microsoft/semantic-kernel · error · KernelException

Type '{this.InnerStepDotnetType}' is not a valid KernelProce

Error message

Type '{this.InnerStepDotnetType}' is not a valid KernelProcessStep type.

What it means

Thrown when the resolved inner step type does not implement KernelProcessStep. After successfully loading the Type from its assembly-qualified name, the runtime verifies it is assignable to KernelProcessStep; a type that fails this check is not a valid process step.

Source

Thrown at dotnet/src/Experimental/Process.Runtime.Dapr/DaprStepInfo.cs:55

    /// </summary>
    public required Dictionary<string, List<KernelProcessEdge>> Edges { get; init; }

    /// <summary>
    /// Builds an instance of <see cref="KernelProcessStepInfo"/> from the current object.
    /// </summary>
    /// <returns>An instance of <see cref="KernelProcessStepInfo"/></returns>
    /// <exception cref="KernelException"></exception>
    public KernelProcessStepInfo ToKernelProcessStepInfo()
    {
        Type? innerStepType = Type.GetType(this.InnerStepDotnetType);
        if (innerStepType is null)
        {
            throw new KernelException($"Unable to create inner step type from assembly qualified name `{this.InnerStepDotnetType}`");
        }

        if (!typeof(KernelProcessStep).IsAssignableFrom(innerStepType))
        {
            throw new KernelException($"Type '{this.InnerStepDotnetType}' is not a valid KernelProcessStep type.");
        }

        return new KernelProcessStepInfo(innerStepType, this.State, this.Edges);
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="DaprStepInfo"/> class from an instance of <see cref="KernelProcessStepInfo"/>.
    /// </summary>
    /// <returns>An instance of <see cref="DaprStepInfo"/></returns>
    public static DaprStepInfo FromKernelStepInfo(KernelProcessStepInfo kernelStepInfo)
    {
        Verify.NotNull(kernelStepInfo, nameof(kernelStepInfo));

        return new DaprStepInfo
        {
            InnerStepDotnetType = kernelStepInfo.InnerStepType.AssemblyQualifiedName!,
            State = kernelStepInfo.State,
            Edges = kernelStepInfo.Edges.ToDictionary(kvp => kvp.Key, kvp => new List<KernelProcessEdge>(kvp.Value)),

View on GitHub (pinned to c028a0c7dc)

Solutions

Confirm the persisted type actually implements KernelProcessStep (directly or via a step base class).
Correct the InnerStepDotnetType in the process definition to point at a real KernelProcessStep subclass.
If the type was refactored, update the process definition and re-persist it.

Example fix

// before
public class MyWorker { } // not a step
// after
public class MyWorker : KernelProcessStep
{
    [KernelProcessStepProcessFunction]
    public ValueTask DoWorkAsync(ProcessMessage msg) => ValueTask.CompletedTask;
}

Defensive patterns

Strategy: validation

Validate before calling

var t = Type.GetType(step.InnerStepDotnetType);
if (t is null || !typeof(KernelProcessStep).IsAssignableFrom(t))
    throw new InvalidOperationException($"Type '{step.InnerStepDotnetType}' is not a valid KernelProcessStep.");

Type guard

public static bool IsValidStepType(string assemblyQualifiedTypeName)
{
    var t = Type.GetType(assemblyQualifiedTypeName);
    return t is not null && typeof(KernelProcessStep).IsAssignableFrom(t);
}

Try / catch

try
{
    var info = daprStepInfo.ToKernelProcessStepInfo();
}
catch (KernelException ex) when (ex.Message.Contains("not a valid KernelProcessStep type"))
{
    _logger.LogError(ex, "Persisted type is not a KernelProcessStep; check definition.");
    throw;
}

Prevention

Only register KernelProcessStep subclasses as step types.
Add a unit test asserting each step type implements KernelProcessStep.
Validate step types at process-definition build time.

When it happens

Trigger: DaprStepInfo.ToKernelProcessStepInfo() checks `typeof(KernelProcessStep).IsAssignableFrom(innerStepType)` and throws when the type is some other class that happens to share the persisted type name or was mistakenly registered as a step.

Common situations: The persisted InnerStepDotnetType was changed to point at a non-step class, the assembly-qualified name collided with a different type after a refactor, or a generic/utility class was accidentally used as a step type.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/1df608c62feb5ab0. Report an issue: GitHub.