windmill-labs/windmill · error

Couldn't create schema validator for script requiring schema

Error message

Couldn't create schema validator for script requiring schema validation: {e}

What it means

When caching a script, if the script has `schema_validation` enabled, the stored JSON schema is compiled into a SchemaValidator at cache time. If the schema string is malformed (not valid JSON Schema) compilation fails and this error wraps the underlying cause. The script cannot be cached/executed until its schema is fixed.

Source

Thrown at backend/windmill-common/src/cache.rs:738

                    language: r.language,
                    envs: r.envs,
                    codebase: if let Some(use_tar) = r.use_tar {
                        let mut sh = hash.to_string();
                        if r.is_esm.unwrap_or(false) {
                            sh = format!("{sh}.esm");
                        }
                        if use_tar {
                            sh = format!("{sh}.tar");
                        }
                        Some(sh)
                    } else {
                        None
                    },
                    schema_validator: if r.schema_validation {
                        r.schema
                            .as_ref()
                            .map(|schema_str| {
                                SchemaValidator::from_schema(schema_str).map_err(|e| anyhow!("Couldn't create schema validator for script requiring schema validation: {e}"))
                            })
                            .transpose()?
                    } else {
                        None
                    },
                    schema: r.schema,
                }),
            })
        })
    }

    /// Invalidate the script cache for the given `hash`.
    pub fn invalidate(hash: ScriptHash) {
        let _ = CACHE.remove(&hash);
    }
}

pub mod app {

View on GitHub (pinned to e474e8803c)

Solutions

1. Validate the script's `schema` field with a JSON Schema validator (e.g. ajv in strict mode) and fix syntax/dialect errors.
2. Ensure the schema is a complete JSON Schema object (type/properties at root), not a fragment or quoted string of JSON.
3. Disable schema_validation on the script if validation is not actually needed.
4. Re-deploy the script after fixing so the cache rebuilds with a valid validator.

Example fix

// before
"schema": "{type: object}"            // not valid JSON
// after
"schema": "{\"type\":\"object\",\"properties\":{\"name\":{\"type\":\"string\"}}}"

const ajv = new (require('ajv'))({ strict: true });
try { ajv.compile(JSON.parse(script.schema)); } catch (e) { throw new Error(`invalid schema on script ${script.path}: ${e.message}`); }

function isValidJsonSchema(schemaStr) { try { const s = JSON.parse(schemaStr); return typeof s === 'object' && s !== null; } catch { return false; } }

match SchemaValidator::from_schema(schema_str) {
    Ok(v) => v,
    Err(e) => { /* fix the script's schema and redeploy */ return Err(...); }
}

When it happens

Trigger: Loading a script into the script cache (backend/windmill-common/src/cache.rs:738) where r.schema_validation is true and SchemaValidator::from_schema rejects the schema string — invalid JSON, unsupported schema keywords/dialect, or empty/corrupted schema.

Common situations: Hand-edited schemas with JSON syntax errors; schemas written for a different validator dialect; payloads pasted from other tools with $ref to external documents; migration leaving a legacy schema string the current validator can't parse.

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

• Missing required arguments: ${required.join(", ")}. Use -d '
• Flow schema must be an object or null
• Field properties should be an object
• `oneOf` needs to be an array
• oneOf variant definition should have a `title` field

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/6a3b53086a059fdb. Report an issue: GitHub.