What causes "error parsing output_dir template: %w" in hashicorp/packer?

Fix the template syntax in `output_dir`; ensure `{{ .Variable }}` style with balanced braces.

error parsing output_dir template: %w

Error message

error parsing output_dir template: %w

What it means

During Configure, the provenance post-processor validates the `output_dir` string with `interpolate.Validate` before any build runs. Malformed template syntax (unbalanced braces, unknown functions, invalid variable names) returns `error parsing output_dir template: %w`. Configure fails immediately, so the build never starts.

Source

Thrown at post-processor/provenance/post-processor.go:158

	if p.config.SBOMFormat == "" {
		p.config.SBOMFormat = string(internalsbom.FormatCycloneDX)
	}
	if p.config.SBOMScope == "" {
		p.config.SBOMScope = internalsbom.ScopeSquashed
	}
	if p.config.SigningMode == "" {
		p.config.SigningMode = internalattestation.SigningModeNone
	}
	if p.config.FulcioURL == "" {
		p.config.FulcioURL = "https://fulcio.sigstore.dev"
	}
	if p.config.RekorURL == "" {
		p.config.RekorURL = "https://rekor.sigstore.dev"
	}

	if p.config.OutputDir != "" {
		if err := interpolate.Validate(p.config.OutputDir, &p.config.ctx); err != nil {
			return fmt.Errorf("error parsing output_dir template: %w", err)
		}
	}

	if _, err := p.signingBackendConfig(); err != nil {
		return err
	}

	if p.generateSBOM == nil {
		p.generateSBOM = func(ctx context.Context, cfg internalsbom.Config) ([]byte, error) {
			return internalsbom.NewGenerator(cfg).Generate(ctx)
		}
	}

	return nil
}

func (p *PostProcessor) PostProcess(ctx context.Context, ui packersdk.Ui, source packersdk.Artifact) (packersdk.Artifact, bool, bool, error) {
	if p.config.Provenance.False() {

View on GitHub (pinned to eb36e3c3e4)

Solutions

Fix the template syntax in `output_dir`; ensure `{{ .Variable }}` style with balanced braces.
Run `packer validate` to check the template before building.
Read the wrapped error after the colon for the exact parse offset/function.
If you don't need templating, use a plain literal path with no `{{ }}` at all.

Example fix

// before
output_dir = "attestations/{{ .BuildName"
// after
output_dir = "attestations/${build.name}"

Defensive patterns

Strategy: validation

Validate before calling

// validate output_dir template at config-load time
if err := interpolate.Validate(cfg.OutputDir, &ctx); err != nil {
    return fmt.Errorf("bad output_dir: %w", err)
}

Try / catch

// Configure already returns the error; just fail fast with context
if err := pp.Configure(raws); err != nil {
    return fmt.Errorf("provenance config rejected: %w", err)
}

Prevention

Run `packer validate` before every build.
Avoid hand-writing `{{ }}` templates; copy known-good examples.
Use plain literal paths when templating isn't needed.
Escape braces if you need literal `{{` in output paths.

When it happens

Trigger: Setting `output_dir` in the provenance post-processor to a value like `out/{{ .BuildName` (unclosed `{{`) or using an unknown function `{{ notAFunction "x" }}`. The error surfaces as a Configure failure at the start of `packer build`.

Common situations: Typos in template functions; HCL2 strings where `{{` must be escaped as `{{` inside quotes plus `%{` for directives confusion; porting JSON-templates syntax into HCL2.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/55c9a65be411e208. Report an issue: GitHub.