Experiments

Terragrunt supports operating in a mode referred to as “Experiment Mode”.

Experiment Mode is a set of controls that can be enabled to opt in to experimental features before they’re stable. These features are subject to change and may be removed or altered at any time. They generally provide early access to new features or changes that are being considered for inclusion in future releases.

Those experiments will be documented here so that you know the following:

1. What the experiment is.
2. What the experiment does.
3. How to provide feedback on the experiment.
4. What criteria must be met for the experiment to be considered stable.

Sometimes, the criteria for an experiment to be considered stable is unknown, as there may not be a clear path to stabilization. In that case, this will be noted in the experiment documentation, and collaboration with the community will be encouraged to help determine the future of the experiment.

Controlling Experiment Mode

The simplest way to enable experiment mode is to set the experiment-mode flag.

This will enable experiment mode for all Terragrunt commands, for all experiments (note that this isn’t generally recommended, unless you are following Terragrunt development closely and are prepared for the possibility of breaking changes).

terragrunt plan --experiment-mode

You can also use the environment variable, which can be more useful in CI/CD pipelines:

TG_EXPERIMENT_MODE='true' terragrunt plan

Instead of enabling experiment mode, you can also enable specific experiments by setting the experiment flag to a value that’s specific to an experiment. This can allow you to experiment with a specific unstable feature that you think might be useful to you.

terragrunt plan --experiment symlinks

Again, you can also use the environment variable, which can be more useful in CI/CD pipelines:

TG_EXPERIMENT='symlinks' terragrunt plan

You can also enable multiple experiments at once.

terragrunt --experiment symlinks plan

Including the environment variable:

TG_EXPERIMENT='symlinks,stacks' terragrunt plan

Active Experiments

The following experiments are available:

• symlinks
• cas
• report
• runner-pool
• auto-provider-cache-dir

symlinks

Support symlink resolution for Terragrunt units.

symlinks - What it does

By default, Terragrunt will ignore symlinks when determining which units it should run. By enabling this experiment, Terragrunt will resolve symlinks and add them to the list of units being run.

symlinks - How to provide feedback

Provide your feedback on the Experiment: Symlinks discussion.

symlinks - Criteria for stabilization

To stabilize this feature, the following need to be resolved, at a minimum:

• Ensure that symlink support continues to work for users referencing symlinks in flags. See #3622.
  • Add integration tests for all filesystem flags to confirm support with symlinks (or document the fact that they cannot be supported).
• Ensure that MacOS integration tests still work. See #3616.
  • Add integration tests for MacOS in CI.

cas

Support for Terragrunt Content Addressable Storage (CAS).

cas - What it does

Allow Terragrunt to store and retrieve Git repositories from a Content Addressable Storage (CAS) system.

The CAS is used to speed up both catalog cloning and OpenTofu/Terraform source cloning by avoiding redundant downloads of Git repositories.

cas - How to provide feedback

Share your experience with this feature in the CAS Feedback GitHub Discussion. Feedback is crucial for ensuring the feature meets real-world use cases. Please include:

• Any bugs or issues encountered (including logs or stack traces if possible).
• Suggestions for additional improvements or enhancements.

cas - Criteria for stabilization

To transition the cas feature to a stable release, the following must be addressed:

• Add support for storing and retrieving catalog repositories from the CAS.
• Add support for storing and retrieving OpenTofu/Terraform modules from the CAS.
• Add support for storing and retrieving Unit/Stack configurations from the CAS.

report

Support for Terragrunt Run Reports and Summaries.

report - What it does

Allow usage of experimental run report generation, and summary displays.

report - How to provide feedback

Provide your feedback on the Run Summary RFC.

report - Criteria for stabilization

To transition the report feature to a stable release, the following must be addressed:

• Add support for generating reports (in CSV format by default).
• Add support for displaying summaries of runs.
• Add ability to disable summary display.
• Add support for generating reports in JSON format.
• Add comprehensive integration tests for the report experiment.
• Finalize the design of run summaries and reports.

runner-pool

Proposes replacing Terragrunt’s group-based execution with a dynamic runner pool that schedules Units as soon as dependencies are resolved. This improves efficiency, reduces bottlenecks, and limits the impact of individual failures.

runner-pool - What it does

Allow usage of experimental runner pool implementation for units execution.

runner-pool - How to provide feedback

Provide your feedback on the Runner Pool.

runner-pool - Criteria for stabilization

To transition the runner-pool feature to a stable release, the following must be addressed:

• Use new discovery and queue packages to discover units.
• Add support for including/excluding external units in the discovery process.
• Add runner pool implementation to execute discovered units.
• Add integration tests to track that the runner pool works in the same way as the current implementation.
• Add performance tests to track that the runner pool implementation is faster than the current implementation.
• Add support for fail fast behavior in the runner pool.
• Improve the UI to queue to apply.
• Add OpenTelemetry support to the runner pool.

auto-provider-cache-dir

Enable native OpenTofu provider caching by setting TF_PLUGIN_CACHE_DIR instead of using Terragrunt’s internal provider cache server.

auto-provider-cache-dir - What it does

When enabled, this experiment automatically configures OpenTofu to use its built-in provider caching mechanism by setting the TF_PLUGIN_CACHE_DIR environment variable. This approach leverages OpenTofu’s native provider caching capabilities, which are more robust for concurrent operations in OpenTofu 1.10+.

