What causes "Error interpolating source: %s" in hashicorp/packer?

Correct the variable name or syntax in the source path

Error interpolating source: %s

Error message

Error interpolating source: %s

What it means

In ProvisionDownload, each source path is interpolated with the provisioner's template context. A failure (unknown variable, bad syntax, missing generated data) aborts the whole download loop with this error. The underlying renderer error is embedded after the colon.

Source

Thrown at provisioner/file/provisioner.go:168

	}

	if p.config.Direction == "download" {
		return p.ProvisionDownload(ui, comm)
	} else {
		return p.ProvisionUpload(ui, comm)
	}
}

func (p *Provisioner) ProvisionDownload(ui packersdk.Ui, comm packersdk.Communicator) error {
	dst, err := interpolate.Render(p.config.Destination, &p.config.ctx)
	if err != nil {
		return fmt.Errorf("Error interpolating destination: %s", err)
	}
	for _, src := range p.config.Sources {
		dst := dst
		src, err := interpolate.Render(src, &p.config.ctx)
		if err != nil {
			return fmt.Errorf("Error interpolating source: %s", err)
		}

		// ensure destination dir exists.  p.config.Destination may either be a file or a dir.
		dir := dst
		// if it doesn't end with a /, set dir as the parent dir
		if !strings.HasSuffix(dst, "/") {
			dir = filepath.Dir(dir)
		} else if !strings.HasSuffix(src, "/") && !strings.HasSuffix(src, "*") {
			dst = filepath.Join(dst, filepath.Base(src))
		}
		ui.Say(fmt.Sprintf("Downloading %s => %s", src, dst))

		if dir != "" {
			err := os.MkdirAll(dir, os.FileMode(0755))
			if err != nil {
				return err
			}
		}

View on GitHub (pinned to eb36e3c3e4)

Solutions

Correct the variable name or syntax in the source path
Use only generated-data keys documented for the builder
Simplify the source path to a literal string if no substitution is needed
Run packer validate / inspect the wrapped error for the exact failing key

Example fix

// before
source = "{{ .BuildName }}/{{ .Nonexistent }}.log"
// after
source = "{{ build_name }}.log"

Defensive patterns

Strategy: try-catch

Validate before calling

// Test source templates against the builder's generated data before building:
// packer validate template.pkr.hcl

Try / catch

if err := provision(ctx, ui, comm, generated); err != nil {
    if strings.Contains(err.Error(), "Error interpolating source") {
        // fix the offending source template variable
    }
}

Prevention

Check builder docs for which {{ . }} fields are available in file sources
Avoid deep variable chains in source globs; keep sources simple
Run packer validate and a smoke build before CI

When it happens

Trigger: direction=download and a source entry contains template placeholders that fail to render, e.g. {{ .HostVars.x }} not exposed by the builder or malformed {{ syntax.

Common situations: Download sources using builder-specific generated data not available (e.g. {{ .Flavor }} only on some builders); typos in variable names; using provisioning-state functions not valid in this context.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/f8b4dee5b2cd3c7d. Report an issue: GitHub.