dagger/dagger · error

failed to set unstable flag %s in deno.json: %w

Error message

failed to set unstable flag %s in deno.json: %w

What it means

While iterating the unstableFlags list, appendIfNotExists failed to add a flag (e.g. the `unstable` field in deno.json is not an array) to the deno.json content. The error names the specific flag and wraps the underlying cause.

Source

Thrown at sdk/typescript/runtime/tsutils/deno_config_updator.go:37

	denoConfig = removeJSONComments(denoConfig)

	// Add imports.typescript
	denoConfig, err := setIfNotExists(denoConfig, "imports.typescript", "npm:typescript@"+tsdistconsts.DefaultTypeScriptVersion)
	if err != nil {
		return "", fmt.Errorf("failed to set typescript in deno.json")
	}

	// Add nodeModulesDir="auto"
	denoConfig, err = sjson.Set(denoConfig, "nodeModulesDir", "auto")
	if err != nil {
		return "", fmt.Errorf("failed to update deno config nodeModulesDir: %w", err)
	}

	// Add unstable flags if they do not exist
	for _, flag := range unstableFlags {
		denoConfig, err = appendIfNotExists(denoConfig, "unstable", flag)
		if err != nil {
			return "", fmt.Errorf("failed to set unstable flag %s in deno.json: %w", flag, err)
		}
	}

	return denoConfig, nil
}

func UpdateDenoConfigForModule(denoConfig string) (string, error) {
	denoConfig, err := updateDenoConfigForDagger(denoConfig)
	if err != nil {
		return "", fmt.Errorf("failed to update deno config for dagger: %w", err)
	}

	// Add compilerOptions.experimentalDecorators=true
	denoConfig, err = sjson.Set(denoConfig, "compilerOptions.experimentalDecorators", true)
	if err != nil {
		return "", fmt.Errorf("failed to update deno config experimentalDecorators: %w", err)
	}

View on GitHub (pinned to 82ba2681db)

Solutions

1. Change `unstable` in deno.json to an array of strings, e.g. {"unstable": ["raw-imports"]}.
2. Remove the `unstable` key entirely and let Dagger recreate it with the required flags.
3. Re-run `dagger develop` to regenerate a correct deno.json.
4. Verify after editing: `deno check` / JSON linter accepts the file.

Example fix

// before deno.json
{"unstable": true}
// after
{"unstable": ["raw-imports"]}

const cfg = JSON.parse(Deno.readTextFileSync('deno.json'));
if ('unstable' in cfg && !Array.isArray(cfg.unstable)) {
  throw new Error('deno.json unstable must be an array of strings');
}

function unstableIsArray(cfg: Record<string, unknown>): cfg is { unstable: string[] } {
  return !('unstable' in cfg) || Array.isArray(cfg.unstable);
}

When it happens

Trigger: updateDenoConfigForDagger processing a deno.json where the `unstable` key exists but holds a non-array value (string, object, null), causing appendIfNotExists/sjson array append to fail.

Common situations: Manually setting `"unstable": true` or a string instead of an array in deno.json; older Deno config formats where unstable flags were booleans; copy-pasted config from Deno docs using deprecated boolean style.

Related errors

• failed to set typescript in deno.json
• failed to update deno config nodeModulesDir: %w
• failed to update deno config for dagger: %w
• failed to update deno config experimentalDecorators: %w
• failed to update deno.json

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/84d63b7a60cf86ff. Report an issue: GitHub.