docker/compose · error

cannot render project into yaml: %w

Error message

cannot render project into yaml: %w

What it means

In the compose-to-Podman-bridge converter (bridge.Convert), the project is serialized back to YAML (project.MarshalYAML with types.WithSecretContent) so transformers can operate on compose-style attribute names. This error means that serialization itself failed. The %w wraps the underlying marshal error, which almost always indicates data in the in-memory project that cannot be represented in the compose YAML schema.

Source

Thrown at pkg/bridge/convert.go:64

	Output          string
	Templates       string
	Transformations []string
}

func Convert(ctx context.Context, dockerCli command.Cli, project *types.Project, opts ConvertOptions) error {
	if len(opts.Transformations) == 0 {
		opts.Transformations = []string{DefaultTransformerImage}
	}
	// Load image references, secrets and configs, also expose ports
	project, err := LoadAdditionalResources(ctx, dockerCli, project)
	if err != nil {
		return err
	}
	// for user to rely on compose.yaml attribute names, not go struct ones, we marshall back into YAML
	raw, err := project.MarshalYAML(types.WithSecretContent)
	// Marshall to YAML
	if err != nil {
		return fmt.Errorf("cannot render project into yaml: %w", err)
	}
	var model map[string]any
	err = yaml.Unmarshal(raw, &model)
	if err != nil {
		return fmt.Errorf("cannot render project into yaml: %w", err)
	}

	if opts.Output != "" {
		_ = os.RemoveAll(opts.Output)
		err := os.MkdirAll(opts.Output, 0o744)
		if err != nil && !os.IsExist(err) {
			return fmt.Errorf("cannot create output folder: %w", err)
		}
	}
	// Run Transformers images
	return convert(ctx, dockerCli, model, opts)
}

View on GitHub (pinned to ddc4b044b6)

Solutions

1. Inspect the wrapped error (%w) to identify which field failed to marshal
2. Reproduce with docker compose config on the same file to see whether the CLI itself rejects the model
3. Upgrade docker/compose and compose-go to matching versions so model structs and YAML schema agree
4. Simplify the project (remove the suspicious service/field) and re-add pieces until the failing attribute is isolated

if err := bridge.Convert(ctx, dockerCli, project, opts); err != nil {
	if strings.Contains(err.Error(), "cannot render project into yaml") {
		// inspect project for non-serializable fields; fall back to a file-loaded project
		log.Printf("conversion input invalid: %v", errors.Unwrap(err))
	}
	return err
}

When it happens

Trigger: Calling bridge.Convert with a programmatically built *types.Project containing values that do not survive marshal/unmarshal (custom types, invalid map keys, unsupported fields); a project mutated by LoadAdditionalResources into a state the YAML schema rejects.

Common situations: Using the bridge converter (docker compose-to-podman style flows) on a project with unusual or manually constructed fields; version mismatches between compose-go model structs and the YAML schema; injecting non-serializable content into service maps before conversion.

Related errors

• cannot create output folder: %w
• healthcheck.start_interval requires healthcheck.start_period
• output folder %s already exists
• cannot create output folder: %w
• saving creds for API socket: %w

AI-assisted analysis of docker/compose@ddc4b044b6 (2026-08-15). Data as JSON: /api/errors/bab542ab70354c57. Report an issue: GitHub.