tofu test: a run block's `variables` cannot reference the test file's top-level `variables`

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OpenTofu Version

OpenTofu v1.12.6
OpenTofu v1.13.0-beta1

Both affected. Terraform 1.8.3 and 1.16.2 accept the same configuration.

OpenTofu Configuration Files

main.tf:

variable "input" {
  type = string
}

output "out" {
  value = var.input
}

main.tftest.hcl:

variables {
  helper = "hello"
}

run "check" {
  command = plan

  variables {
    input = var.helper
  }

  assert {
    condition     = output.out == "hello"
    error_message = "expected hello"
  }
}

Note that helper is declared only in the test file's top-level variables block — it is deliberately not a variable of the module under test.

Debug Output

main.tftest.hcl... fail
  run "check"... fail

Error: No value for required variable

  on main.tf line 1:
   1: variable "input" {

The root module input variable "input" is not set, and has no default value.

Error: Unsupported attribute

  on main.tftest.hcl line 9, in run "check":
   9:     input = var.helper
    ├────────────────
    │ var is object with no attributes

This object does not have an attribute named "helper".

Failure! 0 passed, 1 failed.

Expected Behavior

var.helper inside a run block's variables block resolves to the value defined in the test file's top-level variables block, so the module receives input = "hello" and the test passes. This is what Terraform does — terraform test reports Success! 1 passed, 0 failed. on 1.8.3 and on 1.16.2.

Actual Behavior

var is an object with no attributes inside the run block's variables block, so the reference fails and the module variable ends up unset.

Steps to Reproduce

1. tofu init
2. tofu test

Additional Context

This pattern is useful for expressing a value once and reusing it in both the inputs of a run block and its assertions, which is how we hit it: a shared module's test file defines a set of expected backoff/retry values at file level and references them from several run blocks.

#1213 added support for var. references in the top-level variables block (resolving to module input values supplied via tfvars/TF_VAR_). This report is the other direction: referencing a top-level test variable from a run block. I could not find an existing issue covering it.

References

• #1213 — related, but about var. inside the top-level variables block

Workaround

Verified working on OpenTofu 1.12.6 and 1.13.0-beta1, and still passing on Terraform 1.8.3 and 1.16.2, so the reshaped file stays portable:

• Inline the value at each use site and delete the top-level variables block, or
• if the value is worth defining once, declare it as a variable of the module under test with a default and reference var.<name> — module variables resolve correctly in both tools.

The second option keeps the "define once" property but changes the module's public surface, which is unattractive for a shared module. Neither is a reason not to fix the reference, but both unblock today.