microsoft/semantic-kernel · error · KernelException

The headers parameter '{parameterStyle}' serialization style

Error message

The headers parameter '{parameterStyle}' serialization style is not supported.

What it means

Thrown by RestApiOperation.BuildHeaders when a header parameter's effective style (parameter.Style ?? Simple) is not present in the s_parameterSerializers dictionary. The supported styles are Simple, Form, SpaceDelimited, and PipeDelimited. Any other style (Label, Matrix, DeepObject) for a header parameter is unsupported and rejected at request-build time.

Source

Thrown at dotnet/src/Functions/Functions.OpenApi/Model/RestApiOperation.cs:233

    {
        var headers = new Dictionary<string, string>();

        var parameters = this.Parameters.Where(p => p.Location == RestApiParameterLocation.Header);

        foreach (var parameter in parameters)
        {
            var argument = this.GetArgumentForParameter(arguments, parameter);
            if (argument == null)
            {
                // Skipping not required parameter if no argument provided for it.    
                continue;
            }

            var parameterStyle = parameter.Style ?? RestApiParameterStyle.Simple;

            if (!s_parameterSerializers.TryGetValue(parameterStyle, out var serializer))
            {
                throw new KernelException($"The headers parameter '{parameterStyle}' serialization style is not supported.");
            }

            var node = OpenApiTypeConverter.Convert(parameter.Name, parameter.Type, argument, parameter.Schema);

            //Serializing the parameter and adding it to the headers.
            headers.Add(parameter.Name, serializer.Invoke(parameter, node));
        }

        return headers;
    }

    /// <summary>
    /// Builds the operation query string.
    /// </summary>
    /// <param name="arguments">The operation arguments.</param>
    /// <returns>The query string.</returns>
    internal string BuildQueryString(IDictionary<string, object?> arguments)
    {

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Edit the spec: change the offending header parameter's style to 'simple' (or remove the style field so Simple is used).
2. If the style is genuinely required by the server, contribute a serializer or preprocess the document to remap the style.
3. Filter the parameter out via OpenApiFunctionExecutionParameters.ParameterFilter if it is not needed for your calls.

Example fix

// before - unsupported header style
{ "name": "X-Tag", "in": "header", "style": "label", "schema": { } }

// after - supported (default) style
{ "name": "X-Tag", "in": "header", "style": "simple", "schema": { } }

var supported = new[] { "simple", "form", "spaceDelimited", "pipeDelimited" };
// iterate header params and flag label/matrix/deepObject before importing

static readonly HashSet<string> SupportedHeaderStyles = new() { "simple", "form", "spaceDelimited", "pipeDelimited" };
static bool IsSupportedHeaderStyle(string? style) => style is null || SupportedHeaderStyles.Contains(style.ToLowerInvariant());

try { operation.BuildHeaders(arguments); }
catch (KernelException ex) when (ex.Message.Contains("headers parameter") && ex.Message.Contains("serialization style is not supported"))
{ logger.LogError(ex, "Unsupported header serialization style in spec."); throw; }

When it happens

Trigger: An OpenAPI header parameter declared with style label or style matrix (or deepObject), none of which SK has a serializer for. The default for headers is Simple, so a missing style is fine; only an explicit unsupported style triggers this.

Common situations: A spec authored with an unusual header serialization style; a tool that emits label/matrix styles by default for header params; copying a spec that targets a different client library with broader style support.

Related errors

• The query string parameter '{parameterStyle}' serialization
• The path parameter '{parameterStyle}' serialization style is
• Payload parameters cannot be retrieved from the '{operation.
• The operation path resolves to '{requestAuthority}', which d
• The operation path resolves to '{requestPath}', which is out

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/2288043f23d02190. Report an issue: GitHub.