# Migrate a Deprecated API Across Every Callsite in Terraform HCL with Atlas in 2026

> Atlas ensures a complete migration of deprecated APIs in Terraform HCL by enumerating all callsites and verifying changes with `terraform test`.

To migrate a deprecated API across every callsite in Terraform HCL, Atlas leverages its `lsp` tool to enumerate all references, creates `todowrite` entries for each, and applies changes with `apply_patch`, verifying each step with `terraform test` and `terraform fmt` to ensure a complete and safe transition of your infrastructure code without missing a single caller.

## Key takeaways

- Atlas uses `lsp` and `grep` to find every Terraform HCL callsite of a deprecated API, ensuring 100% coverage.
- `todowrite` ensures no Terraform HCL migration task is missed, providing visible and trackable progress.
- `apply_patch` applies context-aware changes to Terraform HCL files, preventing misapplication and offering unified diffs for review.
- `terraform test` is integrated into the workflow to verify each Terraform HCL migration step immediately after changes.
- Atlas provides comprehensive safety features, including permission prompts and unified diffs, for all Terraform HCL modifications.
- Atlas integrates with `git` to snapshot changes and facilitate rollback, enhancing safety for Terraform HCL refactoring.

## How to find all deprecated Terraform HCL module calls?

In 2026, identifying every callsite of a deprecated Terraform HCL module or function is crucial for a complete migration. Atlas uses its `lsp` tool to find all references from the language server, then cross-checks with `grep` for any dynamic or string-based usages, ensuring 100% coverage.

Atlas begins the migration of a deprecated Terraform HCL API by front-loading the enumeration of all its callsites. It utilizes the `lsp` tool's `findReferences` operation, which queries the language server to yield a complete set of callers for the deprecated symbol within your `.tf` files. This is particularly effective for structured HCL code where symbols are clearly defined. To account for more dynamic or string-based usages that an LSP might miss, Atlas then performs a cross-check using its `grep` tool. This two-pronged approach ensures that every single instance of the deprecated API, whether a direct module call, a variable reference, or a string literal in a `local` block, is identified. Running `atlas` in your root module, alongside your `.tf` files and `.terraform.lock.hcl`, allows it to build a comprehensive index of your resources, variables, outputs, and provider versions, ensuring accurate and exhaustive search results.

## How Atlas manages deprecated Terraform HCL API migration tasks?

Managing a large-scale migration of a deprecated Terraform HCL API across an entire codebase requires meticulous tracking. Atlas addresses this by creating one `todowrite` entry per identified callsite, making partial progress visible and ensuring no single caller is silently skipped during the process in 2026.

Once all callsites of the deprecated Terraform HCL API have been enumerated, Atlas transitions to task management using its `todowrite` tool. For every identified callsite, a distinct `todowrite` entry is created. This approach is vital for migrations that punish half-measures, as it transforms the abstract goal into a concrete, trackable list of tasks. Each entry represents a specific migration point, such as updating a `resource` block in `main.tf` or a `module` call in `modules/vpc/main.tf`. This granular tracking ensures that partial progress is always visible, preventing any callsite from being overlooked or accidentally skipped. As each migration step is completed and verified, the corresponding `todowrite` entry is marked as done, providing a clear audit trail and a definitive measure of completion for the entire codebase migration.

## How to safely apply changes to Terraform HCL files?

Applying changes to Terraform HCL files during an API migration demands precision to avoid unintended modifications. Atlas employs its `apply_patch` tool, which generates a unified diff for every file edit and seeks the hunk's exact context and old lines, failing rather than misapplying to a drifted file in 2026.

The safe application of changes to Terraform HCL files is paramount, given the potential impact on infrastructure. Atlas's `apply_patch` tool is designed with this safety in mind. Instead of blindly applying changes, it operates as a context-anchored patch mechanism. For each migration, Atlas computes a unified diff, which is then surfaced for your approval before any write operation occurs. The `apply_patch` tool specifically seeks the exact context and `old_lines` within the target Terraform HCL file. If the expected lines are not found due to a file drift or concurrent modification, it throws a `Failed to find expected lines` error, preventing misapplication. This robust mechanism ensures that changes are only applied when the file's state precisely matches the patch's expectation. Furthermore, Atlas reads `git` branches, status, and diffs, and can snapshot file changes as `git` patches, allowing edits to be easily diffed, reviewed, and rolled back if necessary, providing multiple layers of safety for your infrastructure code.

## How to verify Terraform HCL API migrations with tests?

Verifying each Terraform HCL API migration step is critical to maintain infrastructure stability. Atlas integrates `terraform test` into its workflow, running tests after each file modification and marking the `todowrite` entry complete only once they pass, aiming for zero remaining deprecated calls by 2026.

