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+---
+title: "Schema Validation Comes to Pulumi ESC with fn::validate"
+date: 2026-02-09
+draft: false
+meta_desc: "Validate configuration values against JSON Schema with fn::validate in Pulumi ESC."
+meta_image: meta.png
+authors:
+    - pablo-terradillos
+    - claire-gaestel
+tags:
+    - esc
+    - features
+schema_type: auto
+social:
+    twitter:
+    linkedin:
+aliases:
+    - /blog/esc-schema-validation-fn-conform/
+---
+
+Pulumi ESC environments can now validate configuration values against JSON Schema with the new `fn::validate` built-in function. Invalid configurations are caught immediately when you save, preventing misconfigurations from reaching your deployments.
+<!--more-->
+
+Configuration errors are often discovered too late during deployment or, worse, in production. With `fn::validate`, you define validation rules directly in your environment, and ESC enforces them at save time. If a value doesn't match its schema, the environment cannot be saved until the issue is resolved.
+
+## 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.
+
+```yaml
+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:
+
+```yaml
+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](/docs/esc/environments/syntax/builtin-properties/environments/) keeps the schema out of your environment's output:
+
+**Schema environment (myorg/schemas)**
+
+```yaml
+values:
+  database-schema:
+    type: object
+    properties:
+      host: { type: string }
+      port: { type: number }
+    required: [host, port]
+```
+
+**Environment using the schema**
+
+```yaml
+values:
+  database:
+    fn::validate:
+      schema: ${environments.myorg.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:
+
+```yaml
+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](/docs/esc/environments/syntax/builtin-functions/fn-validate/).
diff --git a/content/blog/esc-schema-validation-fn-validate/meta.png b/content/blog/esc-schema-validation-fn-validate/meta.png
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diff --git a/content/docs/esc/environments/syntax/builtin-functions/_index.md b/content/docs/esc/environments/syntax/builtin-functions/_index.md
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 ## Functions
 
-- [`fn::fromJSON`](/docs/esc/environments/syntax/builtin-functions/fn-from-json)
+- [`fn::concat`](/docs/esc/environments/syntax/builtin-functions/fn-concat)
+- [`fn::validate`](/docs/esc/environments/syntax/builtin-functions/fn-validate)
 - [`fn::fromBase64`](/docs/esc/environments/syntax/builtin-functions/fn-from-base64)
+- [`fn::fromJSON`](/docs/esc/environments/syntax/builtin-functions/fn-from-json)
 - [`fn::join`](/docs/esc/environments/syntax/builtin-functions/fn-join)
-- [`fn::split`](/docs/esc/environments/syntax/builtin-functions/fn-split)
-- [`fn::concat`](/docs/esc/environments/syntax/builtin-functions/fn-concat)
 - [`fn::open`](/docs/esc/environments/syntax/builtin-functions/fn-open)
 - [`fn::rotate`](/docs/esc/environments/syntax/builtin-functions/fn-rotate)
 - [`fn::secret`](/docs/esc/environments/syntax/builtin-functions/fn-secret)
+- [`fn::split`](/docs/esc/environments/syntax/builtin-functions/fn-split)
 - [`fn::toBase64`](/docs/esc/environments/syntax/builtin-functions/fn-to-base64)
 - [`fn::toJSON`](/docs/esc/environments/syntax/builtin-functions/fn-to-json)
 - [`fn::toString`](/docs/esc/environments/syntax/builtin-functions/fn-to-string)
diff --git a/content/docs/esc/environments/syntax/builtin-functions/fn-validate.md b/content/docs/esc/environments/syntax/builtin-functions/fn-validate.md
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+---
+title: fn::validate
+title_tag: fn::validate
+h1: fn::validate
+meta_desc: Pulumi ESC allows you to compose and manage hierarchical collections of configuration and secrets and consume them in various ways.
+aliases:
+  - /docs/esc/environments/syntax/builtin-functions/fn-conform/
+menu:
+  esc:
+    parent: esc-syntax-builtin-functions
+    identifier: esc-syntax-fn-validate
+    weight: 2
+---
+
+The `fn::validate` built-in function validates a value against a JSON Schema. If the value does not conform to the schema, a validation error is raised and the environment cannot be saved until the issues are resolved.
