What causes "invalid type %T for service build" in docker/cli?

Use a string context: `build: .`

invalid type %T for service build

Error message

invalid type %T for service build

What it means

Thrown by transformBuildConfig, the transformer for a service's `build` field. It accepts a string (context path) or a map (full build config with context/args/dockerfile). Any other type (list, int, bool) is rejected.

Solutions

Use a string context: `build: .`
Or a map: `build: { context: ., dockerfile: Dockerfile }`.

Example fix

# before
web:
  build:
    - .
    - Dockerfile
# after
web:
  build:
    context: .
    dockerfile: Dockerfile

Defensive patterns

Strategy: type-guard

Validate before calling

func validateBuild(build any) error {
    switch build.(type) {
    case string, map[string]any:
        return nil
    default:
        return fmt.Errorf("build must be string or map, got %T", build)
    }
}

Type guard

func isValidBuild(v any) bool {
    switch v.(type) {
    case string, map[string]any:
        return true
    }
    return false
}

Prevention

Use `build: <context>` or a `build: { context: ..., dockerfile: ... }` map.
Don't turn the build block into a list.
Render with `docker compose config`.

When it happens

Trigger: A service defines `build:` as a list (`build: [., Dockerfile]`) or a scalar number/bool.

Common situations: Mis-indenting the build block into a list; templating that yields a non-string/map.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/96cef51053663af9. Report an issue: GitHub.

Appendix: source

Thrown at cli/compose/loader/loader.go:758

var transformStringSourceMap TransformerFunc = func(data any) (any, error) {
	switch value := data.(type) {
	case string:
		return map[string]any{"source": value}, nil
	case map[string]any:
		return data, nil
	default:
		return data, fmt.Errorf("invalid type %T for secret", value)
	}
}

var transformBuildConfig TransformerFunc = func(data any) (any, error) {
	switch value := data.(type) {
	case string:
		return map[string]any{"context": value}, nil
	case map[string]any:
		return data, nil
	default:
		return data, fmt.Errorf("invalid type %T for service build", value)
	}
}

var transformServiceVolumeConfig TransformerFunc = func(data any) (any, error) {
	switch value := data.(type) {
	case string:
		return volumespec.Parse(value)
	case map[string]any:
		return data, nil
	default:
		return data, fmt.Errorf("invalid type %T for service volume", value)
	}
}

var transformServiceNetworkMap TransformerFunc = func(value any) (any, error) {
	if list, ok := value.([]any); ok {
		mapValue := map[any]any{}
		for _, name := range list {

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