microsoft/semantic-kernel · error · ArgumentException

Invalid template format: {config.TemplateFormat}

Error message

Invalid template format: {config.TemplateFormat}

What it means

Thrown by the LiquidPromptTemplate constructor when `config.TemplateFormat` does not equal `LiquidPromptTemplateFactory.LiquidTemplateFormat` (the liquid format identifier). This is a strict guard: the Liquid template engine only handles Liquid-format configs, and passing a mismatched format (e.g. a Handlebars or semantic-kernel format string) is treated as a programming error rather than a render error.

Source

Thrown at dotnet/src/Extensions/PromptTemplates.Liquid/LiquidPromptTemplate.cs:55

#else
    private static Regex RoleRegex() => s_roleRegex;
    private static readonly Regex s_roleRegex = new(@"(?<role>system|assistant|user|function|developer):\s+", RegexOptions.Compiled);
#endif

    /// <summary>Initializes the <see cref="LiquidPromptTemplate"/>.</summary>
    /// <param name="config">Prompt template configuration</param>
    /// <param name="allowDangerouslySetContent">Whether to allow dangerously set content in the template</param>
    /// <exception cref="ArgumentException">throw if <see cref="PromptTemplateConfig.TemplateFormat"/> is not <see cref="LiquidPromptTemplateFactory.LiquidTemplateFormat"/></exception>
    /// <exception cref="ArgumentException">The template in <paramref name="config"/> could not be parsed.</exception>
    /// <exception cref="ArgumentNullException">throw if <paramref name="config"/> is null</exception>
    /// <exception cref="ArgumentNullException">throw if the template in <paramref name="config"/> is null</exception>
    public LiquidPromptTemplate(PromptTemplateConfig config, bool allowDangerouslySetContent = false)
    {
        Verify.NotNull(config, nameof(config));
        Verify.NotNull(config.Template, nameof(config.Template));
        if (config.TemplateFormat != LiquidPromptTemplateFactory.LiquidTemplateFormat)
        {
            throw new ArgumentException($"Invalid template format: {config.TemplateFormat}");
        }

        this._allowDangerouslySetContent = allowDangerouslySetContent;
        this._config = config;

        // Parse the template now so we can check for errors, understand variable usage, and
        // avoid having to parse on each render.
        if (!s_parser.TryParse(config.Template, out this._liquidTemplate, out string error))
        {
            throw new ArgumentException(error is not null ?
                $"The template could not be parsed:{Environment.NewLine}{error}" :
                 "The template could not be parsed.");
        }

        // Ideally the prompty author would have explicitly specified input variables. If they specified any,
        // assume they specified them all. If they didn't, heuristically try to find the variables, looking for
        // variables that are read but never written and that appear to be simple values rather than complex objects.
        if (config.InputVariables.Count == 0)

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Set `config.TemplateFormat = LiquidPromptTemplateFactory.LiquidTemplateFormat` before constructing.
2. Use `LiquidPromptTemplateFactory` to create the template so the format is set correctly for you.
3. Verify the format string spelling against the constant rather than hard-coding a literal.

Example fix

// before
var config = new PromptTemplateConfig { TemplateFormat = "handlebars", Template = "{{user}}" };
var t = new LiquidPromptTemplate(config);
// after
var config = new PromptTemplateConfig { TemplateFormat = LiquidPromptTemplateFactory.LiquidTemplateFormat, Template = "{{user}}" };
var t = new LiquidPromptTemplate(config);

if (config.TemplateFormat != LiquidPromptTemplateFactory.LiquidTemplateFormat) {
    config.TemplateFormat = LiquidPromptTemplateFactory.LiquidTemplateFormat;
}
var template = new LiquidPromptTemplate(config);

try { var t = new LiquidPromptTemplate(config); }
catch (ArgumentException ex) when (ex.Message.Contains("Invalid template format")) {
    // log and surface a clear config error to the user
}

When it happens

Trigger: Constructing `new LiquidPromptTemplate(config)` where `config.TemplateFormat` was set to `PromptTemplateConfig.SemanticKernelTemplateFormat` or a custom string, or reusing a config originally built for another factory.

Common situations: Mixing template engines in one app; copy-pasting a PromptTemplateConfig from a Handlebars example into Liquid code; reading config from JSON/YAML where the format field is wrong or misspelled.

Related errors

• Message must have a role.
• The template could not be parsed:{Environment.NewLine}{error
• Argument '{propertyName}' has a value that doesn't support a
• Configuration not found, please setup the notebooks first us
• AZURE_OPENAI_ENDPOINT is not set.

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/b8feb26cad1f6acd. Report an issue: GitHub.