What causes "BuildFlagsList to reflect value: %s" in kubernetes/kops?

Read the wrapped (%s) inner error to identify which field's conversion or traversal failed

BuildFlagsList to reflect value: %s

Error message

BuildFlagsList to reflect value: %s

What it means

BuildConfigYaml walks the options struct with reflectutils.ReflectRecursive to copy tagged fields into the target config. This error wraps any failure returned by that recursive reflection (including walker errors like the conversion errors above). It is the top-level wrapper meaning "reflecting the options struct into the config file failed".

Source

Thrown at pkg/configbuilder/buildconfigfile.go:81

			}
		}
		switch v := val.Interface().(type) {
		case *resource.Quantity:
			floatVal, err := strconv.ParseFloat(v.AsDec().String(), 64)
			if err != nil {
				return fmt.Errorf("unable to convert from Quantity %v to float", v)
			}
			targetValue.Set(reflect.ValueOf(&floatVal))
		default:
			targetValue.Set(val)
		}

		return reflectutils.SkipReflection
	}

	err := reflectutils.ReflectRecursive(reflect.ValueOf(options), walker, &reflectutils.ReflectOptions{DeprecatedDoubleVisit: true})
	if err != nil {
		return nil, fmt.Errorf("BuildFlagsList to reflect value: %s", err)
	}

	configFile, err := yaml.Marshal(target)
	if err != nil {
		return nil, err
	}

	return configFile, nil
}

func getValueFromStruct(keyWithDots string, object interface{}) (*reflect.Value, error) {
	keySlice := strings.Split(keyWithDots, ".")
	v := reflect.ValueOf(object)
	// iterate through field names, ignoring the first name as it might be the current instance name
	// you can make it recursive also if want to support types like slice,map etc along with struct
	for _, key := range keySlice {
		for v.Kind() == reflect.Ptr {
			v = v.Elem()

View on GitHub (pinned to 4c8573c808)

Solutions

Read the wrapped (%s) inner error to identify which field's conversion or traversal failed
Fix the offending configfile tag or target struct field indicated by the inner error
Run the existing tests (TestParseDefault/TestParse, TestRewriteChannelsManifestForEnroll-style coverage) for BuildConfigYaml to reproduce locally
If reflectutils traversal itself fails on a new type, add a `configfile:"-"` tag to skip that subtree

Example fix

// before
err := reflectutils.ReflectRecursive(reflect.ValueOf(options), walker, &reflectutils.ReflectOptions{DeprecatedDoubleVisit: true})
if err != nil {
	return nil, fmt.Errorf("BuildFlagsList to reflect value: %s", err)
}
// after
err := reflectutils.ReflectRecursive(reflect.ValueOf(options), walker, &reflectutils.ReflectOptions{DeprecatedDoubleVisit: true})
if err != nil {
	return nil, fmt.Errorf("building scheduler config from options: %w", err)
}

Defensive patterns

Strategy: try-catch

Try / catch

cfg, err := configbuilder.BuildConfigYaml(options, target)
if err != nil {
	if strings.Contains(err.Error(), "BuildFlagsList to reflect value") {
		// log the wrapped inner cause to find the offending field/tag
		return nil, fmt.Errorf("config build failed: %w", err)
	}
	return nil, err
}

Prevention

Keep KubeSchedulerConfig tags and the target component-config struct versioned together
Run TestParse/TestParseDefault whenever either struct changes
Use %w instead of %s in wrappers so callers can errors.As/errors.Is on inner causes
Skip unsupported subtrees with configfile:"-" tags before they reach the reflection walk

When it happens

Trigger: Any walker error during ReflectRecursive over *kops.KubeSchedulerConfig — e.g. a configfile tag naming a missing target field, an invalid Quantity conversion, or reflectutils failing to traverse an unsupported type in the options tree.

Common situations: Generating kube-scheduler component config during `kops toolbox`/cluster rendering after a field or tag mismatch was introduced in KubeSchedulerConfig; upstream component-config type changes breaking the tag-to-target mapping.

Related errors

AI-assisted analysis of kubernetes/kops@4c8573c808 (2026-09-05). Data as JSON: /api/errors/027faecd0db9d3cc. Report an issue: GitHub.