hashicorp/packer · error

Error interpolating builder '%s': %s

Error message

Error interpolating builder '%s': %s

What it means

During Core initialization (packer/core.go initialize), builder names from the template are rendered through HCL2/interpolation with the core's context. If rendering a builder's Name fails (e.g. an unknown or unresolvable user variable), initialize returns "Error interpolating builder '%s': %s" naming the builder and the interpolation error. Core initialization then aborts before any build runs.

Source

Thrown at packer/core.go:187

func (core *Core) initialize() error {
	if err := core.validate(); err != nil {
		return err
	}
	if err := core.init(); err != nil {
		return err
	}
	for _, secret := range core.secrets {
		RegisterSecret(secret)
	}

	// Go through and interpolate all the build names. We should be able
	// to do this at this point with the variables.
	core.builds = make(map[string]*template.Builder)
	for _, b := range core.Template.Builders {
		v, err := interpolate.Render(b.Name, core.Context())
		if err != nil {
			return fmt.Errorf(
				"Error interpolating builder '%s': %s",
				b.Name, err)
		}

		core.builds[v] = b
	}

	return nil
}

func (c *Core) PluginRequirements() (plugingetter.Requirements, hcl.Diagnostics) {
	return nil, hcl.Diagnostics{
		&hcl.Diagnostic{
			Summary:  "Packer plugins currently only works with HCL2 configuration templates",
			Detail:   "Please manually install plugins with the plugins command or use a HCL2 configuration that will do that for you.",
			Severity: hcl.DiagError,
		},
	}

View on GitHub (pinned to eb36e3c3e4)

Solutions

1. Read the inner interpolation error — it names the unresolved variable
2. Provide the missing value: packer build -var 'name=myname' ... or a var-file
3. Correct the variable/function spelling in the builder name in the template
4. Run `packer init` and validate the template (`packer validate`) to catch undefined variables before building

Example fix

// before (shell)
packer build template.pkr.hcl   # builder name uses ${local.env_name}, undefined
// after (shell)
packer build -var 'env_name=prod' template.pkr.hcl

// Validate the template and all variables before building:
//   packer validate -var 'name=myname' template.pkr.hcl
// In code, render names with the same context first:
rendered, err := interpolate.Render(builderName, core.Context())
if err != nil {
    return fmt.Errorf("builder %q uses an unresolvable variable: %w", builderName, err)
}

if err := core.Initialize(); err != nil {
    if strings.Contains(err.Error(), "Error interpolating builder") {
        return fmt.Errorf("template references undefined variables; check -var/-var-file: %w", err)
    }
    return err
}

When it happens

Trigger: Calling packer's build/init path with a template whose builder name contains variables (e.g. "{{user `name`}}") that are undefined, misspelled, or not provided via -var/-var-file/env, so interpolate.Render(b.Name, core.Context()) errors.

Common situations: Renaming builders to include user variables and forgetting to pass -var; typos in variable names; using functions/variables unavailable at name-interpolation time; legacy JSON templates referencing variables removed in a template refactor.

Related errors

• Error interpolating output value: %s
• Failed to interpolate %s: Please make sure that the variable
• Error parsing target template: %s
• Error parsing target template: %s
• Error interpolating custom data: %s

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/cd6b819f6db97c7a. Report an issue: GitHub.