microsoft/aspire · error · InvalidOperationException

The Next.js config file

Error message

The Next.js config file '{configFileName}' does not contain 'output: "standalone"'. AddNextJsApp requires Next.js standalone output mode to generate a working Dockerfile. Add 'output: "standalone"' to the nextConfig object in your Next.js config file.

What it means

AddNextJsApp generates a Dockerfile that depends on Next.js standalone output (which emits a self-contained server into .next/standalone). The library reads the Next.js config file at model-build time and throws if it cannot find an 'output: "standalone"' setting, because without it the published container would not run.

Solutions

1. Add output: "standalone" to the nextConfig object in next.config.js/.mjs/.ts.
2. Quote the standalone value (output: "standalone" or output: 'standalone') — unquoted output: standalone will not satisfy the string check in some validation paths.
3. Re-run the AppHost after saving the config change.

Example fix

// before (next.config.js)
const nextConfig = {};
// after
const nextConfig = { output: "standalone" };

var config = File.ReadAllText(Path.Combine(appDir, "next.config.mjs"));
if (!config.Contains("\"standalone\"") && !config.Contains("'standalone'"))
    throw new InvalidOperationException("next.config must set output: \"standalone\" (quoted).");

try { builder.AddNextJsApp("web", "./web"); } catch (InvalidOperationException ex) when (ex.Message.Contains("standalone")) { /* fix next.config and re-run */ }

When it happens

Trigger: Calling AddNextJsApp when next.config.js/.mjs/.ts exists but lacks output:"standalone" (or uses unquoted standalone — the check only matches "standalone" or 'standalone' literals, so output: standalone without quotes fails the Contains check).

Common situations: Scaffolding a new Next.js app with create-next-app (defaults to no standalone output); manually-written config with output: standalone unquoted (TypeScript-valid but fails this string check); config file unreadable is skipped, but a readable file missing the flag throws.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• No Next.js configuration file found. AddNextJsApp expects…
• The path must be a relative path.
• The path must not contain ".." segments.
• Error applying Aspire Vite configuration. Falling back to…
• The apiPath must contain only URL-safe path characters…

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/fc3c9b8a3807bf95. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.JavaScript/JavaScriptHostingExtensions.cs:3174

    /// </summary>
    internal static void ValidateNextJsStandaloneOutput(string appDirectory)
    {
        foreach (var configFileName in s_nextConfigFileNames)
        {
            var configPath = Path.Combine(appDirectory, configFileName);
            if (!File.Exists(configPath))
            {
                continue;
            }

            try
            {
                var content = File.ReadAllText(configPath);

                // Check for quoted "standalone" (double or single quotes) to reduce false positives
                if (!content.Contains("\"standalone\"") && !content.Contains("'standalone'"))
                {
                    throw new InvalidOperationException(
                        $"The Next.js config file '{configFileName}' does not contain 'output: \"standalone\"'. " +
                        "AddNextJsApp requires Next.js standalone output mode to generate a working Dockerfile. " +
                        "Add 'output: \"standalone\"' to the nextConfig object in your Next.js config file.");
                }
            }
            catch (IOException)
            {
                // If we can't read the config, skip the check — the Docker build will surface the error.
            }

            return;
        }

        throw new InvalidOperationException(
            "No Next.js configuration file found. AddNextJsApp expects one of: " +
            string.Join(", ", s_nextConfigFileNames));
    }

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