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

Replace the failing variable in output_path with one available at post-process time (BuildName/BuilderType/timestamp)

Error interpolating output value: %s

Error message

Error interpolating output value: %s

What it means

In PostProcess, the compress post-processor renders the validated output_path template with interpolate.Render. Validation in Configure passed, but rendering failed at runtime — usually because a template variable (e.g. build name, generated data) could not be resolved in the current context.

Source

Thrown at post-processor/compress/post-processor.go:136

	stateData := artifact.State("generated_data")
	if stateData != nil {
		// Make sure it's not a nil map so we can assign to it later.
		generatedData = stateData.(map[interface{}]interface{})
	}
	// If stateData has a nil map generatedData will be nil
	// and we need to make sure it's not
	if generatedData == nil {
		generatedData = make(map[interface{}]interface{})
	}

	// These are extra variables that will be made available for interpolation.
	generatedData["BuildName"] = p.config.PackerBuildName
	generatedData["BuilderType"] = p.config.PackerBuilderType
	p.config.ctx.Data = generatedData

	target, err := interpolate.Render(p.config.OutputPath, &p.config.ctx)
	if err != nil {
		return nil, false, false, fmt.Errorf("Error interpolating output value: %s", err)
	} else {
		fmt.Println(target)
	}

	newArtifact := &Artifact{Path: target}

	if err = os.MkdirAll(filepath.Dir(target), os.FileMode(0755)); err != nil {
		return nil, false, false, fmt.Errorf(
			"Unable to create dir for archive %s: %s", target, err)
	}
	outputFile, err := os.Create(target)
	if err != nil {
		return nil, false, false, fmt.Errorf(
			"Unable to create archive %s: %s", target, err)
	}
	defer outputFile.Close()

	// Setup output interface. If we're using compression, output is a

View on GitHub (pinned to eb36e3c3e4)

Solutions

Replace the failing variable in output_path with one available at post-process time (BuildName/BuilderType/timestamp)
Run within a standard packer build so generated data (PackerBuildName, PackerBuilderType) is populated
Simplify output_path to a static path plus timestamp if dynamic variables are not required
Check the underlying error text to identify the unresolved variable

Example fix

// before
output = "out/{{ .SomeUnknownVar }}.tar.gz"
// after
output = "out/{{ .BuildName }}.tar.gz"

Defensive patterns

Strategy: try-catch

Validate before calling

// pre-render with the same context data to surface unknown variables early
if _, err := interpolate.Render(cfg.OutputPath, ctx); err != nil {
    return fmt.Errorf("output_path cannot be rendered: %w", err)
}

Try / catch

target, err := interpolate.Render(cfg.OutputPath, ctx)
if err != nil {
    // fall back to a static, timestamped path
    target = fmt.Sprintf("output-%d.bin", time.Now().Unix())
}

Prevention

Use only variables populated in the post-process context (BuildName, BuilderType, timestamp)
Run inside a standard packer build so generated data is set
Prefer static paths plus timestamp over exotic template variables

When it happens

Trigger: interpolate.Render(p.config.OutputPath, &p.config.ctx) errors during PostProcess — typically an unknown variable that only appears at runtime, or context data missing when invoked programmatically.

Common situations: Using {{ .BuildName }} or similar when the generated-data key isn't populated by the invoking pipeline; mixing legacy JSON templates with HCL2-only variables; calling the post-processor outside a normal packer build flow.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/20ce1b5f2abefe2d. Report an issue: GitHub.