Schema Validation Comes to Pulumi ESC with fn::validate

Source: Pulumi Blog

How it works

The fn::validate function takes a JSON Schema and a value. If the value conforms to the schema, it passes through unchanged. If not, ESC raises a validation error.

values:
  port:
    fn::validate:
      schema: { type: number, minimum: 1, maximum: 65535 }
      value: 8080

This validates that port is a number between 1 and 65535. The evaluated result is simply 8080.

Validating objects with required fields

For complex configurations, you can enforce structure and required fields:

values:
  database:
    fn::validate:
      schema:
        type: object
        properties:
          host: { type: string }
          port: { type: number }
          name: { type: string }
        required: [host, port, name]
      value:
        host: "db.example.com"
        port: 5432
        name: "myapp"

If any required field is missing or has the wrong type, the environment cannot be saved.

Reusing schemas across environments

Define schemas once and reference them across multiple environments. Using the environments built‑in property keeps the schema out of your environment’s output.

Schema environment (my-project/schemas)

values:
  database-schema:
    type: object
    properties:
      host: { type: string }
      port: { type: number }
    required: [host, port]

Environment using the schema

values:
  database:
    fn::validate:
      schema: ${environments.my-project.schemas.database-schema}
      value:
        host: "prod-db.example.com"
        port: 5432

This pattern ensures consistent validation rules across teams and projects.

What happens when validation fails

When a value doesn’t conform to its schema, ESC returns a clear error message:

values:
  port:
    fn::validate:
      schema: { type: string }
      value: 8080

This raises: expected string, got number. The environment cannot be saved until you fix the value or update the schema.

When to use schema validation

Enable fn::validate for:

• Values with specific type requirements (numbers, strings, arrays)
• Objects that must have certain fields present
• Numbers that must fall within a valid range
• Configurations shared across multiple environments
• Any value where catching errors early prevents downstream issues

Getting started

The fn::validate function is available now in all Pulumi ESC environments. Add schema validation to your existing environments or use it when creating new ones.

For more information, see the fn::validate documentation.