hashicorp/nomad · error

failed to msgpack encode config: %v

Error message

failed to msgpack encode config: %v

What it means

After decoding the user's config via the plugin's schema, validatePluginConfig msgpack-encodes the resulting value (msgpack.Marshal(val, val.Type())) so it can be shipped to the plugin over the go-plugin msgpack connection. If encoding fails — typically because the decoded value contains types msgpack cannot serialize — the loader wraps the marshal error with this message.

Source

Thrown at helper/pluginutils/loader/init.go:501

	// If there is no config, initialize it to an empty map so we can still
	// handle defaults
	if info.config == nil {
		info.config = map[string]interface{}{}
	}

	// Parse the config using the spec
	val, diag, diagErrs := hclutils.ParseHclInterface(info.config, spec, nil)
	if diag.HasErrors() {
		_ = multierror.Append(&mErr, diagErrs...)
		return nil, multierror.Prefix(&mErr, "failed to parse config: ")

	}

	// Marshal the value
	cdata, err := msgpack.Marshal(val, val.Type())
	if err != nil {
		return nil, fmt.Errorf("failed to msgpack encode config: %v", err)
	}

	// Store the marshalled config
	info.msgpackConfig = cdata

	// Dispense the plugin and set its config and ensure it is error free
	instance, err := l.Dispense(id.Name, id.PluginType, nil, l.logger)
	if err != nil {
		return nil, fmt.Errorf("failed to dispense plugin: %v", err)
	}
	defer instance.Kill()

	b, ok := instance.Plugin().(base.BasePlugin)
	if !ok {
		return nil, fmt.Errorf("dispensed plugin %s doesn't meet base plugin interface", id)
	}

	c := &base.Config{

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Fix the plugin's ConfigSchema so it declares valid, encodable types (string/number/bool/list/map/object) — redecode your config against it.
2. Simplify the offending config value (avoid exotic nested structures or nulls) until encoding succeeds, to isolate the failing attribute.
3. Rebuild host and plugin against the same versions of the sdk/base and msgpack libraries.
4. Report/inspect via the wrapped %v cause — it names the exact type or field that failed to encode.

Example fix

// before (plugin schema uses a type decoding can't round-trip)
spec = hclspec.NewAttr("opts", "any", false)
// after (use a concrete encodable type)
spec = hclspec.NewAttr("opts", "map(string)", false)

// Validate config values against the schema and keep to plain encodable types
val, err := schema.Decode(userConfig)
if err != nil {
    return fmt.Errorf("config does not match plugin schema: %w", err)
}
if err := val.Canonicalize(); err != nil {
    return fmt.Errorf("config value not encodable: %w", err)
}

if err := loader.Load(cfg); err != nil {
    if strings.Contains(err.Error(), "failed to msgpack encode config") {
        return fmt.Errorf("config for %s contains a value the plugin cannot accept; check schema types: %w", name, err)
    }
    return err
}

When it happens

Trigger: validatePluginConfig obtains a decoded cty value from schema-implied decoding (hcldec) and msgpack.Marshal of that value errors — e.g. schema produces a type the codec can't encode, or an internal mismatch between the schema-decoded value type and the encoder's expected type.

Common situations: Plugin schema (hclspec) is malformed or declares types inconsistent with what decoding produced; custom/unknown cty types from user config; host and plugin built with different versions of the base/msgpack encoding libraries.

Related errors

• failed to encode passed object: %v
• failed to decode data into passed object: %v
• configuration not allowed but config passed
• setting config on plugin failed: %v
• invalid plugin loader configuration passed: %v

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/d2de7367f3b56f73. Report an issue: GitHub.