+
+## Declaration
+
+```yaml
+fn::validate:
+  schema: json-schema
+  value: value-to-validate
+```
+
+### Parameters
+
+| Property | Type   | Description                                          |
+|----------|--------|------------------------------------------------------|
+| `schema` | object | A JSON Schema definition to validate the value against.
+| `value`  | any    | The value to validate.
+
+### Returns
+
+If validation passes, the original `value` is returned. If validation fails, an error is raised and the environment cannot be saved.
+
+## Example
+
+### Basic type validation
+
+#### Definition
+
+```yaml
+values:
+  valid-string:
+    fn::validate:
+      schema: { type: string }
+      value: "hello"
+```
+
+#### Evaluated result
+
+```json
+{
+  "valid-string": "hello"
+}
+```
+
+### Number constraints
+
+#### Definition
+
+```yaml
+values:
+  validated-number:
+    fn::validate:
+      schema: { type: number, minimum: 0 }
+      value: 42
+```
+
+#### Evaluated result
+
+```json
+{
+  "validated-number": 42
+}
+```
+
+### Object with required fields
+
+#### Definition
+
+```yaml
+values:
+  user:
+    fn::validate:
+      schema:
+        type: object
+        properties:
+          name: { type: string }
+          email: { type: string }
+        required: [name, email]
+      value:
+        name: "Alice"
+        email: "alice@example.com"
+```
+
+#### Evaluated result
+
+```json
+{
+  "user": {
+    "name": "Alice",
+    "email": "alice@example.com"
+  }
+}
+```
+
+### Dynamic schema from reference
+
+You can define schemas as values and reference them dynamically:
+
+#### Definition
+
+```yaml
+values:
+  my-schema:
+    type: string
+    minLength: 1
+  validated:
+    fn::validate:
+      schema: ${my-schema}
+      value: "hello"
+```
+
+#### Evaluated result
+
+```json
+{
+  "my-schema": {
+    "type": "string",
+    "minLength": 1
+  },
+  "validated": "hello"
+}
+```
+
+### Array validation
+
+#### Definition
+
+```yaml
+values:
+  tags:
+    fn::validate:
+      schema:
+        type: array
+        items: { type: string }
+      value: ["production", "us-east-1"]
+```
+
+#### Evaluated result
+
+```json
+{
+  "tags": ["production", "us-east-1"]
+}
+```
+
+### Reusing schemas from other environments
+
+You can define schemas in a separate environment and reference them for reuse across multiple environments. This allows you to centralize schema definitions and ensure consistent validation.
+
+Using the [`environments` built-in property](/docs/esc/environments/syntax/builtin-properties/environments/) to reference the schema is the recommended approach because it doesn't merge the schema into your environment's output.
+
+#### Schema environment (myproj/schemas)
+
+```yaml
+values:
+  user-schema:
+    type: object
+    properties:
+      name: { type: string }
+      email: { type: string }
+    required: [name, email]
+```
+
+#### Environment using the schema reference
+
+```yaml
+values:
+  user:
+    fn::validate:
+      schema: ${environments.myproj.schemas.user-schema}
+      value:
+        name: "Alice"
+        email: "alice@example.com"
+```
+
+#### Evaluated result
+
+```json
+{
+  "user": {
+    "name": "Alice",
+    "email": "alice@example.com"
+  }
+}
+```
+
+This pattern is useful for enforcing consistent validation rules across teams and projects.
+
+## Validation errors
+
+When a value does not conform to its schema, `fn::validate` raises an error that prevents the environment from being saved. This ensures configuration issues are caught before the environment is used.
+
+### Example: type mismatch
+
+```yaml
+values:
+  type-error:
+    fn::validate:
+      schema: { type: string }
+      value: 42
+```
+
+This raises an error: `expected string, got number`. The environment cannot be saved until the value conforms to the schema.
+
+### Example: missing required field
+
+```yaml
+values:
+  missing-required:
+    fn::validate:
+      schema:
+        type: object
+        properties:
+          name: { type: string }
+        required: [name]
+      value:
+        other: "value"
+```
+
+This raises an error indicating that the required field `name` is missing. The environment cannot be saved until the required field is provided.