AWS CloudFormation language server

The AWS CloudFormation language server provides capabilities to accelerate authoring infrastructure-as-code (IaC) and deploying AWS resources safely and with confidence. It follows the language server protocol (LSP) to provide documentation on hover, auto-completion, diagnostics via static validation, go to definition and code actions. In addition to these traditional language server capabilities, the server adds online features to explore and deploy AWS resources via CloudFormation. This includes the ability to validate and deploy templates using change sets; view stack diffs, events, resources, and outputs; list stacks and browse resources by type; and insert live resource state directly into CloudFormation templates. Developer tooling and integrated development environments (IDE) can leverage the language server capabilities to enhance the experience of IaC developers on CloudFormation.

IDEs integrating with the AWS CloudFormation language server

The AWS Toolkit extension integrates the CloudFormation language server to enhance the authoring experience. This integration is currently supported in VS Code based IDEs, including:

• Visual Studio Code

• Kiro

• Cursor

Getting started

Topics

• Prerequisites

• Step 1: Install or upgrade the AWS Toolkit

• Step 2: Access CloudFormation in the AWS Toolkit panel

• Step 3: Validate, test, and refine your template

• Step 4: Navigate through the template

• Step 5: Validate and deploy

Prerequisites

Before you begin, make sure that:

• You are using a VS Code-based IDE on a supported operation system (macOS, Windows, or Linux).

• You have installed or upgraded to the latest version of the AWS Toolkit for your IDE.

Some features in the AWS CloudFormation language server require an active AWS account and configured credentials. You must be signed in to your AWS account through the AWS Toolkit using valid credentials.

Step 1: Install or upgrade the AWS Toolkit

• Open your IDE's Extensions or Plugin Manager.

• Search for AWS Toolkit.

• Install or update to version 3.85.0 or later of the AWS Toolkit for your IDE.

• Restart your IDE after installation.

Note: After installation, the AWS Toolkit automatically enables CloudFormation IDE support. When you first install or upgrade the AWS Toolkit with the AWS CloudFormation language server extension, you're prompted to grant permission for AWS to collect anonymous usage data. This data helps AWS improve the CloudFormation language server and enhances the authoring experience. No sensitive information is collected and AWS does not record or store template content, resource configurations, or any identifiable customer data. You can change your telemetry preferences at any time from the IDE settings. The usage data collected focuses only on feature interactions and performance metrics. These insights help AWS identify and prioritize improvements such as faster validation, enhanced autocomplete, and better error diagnostics.

Step 2: Access CloudFormation in the AWS Toolkit panel

In your IDE, open the AWS Toolkit panel from the activity bar. Under AWS Toolkit, choose CLOUDFORMATION.

Panel sections:

• Region: Displays the current AWS Region. You can change it by selecting the Region name or by using the AWS CloudFormation: Select Region command from the command palette.

• Environment: Indicates the environment you selected during CFN init (for example, dev, test, or prod). This value appears only after completing the environment selection step.

• Stacks: Displays a paginated list of CloudFormation stacks in your account.

  • Click on the refresh icon to update the list of stacks.

  • Use the + icon to deploy a new template as a stack.

  • Each stack entry includes:

    • Overview: Displays stack summary and status

    • Stack ID

    • Description

    • Created time and Updated time

    • Status and Status reason

    • Events, Outputs, and Resources

  • Change set

• Resources: After you select a resource type, the panel displays the AWS resources of that type in your account. You can view, refresh, copy, or import them into your template.

Step 3: Validate, test, and refine your template

As you write your CloudFormation template, the IDE provides intelligent authoring assistance to help you create accurate and compliant infrastructure faster. The CloudFormation language server runs in the background and provides the following authoring features:

• Code completion: Suggests resource types, parameters, and properties based on CloudFormation schemas.

• Add existing AWS resources: Allows you to import existing resources from your AWS account into your template. The IDE uses the AWS Cloud Control API (CCAPI) to retrieve the live configuration and properties of the resource, helping you clone or reuse existing infrastructure within your template.

To add resources to your template

• Expand the CloudFormation Resources panel: In the AWS Toolkit side panel, expand CloudFormation, then under Resources, click the Add + icon.

