Explicit inherit requirement introduced in Terramate 0.12 for stack variable propagation

Answer

Starting with Terramate 0.12, variable propagation from a parent stack to a child stack is no longer implicit. A child stack must declare an explicit inherit block to receive any variables that are defined in a parent’s vars block. This change affects all Terramate HCL files and will cause “variable not found” errors if the inherit block is omitted.

Confirmed facts

• Variables defined in a parent stack’s vars block are only visible to a child when the child contains an inherit block that lists those variable names (or uses inherit {} to inherit all).
• The requirement applies to the root terramate.hcl file as well: if the root defines a vars block, child stacks still need an inherit block to access those variables.
• Terraform input variables (declared in .tf files) and environment variables are managed outside Terramate’s variable‑propagation mechanism; they do not require an inherit block.
• Values injected by Terramate generators or scripts into generated files are written as static content or references; the inherit block governs only the Terramate configuration layer, not the resulting file content.

Likely explanation / remaining uncertainty

The documentation states that the inherit block is required for “variables defined in parent stacks.” Based on the internal design, this rule is limited to variables that originate from a vars block anywhere in the inheritance chain. Variables that are:

• Set by Terraform CLI (e.g., -var flags) or declared in .tf files,
• Read from the process environment (e.g., $MY_VAR), or
• Produced by a generator/script and embedded directly into a template,

are not subject to the inherit requirement because they never travel through Terramate’s variable‑propagation system.

Verification steps

1. Check the Terramate version: terramate version (must be 0.12 or higher).
2. Create a parent stack with a vars block (e.g., vars { my_var = "value" }).
3. In a child stack, reference my_var without an inherit block and run terramate run; you should see a “variable not found” error.
4. Add an inherit block to the child stack (inherit {} or inherit { my_var }) and re‑run; the variable should be accessible.
5. Repeat the test with a Terraform input variable (-var"my_var=…") or an environment variable; the child stack should read it without any inherit block.

Missing diagnostic detail

If you are observing errors with variables that come from a generator or script output, please confirm whether the variable is being read from the generated file (static value) or accessed via Terramate’s {{ var }} syntax, as this determines whether the inherit block is needed.