elsa-workflows/elsa-core · error · InvalidOperationException

A workflow resumer is required to complete a User Task.

Error message

A workflow resumer is required to complete a User Task.

What it means

DefaultUserTaskWorkflowResumer.ResumeAsync completes a User Task by resuming the suspended workflow through an IWorkflowResumer resolved from the service provider. If no IWorkflowResumer is registered in DI, the resumer cannot complete the task and throws InvalidOperationException. This is a missing-infrastructure error, not a task-state error.

Solutions

1. Register the Elsa workflow runtime feature/module that provides IWorkflowResumer in the host's DI configuration.
2. If building a minimal host, register an IWorkflowResumer implementation explicitly.
3. Verify with services.GetService<IWorkflowResumer>() != null at startup and fail fast with a clear message.

Example fix

// before
services.AddUserTasks(); // resumer not registered
// after
services.AddElsa(elsa => elsa.UseWorkflowRuntime()); // registers IWorkflowResumer alongside User Tasks

if (services.GetService<Elsa.Workflows.Runtime.IWorkflowResumer>() is null)
    throw new InvalidOperationException("IWorkflowResumer is not registered; add the workflow runtime module.");

var canResume = serviceProvider.GetService<Elsa.Workflows.Runtime.IWorkflowResumer>() is not null;

try { await resumer.ResumeAsync(task, stimulus, ct); }
catch (InvalidOperationException ex) when (ex.Message.Contains("workflow resumer is required")) { /* fix host DI registration */ }

When it happens

Trigger: Calling IUserTaskWorkflowResumer.ResumeAsync (directly or via a 'complete task' endpoint/service) in an application whose DI container lacks an IWorkflowResumer registration — typically an Elsa host assembled without the workflow runtime resumer feature/module.

Common situations: Custom minimal Elsa hosts that registered the User Tasks module but not the workflow runtime pieces; trimming or feature-flagged setups that dropped the resumer registration; test hosts with a partial service collection.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

• Build() must be called before accessing services
• External Authentication handle-hashing settings are…
• Invitation actions must be configured User Task actions.
• Invitation verifier names and allowed actions are required.
• Multiple handlers were found to handle the

AI-assisted analysis of elsa-workflows/elsa-core@fe9217bdfa (2026-09-13). Data as JSON: /api/errors/8bb736357a2919a1. Report an issue: GitHub.

Appendix: source

Thrown at src/modules/Elsa.UserTasks/Services/DefaultUserTaskWorkflowResumer.cs:14

using Elsa.UserTasks.Contracts;
using Elsa.UserTasks.Models;
using Elsa.Workflows.Runtime;
using Microsoft.Extensions.DependencyInjection;

namespace Elsa.UserTasks.Services;

public sealed class DefaultUserTaskWorkflowResumer(IServiceProvider services) : IUserTaskWorkflowResumer
{
    public async Task ResumeAsync(UserTask task, UserTaskStimulus stimulus, CancellationToken cancellationToken = default)
    {
        var resumer = services.GetService<IWorkflowResumer>();
        if (resumer == null)
            throw new InvalidOperationException("A workflow resumer is required to complete a User Task.");

        await resumer.ResumeAsync(task.BookmarkId, new Dictionary<string, object>
        {
            [Elsa.UserTasks.Activities.UserTask.InputKey] = stimulus
        }, cancellationToken);
    }
}

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