• Search for a resource type: To find a specific AWS resource type, in the search bar type the specific AWS resource type you want to add. Example:

  • AWS::S3::Bucket

  • AWS::Lambda::Function

  You can select resource type using AWS CloudFormation: Add Resource Types in the command palette.

• Adding resources to your template: Under the Resources panel, a paginated list of detected AWS resources in your account is displayed. If you have many resources, only first page is shown. Use the navigation controls at the bottom of the panel to move through additional pages and view all available resources.

• Choose the resource you want to include in your template.

• You can insert a resource into your template in two ways, depending on your goal:

  • Clone an existing resource: Create a new resource in your template using the live configuration and properties of an existing AWS resource.

  • Import an existing resource: Insert the actual resource into your stack by adding it to your template using its live state.

Tips

• You can refresh the Resources panel at any time to view the latest list of resources available in your account or Region.

• If you are importing resources, do not add a resource that already belongs to an existing CloudFormation stack in the same account.

• To confirm if a resource is already managed by CloudFormation, click the i (information) icon next to the resource name.

• Alternatively, you can use the command AWS CloudFormation: Get Stack Management Info to check whether a resource belongs to a stack.

Add related resources

You can add related resources to the selected resource by using the command AWS CloudFormation: Add Related Resources by Type. Once you select a resource type from the ones already defined in your template, the IDE displays a list of resources that are typically associated with or dependent on that type. For example, if you select an AWS::EC2::Instance, the IDE may suggest adding related resources such as AWS::EC2::SecurityGroup or AWS::EC2::Subnet. This feature helps you quickly build connected infrastructure components without manually searching for compatible resource types.

Static validation

The CloudFormation IDE provides built-in static validation powered by AWS CloudFormation Linter (cfn-lint) and AWS CloudFormation Guard. These validations run behind the scenes as you author templates, helping you identify syntax errors, compliance gaps, and best practice issues before deployment.

Static validation overview

You will see two types of real-time static validations in the IDE:

• CloudFormation Linter (cfn-lint): Validates your template against CloudFormation resource specifications and schema rules.

• Guard (cfn-guard): Validates your template against compliance rules and organizational policy packs.

CloudFormation Linter (cfn-lint)

The CloudFormation Linter is integrated into the IDE to automatically check your template syntax and structure as you type.

• Schema validation: Detects syntax and schema errors to ensure your templates conform to CloudFormation resource schema.

• Error highlighting: Displays red or yellow squiggly lines under issues, representing deployment blockers or warnings.

• Hover over help: When you hover over an error, the IDE shows the diagnostic message associated with that issue. If a quick fix is available, it will also be offered in the hover panel.

Guard integration

Guard validates your templates against rule sets that define compliance and security policies. The IDE runs Guard validations in real time through the CloudFormation language server, giving you immediate feedback while you author templates.

• Default rule packs: The IDE includes a pre-registered set of Guard rules focused on foundational best practices for resource security and configuration hygiene. To learn more link to the guard rule registry.

• Adding rule packs: To add or modify rule sets, open Settings, Guard and select or upload additional Guard rule packs.

Tips: Understanding squiggly line indicators

• Blue squiggles: Best practice hints or optimization recommendations.

• Yellow squiggles: Warnings for non-blocking issues (for example, missing tags or parameters).

• Red squiggles: Deployment blockers such as invalid property names, missing required fields, or schema mismatches.

Step 4: Navigate through the template

When you click on a template file in the Explorer, the Outline panel automatically displays a structured, hierarchical view of your CloudFormation template. It organizes the template into sections such as Parameters, Resources, Outputs, and Mappings, and shows each resource type and logical ID. This makes it easy to quickly locate and navigate to specific resources or parameters within large templates.

You can use Go to Definition for intrinsic functions such as GetAtt and Ref, allowing you to jump directly to the referenced resource or parameter in your template. This helps you trace dependencies, understand resource relationships, and make edits more efficiently.

Step 5: Validate and deploy

When you're ready to deploy your CloudFormation template, open the command palette and run AWS CloudFormation: Validate and Deploy. The command validates your template, and if no blocking errors are found, it proceeds to the deployment phase. Before confirming, the IDE shows a drift-aware change set summary and a diff view so you can review all proposed changes.

