microsoft/semantic-kernel · error · KernelException

Parameter style of {parameter.Name} parameter of {operationI

Error message

Parameter style of {parameter.Name} parameter of {operationId} operation is undefined.

What it means

Thrown by CreateRestApiOperationParameters when an OpenAPI parameter's 'style' field is null. Although 'style' has per-location defaults in the OpenAPI spec, this parser requires it to be explicitly set (non-null) so it can Enum.Parse it into a RestApiParameterStyle; an absent style is rejected rather than defaulted.

Source

Thrown at dotnet/src/Functions/Functions.OpenApi/OpenApi/OpenApiDocumentParser.cs:437

    /// Creates REST API parameters.
    /// </summary>
    /// <param name="operationId">The operation id.</param>
    /// <param name="parameters">The OpenAPI parameters.</param>
    /// <returns>The parameters.</returns>
    private static List<RestApiParameter> CreateRestApiOperationParameters(string operationId, IEnumerable<OpenApiParameter> parameters)
    {
        var result = new List<RestApiParameter>();

        foreach (var parameter in parameters)
        {
            if (parameter.In is null)
            {
                throw new KernelException($"Parameter location of {parameter.Name} parameter of {operationId} operation is undefined.");
            }

            if (parameter.Style is null)
            {
                throw new KernelException($"Parameter style of {parameter.Name} parameter of {operationId} operation is undefined.");
            }

            var restParameter = new RestApiParameter(
                parameter.Name,
                parameter.Schema.Type,
                parameter.Required,
                parameter.Explode,
                (RestApiParameterLocation)Enum.Parse(typeof(RestApiParameterLocation), parameter.In.ToString()!),
                (RestApiParameterStyle)Enum.Parse(typeof(RestApiParameterStyle), parameter.Style.ToString()!),
                parameter.Schema.Items?.Type,
                GetParameterValue(parameter.Schema.Default, "parameter", parameter.Name),
                parameter.Description,
                parameter.Schema.Format,
                parameter.Schema.ToJsonSchema()
            );

            result.Add(restParameter);
        }

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Add an explicit 'style' to each parameter matching its default per location: 'simple' for path/header, 'form' for query/cookie, or the intended array style.
2. Post-process the spec to fill in defaults: path/header -> simple, query/cookie -> form.
3. Regenerate the spec with a tool that always emits the 'style' field.
4. Validate with a linter configured to flag missing style.

Example fix

// before - query parameter without style
{ "name": "tags", "in": "query", "schema": { "type": "array", "items": { "type": "string" } } }

// after - explicit form style
{ "name": "tags", "in": "query", "style": "form", "explode": true, "schema": { "type": "array", "items": { "type": "string" } } }

foreach (var (path, item) in doc.Paths)
    foreach (var p in item.Operations.SelectMany(kv => kv.Value.Parameters))
        if (p.Style is null)
            throw new InvalidDataException($"Parameter '{p.Name}' on {path} is missing 'style'.");

static bool AllParametersHaveStyle(OpenApiDocument doc)
    => doc.Paths.SelectMany(p => p.Value.Operations.SelectMany(o => o.Value.Parameters)).All(p => p.Style is not null);

When it happens

Trigger: A parameter object in the spec that omits the 'style' property. The guard runs right after the 'in' check, so only parameters that already have a valid location reach this point. The subsequent Enum.Parse on parameter.Style would otherwise throw on null.

Common situations: A spec authored without explicit 'style' on its parameters (relying on OpenAPI defaults); a generated spec whose serializer omitted style when it matched the default; parameters imported from an older converter that did not emit style.

Related errors

• Parameter location of {parameter.Name} parameter of {operati
• Payload parameters cannot be retrieved from the '{operation.
• The headers parameter '{parameterStyle}' serialization style
• The query string parameter '{parameterStyle}' serialization
• The path parameter '{parameterStyle}' serialization style is

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/ccea23001488660d. Report an issue: GitHub.