf` field, or be a resource

What it means

An object-typed schema in Windmill must be one of: having `properties`, having `anyOf`, or being a resource (its `format` starts with `resource-`). A bare `{ "type": "object" }` with none of these gives the parser no rules to build, so it is rejected.

Source

Thrown at backend/windmill-common/src/schema.rs:120

                                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"
                        ));
                    }
                }
            }
            JsonPrimitiveType::Array => {
                let items = val
                    .get("items")
                    .ok_or(anyhow!("Array type should have field `items`"))?;

                let arr_rules = SchemaValidationRule::from_value(items)?;

                schema_rules.push(SchemaValidationRule::IsArray(arr_rules));
            }
            JsonPrimitiveType::Boolean => {
                schema_rules.push(SchemaValidationRule::IsBool);
            }
            JsonPrimitiveType::Null => {

View on GitHub (pinned to e474e8803c)

Solutions

If arbitrary key/value input is needed, use `"format": "resource-windmill-u-py"`-style resource typing or define the fields under `properties`.
Add a `properties` object describing the expected fields (even one).
Use `anyOf` with concrete object variants.
Change the primitive to `string` if the value is actually unstructured.

Example fix

// before
{ "type": "object" }
// after
{ "type": "object", "properties": { "name": { "type": "string" } } }

Defensive patterns

Strategy: validation

Validate before calling

const s = JSON.parse(schemaRaw);
const isResource = typeof s.format === "string" && s.format.startsWith("resource");
if (s.type === "object" && !s.properties && !s.anyOf && !isResource) {
  throw new Error("Object schema needs `properties`, `anyOf`, or a resource-* format");
}

Type guard

function isWellFormedObjectSchema(s) {
  const isResource = typeof s?.format === "string" && s.format.startsWith("resource");
  return s?.type !== "object" || Boolean(s.properties || s.anyOf || isResource);
}

Try / catch

try {
  await deploy(schema);
} catch (e) {
  if (String(e.message).includes("should have a `properties` or `anyOf` field")) {
    throw new Error("Declare object fields under properties, use anyOf, or set a resource-* format");
  } else throw e;
}

Prevention

In Windmill, a bare `type: object` is never valid — always add properties/anyOf or a resource format.
For free-form dicts, check current Windmill docs for the accepted pattern instead of assuming plain JSON Schema.
Lint every input/output schema before deploying.

When it happens

Trigger: Declaring `"type": "object"` in a script/flow input schema without `properties`, without `anyOf`, and without `"format": "resource-..."`.

Common situations: Accepting arbitrary free-form JSON input; schemas copied from generic JSON Schema docs; removing properties and leaving an empty object node; wanting a dict but not knowing Windmill requires a resource format or properties.

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

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/6453b46f0a176e5a. Report an issue: GitHub.