hashicorp/packer · error

error interpolating default value for '%s': %s

Error message

error interpolating default value for '%s': %s

What it means

`renderVarsRecursively` interpolates each variable's default with `interpolate.RenderRegex` and switches on the error type. `nil` means success, `ttmp.ExecError` containing 'variable not set' triggers a retry loop, but any other error type is unexpected and aborts immediately with `error interpolating default value for '%s': %s`, naming the variable key and the underlying interpolation error. This indicates malformed template-engine syntax or a render failure that is not simply an unset user variable.

Source

Thrown at packer/core.go:1016

				// variables that still need interpolating for a repeat.
				done, err := isDoneInterpolating(kv.Value)
				if err != nil {
					return ctx, err
				}
				if done {
					deleteKeys = append(deleteKeys, kv.Key)
				} else {
					shouldRetry = true
				}
			case ttmp.ExecError:
				if strings.Contains(err.Error(), interpolate.ErrVariableNotSetString) {
					shouldRetry = true
					failedInterpolation = fmt.Sprintf(`"%s": "%s"; error: %s`, kv.Key, kv.Value, err)
				} else {
					return ctx, err
				}
			default:
				return ctx, fmt.Errorf(
					// unexpected interpolation error: abort the run
					"error interpolating default value for '%s': %s",
					kv.Key, err)
			}
		}
		if !shouldRetry {
			break
		}

		// Clear completed vars from sortedMap before next loop. Do this one
		// key at a time because the indices are gonna change ever time you
		// delete from the map.
		for _, k := range deleteKeys {
			for ind, kv := range sortedMap {
				if kv.Key == k {
					sortedMap = append(sortedMap[:ind], sortedMap[ind+1:]...)
					break
				}

View on GitHub (pinned to eb36e3c3e4)

Solutions

  1. Look at the variable key named in the error and inspect its default value's `{{ }}` syntax for typos or unclosed braces/backticks.
  2. Fix or remove the invalid template expression in the variable definition.
  3. Confirm the function used exists in Packer's interpolation function list (e.g. `timestamp`, `env`, `user`) and is spelled correctly.
  4. If the variable genuinely references an unset user variable, that case is handled separately — supply the value with `-var` so it resolves instead of erroring.

Example fix

// before
"variables": {
  "name": "{{user `base`}}-{{timestampa}}"
}
// after
"variables": {
  "name": "{{user `base`}}-{{timestamp}}"
}
Defensive patterns

Strategy: try-catch

Validate before calling

// sanity-check a variable default's template syntax before building
for name, val := range variables {
    if strings.Count(val, "{{") != strings.Count(val, "}}") ||
       strings.Count(val, "`")%2 != 0 {
        return fmt.Errorf("variable %s has unbalanced template syntax", name)
    }
}

Try / catch

// Go: distinguish unexpected interpolation failures
if err := core.Initialize(); err != nil {
    if strings.Contains(err.Error(), "error interpolating default value for") {
        // extract the key between quotes and surface a config-fix message
        return fmt.Errorf("fix variable template syntax: %w", err)
    }
    return err
}

Prevention

When it happens

Trigger: Called during `Core.init()`; `interpolate.RenderRegex(kv.Value, ctx, renderFilter)` returns an error that is neither nil nor a `ttmp.ExecError` with the variable-not-set string — e.g. badly nested/unclosed `{{ }}` template blocks, unknown template functions, or other text-template parse/exec failures in a variable default.

Common situations: Typo in a template function inside a variable default (e.g. `{{timestampa}}`); unbalanced backticks or braces in `{{user `foo`}}`; copying shell-style `${}` syntax into a JSON template variable; escaping problems after migrating a template between JSON and HCL2.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/71e0bd486ab5964c. Report an issue: GitHub.