After each individual Terraform HCL file modification, Atlas mandates immediate verification using the `terraform test` runner. This ensures that the changes introduced by the API migration have not inadvertently broken existing functionality or introduced new issues. Atlas uses its `bash` tool to execute `terraform test` in the affected directory. Only once these tests pass successfully is the corresponding `todowrite` entry marked as complete. This iterative testing approach minimizes the risk of accumulating errors and simplifies debugging. Once all `todowrite` entries are completed, Atlas performs a final `grep` for the deprecated symbol across the entire codebase, confirming zero remaining hits. This final check, combined with running `terraform fmt -recursive` to ensure consistent formatting, guarantees a clean and fully migrated codebase, ready for `terraform validate` and `terraform plan` with confidence.

## What are Atlas's safety and review mechanisms for Terraform HCL?

Atlas incorporates multiple safety and review mechanisms to protect your Terraform HCL codebase during complex migrations in 2026. Every Atlas tool call is permission-gated, and a read-only plan agent drafts changes, presenting a unified diff for approval before any file is written, ensuring controlled and transparent operations.

Atlas prioritizes safety and transparency throughout the Terraform HCL migration process. All Atlas tool calls, including `lsp`, `grep`, `todowrite`, `apply_patch`, and `bash`, are permission-gated against `allow`, `ask`, and `deny` rules. This means you retain explicit control over what actions Atlas can perform. Before any modifications are made, Atlas drafts a plan in a read-only plan agent and asks for your explicit approval. For every proposed file edit, Atlas computes and surfaces a unified diff, allowing you to review the exact changes before they are written to disk. Atlas also reads `git` branches, status, and diffs, and can stage and create commits on your behalf, integrating direct with your existing version control workflow. Furthermore, Atlas snapshots file changes as `git` patches, providing a robust mechanism for rolling back edits if needed. When working with Terraform HCL, Atlas can even run `terraform validate` and `terraform plan` behind a permission prompt, then read the plan diff with you before anything is applied, offering an unparalleled level of insight and control over potential infrastructure changes.

## Steps

1. Run `atlas` in your Terraform root module, ensuring it contains `.tf` files and a `.terraform.lock.hcl`.
2. Use `atlas lsp findReferences` on the deprecated Terraform HCL symbol to enumerate all callsites, then cross-check with `atlas grep` for dynamic or string-based usages.
3. Create a `todowrite` entry for each identified callsite using `atlas todowrite add "Migrate deprecated_module.call in path/to/main.tf"` to track progress.
4. For each `todowrite` entry, use `atlas apply_patch` to generate and apply the migration, carefully reviewing the unified diff for approval.
5. After each file modification, run `terraform test` to verify functionality and `terraform fmt -recursive` to ensure consistent formatting.
6. Mark the `todowrite` entry as complete only after `terraform test` passes successfully.
7. Once all callsites are migrated, run `atlas grep "deprecated_symbol"` to confirm zero remaining hits across the codebase.
8. Delete the old deprecated Terraform HCL implementation from your project.

## FAQ

### How do I find all usages of a deprecated Terraform module or resource?

Atlas uses its `lsp` tool to query the language server for all references to the deprecated Terraform HCL symbol. It then cross-checks these results with `grep` to catch any dynamic or string-based usages that might be missed, ensuring a complete enumeration of every callsite in your `.tf` files.

### Can Atlas handle complex refactoring in Terraform HCL, like extracting blocks?

Yes, Atlas is designed to handle complex Terraform HCL refactoring. Beyond migrations, you can ask Atlas to extract a repeated block into a module or add a `for_each` instead of counted resources, leveraging its understanding of HCL AST declarations through tree-sitter.

### How does Atlas prevent breaking my Terraform infrastructure during a migration?

Atlas employs several safety measures: all tool calls are permission-gated, a read-only plan agent drafts changes for approval, and `apply_patch` uses context-anchored logic that fails rather than misapplying. Crucially, it runs `terraform test` after each file change and can show you `terraform plan` diffs before applying anything.

### What if a Terraform HCL file changes before Atlas applies a patch?

Atlas's `apply_patch` tool is designed to prevent misapplication. It seeks the exact context and `old_lines` within the target Terraform HCL file. If the file has drifted and the expected lines are not found, `apply_patch` will throw a `Failed to find expected lines` error, ensuring changes are only applied to the correct file state.

### Does Atlas integrate with my existing Terraform HCL development workflow?

Absolutely. Atlas is terminal-native and integrates direct. It reads your `git` branches and status, can stage and create commits, and uses your actual Terraform toolchain commands like `terraform test`, `terraform init (module registry)`, and `terraform fmt` within its workflow.

### How does Atlas ensure all deprecated calls are migrated and none are missed?

Atlas ensures completeness by first enumerating every callsite with `lsp` and `grep`. It then creates a `todowrite` entry for each, making every migration task explicit and trackable. The workflow concludes with a final `grep` for the deprecated symbol to confirm zero remaining hits before deletion.

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Canonical HTML: https://runatlas.sh/resources/stacks/migrate-a-deprecated-api-across-callsites-in-terraform
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Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.