Requirements:

• OpenTofu version >= 1.10 is required
• Only works when using OpenTofu (not Terraform)
• If the requirements are not met, the experiment silently does nothing

Usage:

terragrunt run --all apply --experiment auto-provider-cache-dir

Or with environment variables:

TG_EXPERIMENT='auto-provider-cache-dir' \
terragrunt run --all apply

Disabling the feature:

Even when the experiment is enabled, you can still disable the auto-provider-cache-dir feature for specific runs using the --no-auto-provider-cache-dir flag:

terragrunt run --all apply --experiment auto-provider-cache-dir --no-auto-provider-cache-dir

This will be most important post-stabilization, when the feature is enabled by default.

auto-provider-cache-dir - How to provide feedback

Please provide feedback through GitHub issues with the experiment: auto-provider-cache-dir label.

auto-provider-cache-dir - Criteria for stabilization

To transition the auto-provider-cache-dir feature to a stable release, the following must be addressed:

• Comprehensive testing to confirm the safety of concurrent runs using the same provider cache directory.
• Performance comparison with the existing provider cache server approach.
• Documentation and examples of best practices for usage.
• Community feedback on real-world usage and any edge cases discovered.

Note that the current plan for stabilization is to have the feature be enabled by default, and to allow users to opt-out if they need to, or use the provider cache server if they want to do something more advanced, like store their provider cache in a different filesystem.

Completed Experiments

• cli-redesign
• stacks

cli-redesign

Support for the new Terragrunt CLI design.

cli-redesign - What it does

Enabled features from the CLI Redesign RFC.

This experiment flag is no longer needed, as the CLI Redesign is now the default.

cli-redesign - How to provide feedback

Now that the CLI Redesign experiment is complete, please provide feedback in the form of standard GitHub issues.

cli-redesign - Criteria for stabilization

To transition cli-redesign features to a stable the following have been completed:

• Add support for run command.
  • Add support for basic usage of the run command (e.g., terragrunt run plan, terragrunt run -- plan -no-color).
  • Add support for the --all flag.
  • Add support for the --graph flag.
• Add support for exec command.
• Rename legacy --terragrunt- prefixed flags so that they no longer need the prefix.
• Add the hcl command, replacing commands like hclfmt, hclvalidate and validate-inputs.
• Add OpenTofu commands as explicit shortcuts in the CLI instead of forwarding all unknown commands to OpenTofu/Terraform.
• Add support for the backend command.
• Add support for the render command.
• Add support for the info command.
• Add support for the dag command.
• Add support for the find command.
  • Add support for find without flags.
  • Add support for find with colorful output.
  • Add support for find with --format=json flag.
  • Add support for find with stdout redirection detection.
  • Add support for find with --hidden flag.
  • Add support for find with --sort=alpha flag.
  • Add support for find with --sort=dag flag.
  • Add support for find with the exclude block used to exclude units from the search.
  • Add integration with symlinks experiment to support finding units/stacks via symlinks.
  • Add handling of broken configurations or configurations requiring authentication.
  • Add integration test for find with --sort=dag flag on all the fixtures in the test/fixtures directory.
• Add support for the list command.
  • Add support for list without flags.
  • Add support for list with colorful output.
  • Add support for list with --format=tree flag.
  • Add support for list with --format=long flag.
  • Add support for list with stdout redirection detection.
  • Add support for list with --hidden flag.
  • Add support for list with --sort=alpha flag.
  • Add support for list with --sort=dag flag.
  • Add support for list with --group-by=fs flag.
  • Add support for list with --group-by=dag flag.
  • Add support for list with the exclude block used to exclude units from the search.
  • Add integration with symlinks experiment to support listing units/stacks via symlinks.
  • Add handling of broken configurations or configurations requiring authentication.
  • Add integration test for list with --sort=dag flag on all the fixtures in the test/fixtures directory.

stacks

Support for Terragrunt stacks.

What it does

Enable stack command to manage Terragrunt stacks.

stacks - Criteria for stabilization

To transition the stacks feature to a stable release, the following must be addressed:

• Add support for stack run * command
• Add support for stack output commands to extend stack-level operations.
• Integration testing for recursive stack handling across typical workflows, ensuring smooth transitions during plan, apply, and destroy operations.
• Confirm compatibility with parallelism flags (e.g., --parallel), especially for stacks with dependencies.
• Ensure that error handling and failure recovery strategies work as intended across large and nested stacks.