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Terragrunt CAS `mutable` attribute for editable fetched sources

Introduced 2026-05-18 · score 0.94

HOW TO USE IT

When a source is fetched through CAS and you need to edit its materialized files, set `mutable = true` in the applicable `terraform`, `unit`, or `stack` block. This copies files into the block’s working directory instead of hardlinking them from CAS.

terraform { source = "git::https://github.com/acme/infrastructure-modules.git//vpc?ref=v1.0.0" mutable = true }

WHY THIS HELPS

When you need to edit source files materialized through CAS, configure the applicable block as mutable.

REQUIREMENTS AND LIMITATIONS

  • Version: v1.0.5
  • mutable: Boolean on `terraform`, `unit`, or `stack`; `true` copies CAS-fetched files so they are editable. Defaults to `false`, which hardlinks files and marks them read-only.
  • Prerequisite: The source must be fetched through CAS; `mutable` has no effect with the standard download path.
  • The default is `false`; files are materialized read-only so an accidental edit cannot reach back into the shared CAS store.
  • The attribute is orthogonal to `update_source_with_cas`.
  • The attribute has no effect when content is fetched through the standard download path, which already produces an independent copy.

FIRST-PARTY EVIDENCE

Check the installed version and the source before using it. Entry JSON

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