microsoft/semantic-kernel · error · KernelException

The query string parameter '{parameterStyle}' serialization

Error message

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

What it means

Thrown by RestApiOperation.BuildQueryString when a query parameter's effective style (parameter.Style ?? Form) is not in s_parameterSerializers. Supported styles are Simple, Form, SpaceDelimited, and PipeDelimited. Unsupported styles for query (Label, Matrix, DeepObject) are rejected when the request query string is built.

Source

Thrown at dotnet/src/Functions/Functions.OpenApi/Model/RestApiOperation.cs:269

    {
        var segments = new List<string>();

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

        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.Form;

            if (!s_parameterSerializers.TryGetValue(parameterStyle, out var serializer))
            {
                throw new KernelException($"The query string 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 query string if there's an argument for it.
            segments.Add(serializer.Invoke(parameter, node));
        }

        return string.Join("&", segments);
    }

    /// <summary>
    /// Makes the current instance unmodifiable.
    /// </summary>
    internal void Freeze()
    {
        this._freezable.Freeze();
        this.Payload?.Freeze();

View on GitHub (pinned to c028a0c7dc)

Solutions

Change the query parameter style to 'form' (default) or 'spaceDelimited'/'pipeDelimited' if the array shape fits.
For deepObject needs, flatten the nested fields into separate simple query parameters in the spec.
If unsupported at the spec level, preprocess the document or use a ParameterFilter to drop/replace the parameter.

Example fix

// before - deepObject style is not serialized
{ "name": "filter", "in": "query", "style": "deepObject", "schema": { } }

// after - flatten to form-style parameters
{ "name": "filter[name]", "in": "query", "style": "form", "schema": { } }

Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

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

Try / catch

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

Prevention

Use form (or omit style) for query parameters.
Flatten deepObject-style nested query params into simple form params.
Pre-scan the spec for unsupported query styles.

When it happens

Trigger: An OpenAPI query parameter declared with style label, style matrix, or style deepObject. The default for query is Form, so an absent style is safe; only an explicit unsupported style triggers this.

Common situations: A spec using deepObject for nested query serialization (common in some API styles) which SK does not implement; matrix or label styles copied from another toolchain.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/541bd84d9e558dfb. Report an issue: GitHub.