What causes "Error parsing target template: %s" in hashicorp/packer?

Fix the template syntax — every placeholder must be {{.Variable}} with balanced braces

Error parsing target template: %s

Error message

Error parsing target template: %s

What it means

Configuration-time validation for the checksum post-processor: the `output` path is a template processed by Packer's HCL2-style interpolation engine, and interpolate.Validate rejects it when the template is syntactically invalid (unbalanced braces, unknown/misspelled variables used in ways validation rejects, malformed functions).

Source

Thrown at post-processor/checksum/post-processor.go:91

	if p.config.ChecksumTypes == nil {
		p.config.ChecksumTypes = []string{"md5"}
	}

	for _, k := range p.config.ChecksumTypes {
		if h := getHash(k); h == nil {
			errs = packersdk.MultiErrorAppend(errs,
				fmt.Errorf("Unrecognized checksum type: %s", k))
		}
	}

	if p.config.OutputPath == "" {
		p.config.OutputPath = "packer_{{.BuildName}}_{{.BuilderType}}_{{.ChecksumType}}.checksum"
	}

	if err = interpolate.Validate(p.config.OutputPath, &p.config.ctx); err != nil {
		errs = packersdk.MultiErrorAppend(
			errs, fmt.Errorf("Error parsing target template: %s", err))
	}

	if len(errs.Errors) > 0 {
		return errs
	}

	return nil
}

func (p *PostProcessor) PostProcess(ctx context.Context, ui packersdk.Ui, artifact packersdk.Artifact) (packersdk.Artifact, bool, bool, error) {
	files := artifact.Files()
	var h hash.Hash

	var generatedData map[interface{}]interface{}
	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{})

View on GitHub (pinned to eb36e3c3e4)

Solutions

Fix the template syntax — every placeholder must be {{.Variable}} with balanced braces
Only use the supported variables: .BuildName, .BuilderType, .ChecksumType
Test the path with packer validate before running the build

Example fix

// before
output = "packer/{{.BuildName}.checksum"
// after (closed placeholder / valid vars)
output = "packer/{{.BuildName}}_{{.ChecksumType}}.checksum"

Defensive patterns

Strategy: validation

Validate before calling

// Go: pre-validate the output template the same way Packer does
ctx := &interpolate.Context{BuildName: "b", BuilderType: "t"}
if err := interpolate.Validate("packer_{{.BuildName}}_{{.ChecksumType}}.checksum", ctx); err != nil {
    log.Fatalf("invalid checksum output template: %v", err)
}

Try / catch

if err := pp.Configure(raws); err != nil {
    if strings.Contains(err.Error(), "Error parsing target template") {
        // fix output placeholder syntax: {{.Var}} with balanced braces
    }
    return err
}

Prevention

Use balanced {{.Variable}} placeholders only
Stick to the supported variables: .BuildName, .BuilderType, .ChecksumType
Don't paste shell ${VAR} syntax into Packer output templates
Run packer validate on every template change

When it happens

Trigger: Setting an `output` value like "dir/{{.BuildName" (unclosed placeholder), "{{.Foo}}" (unknown variable), or invoking a nonexistent function — interpolate.Validate against the config context fails and the error is appended to errs.

Common situations: Hand-edited output paths with typos; copying shell `${VAR}` syntax instead of Packer's {{.Var}}; using variables not in the checksum context (BuildName, BuilderType, ChecksumType).

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/055e0ba1b8021dd4. Report an issue: GitHub.