How validation works

The IDE automatically performs a validation check before deployment and validates your template against common failure causes, including:

• Invalid property syntax or schema mismatches: These issues are typically caught by cfn-lint during authoring, but if a user proceeds to deploy without addressing them, CloudFormation's deployment-time validation will surface the same errors before the stack is created or updated.

• Resource name conflicts with existing resources in your account.

• Service-specific constraints, such as S3 bucket name conflicts or missing encryption.

If the validation detects errors, the IDE highlights the issues directly in your template using red or yellow squiggly lines and lists the errors in the PROBLEMS panel. Each issue includes the specific property or resource that caused the failure, along with a suggested fix. If there are no blocking errors, you can proceed to the deployment phase.

If warnings are found (non-blocking issues), a dialog appears allowing you to either proceed with deployment or cancel and make corrections.

The IDE opens a drift-aware change set that displays any differences between your current template and the deployed stack configuration. This allows you to review, confirm, or cancel the change set before execution.

Note: Drift-aware change sets enhance the CloudFormation deployment process by allowing you to handle stack drift safely. Stack drift occurs when the actual state of your resources differs from what's defined in your CloudFormation template, often due to manual changes made through the AWS Management Console, CLI, or SDK. CloudFormation drift-aware change set compare your processed stack configuration with the live resource state, and the IDE surfaces these differences so you can bring resources back into compliance before deployment.

View stack events

When the deployment starts, you can navigate to the CloudFormation tab in the panel to monitor progress in real time. Under Stack Events, you'll see a list of operations performed during the deployment. Each event includes details such as:

• Timestamp: The time the event occurred

• Resource: The specific AWS resource being created, updated, or deleted

• Status: The current state of the operation (for example, CREATE_IN_PROGRESS, UPDATE_COMPLETE, or ROLLBACK_IN_PROGRESS)

• Reason: Additional context or error messages, if applicable

You can also view the stack's Resources and Outputs from this panel. If you want to open the stack in the AWS Management Console, use the external-link icon next to the stack name. The Stack Events view helps you track deployment progress, identify potential issues, and confirm when your stack has completed successfully.

Initializing a CloudFormation project in the IDE

Initializing a CloudFormation project in the IDE helps you set up a structured workspace with the correct folders, environment configuration, and AWS credentials so you can validate and deploy your templates reliably. You can initialize a new CloudFormation project directly from the IDE to create this recommended setup.

To initialize a CloudFormation project:

• Open the command palette

  • From your IDE, open the command palette (Ctrl+Shift+P or Cmd+Shift+P on macOS).

  • Choose AWS CloudFormation: CFN Init: Initialize Project.

• Choose a project directory

  • By default, the IDE uses your current working directory.

  • You can change this path to any folder where you want to store your CloudFormation templates.

• Select your AWS credential profile

  • You'll be prompted to choose an AWS credential profile. The selected profile will be used for environment detection, validations, and deployments.

• Set up your environment

  • You'll be prompted to create or select an environment.

  • Environments define where and how your templates will be deployed or validated (for example, dev, beta, or production). You can use AWS CloudFormation: CFN Init: Add Environment to select or change your environment.

  • You can use AWS CloudFormation: CFN Init: Remove Environment to remove the environment you've selected.

• (Optional) Import parameter files

  • If you already have existing parameter files, the IDE allows you to import them during initialization.

  • The IDE automatically detects compatible files and links them to your project for use in template validation and deployment.

• Name and finalize the project

  • Provide a project name, such as beta-environment, and complete the setup.

  • The IDE creates the initial project structure and configuration file for you.

You can run validations, preview deployments, or switch between environments directly from the IDE.

Open source

The AWS CloudFormation language server is open-sourced under the Apache-2.0 License, giving customers full transparency into how template diagnostics, schema validation, and static analysis are performed. This reduces security and compliance friction for customers who require source-level visibility before adopting tooling.

The code base is publicly available on GitHub: https://github.com/aws-cloudformation/cloudformation-languageserver/.

Need help?

Try the CloudFormation community on AWS re:Post.