What causes "Application is already running." in microsoft/semantic-kernel?

Call StartAsync exactly once, or rely solely on PublishMessageAsync's auto-start and skip explicit StartAsync.

microsoft/semantic-kernel · error · InvalidOperationException

Application is already running.

Error message

Application is already running.

What it means

AgentsApp.StartAsync uses Interlocked.Exchange on _runningCount as a single-entry guard: if it was already 1, it throws InvalidOperationException. The app is a singleton-style host wrapper, so it may only be started once until ShutdownAsync resets the counter to 0.

Source

Thrown at dotnet/src/Agents/Runtime/Core/AgentsApp.cs:56

    /// <summary>
    /// Gets the application lifetime object to manage startup and shutdown events.
    /// </summary>
    public IHostApplicationLifetime ApplicationLifetime => this.Services.GetRequiredService<IHostApplicationLifetime>();

    /// <summary>
    /// Gets the agent runtime responsible for handling agent messaging and operations.
    /// </summary>
    public IAgentRuntime AgentRuntime => this.Services.GetRequiredService<IAgentRuntime>();

    /// <summary>
    /// Starts the application by initiating the host.
    /// Throws an exception if the application is already running.
    /// </summary>
    public async ValueTask StartAsync()
    {
        if (Interlocked.Exchange(ref this._runningCount, 1) != 0)
        {
            throw new InvalidOperationException("Application is already running.");
        }

        await this.Host.StartAsync().ConfigureAwait(false);
    }

    /// <summary>
    /// Shuts down the application by stopping the host.
    /// Throws an exception if the application is not running.
    /// </summary>
    public async ValueTask ShutdownAsync()
    {
        if (Interlocked.Exchange(ref this._runningCount, 0) != 1)
        {
            throw new InvalidOperationException("Application is already stopped.");
        }

        await this.Host.StopAsync().ConfigureAwait(false);
    }

View on GitHub (pinned to c028a0c7dc)

Solutions

Call StartAsync exactly once, or rely solely on PublishMessageAsync's auto-start and skip explicit StartAsync.
Guard with your own started flag or check the application lifetime state before calling StartAsync.
Ensure ShutdownAsync completes before attempting to restart.

Example fix

// before
await app.StartAsync();
await app.PublishMessageAsync(msg, topic); // fine, already running
await app.StartAsync(); // throws

// after
await app.StartAsync();
// do not start again; publish directly
await app.PublishMessageAsync(msg, topic);

Defensive patterns

Strategy: validation

Validate before calling

// AgentsApp exposes no public IsRunning; track via your own flag or rely on PublishMessageAsync auto-start.
bool started = false;
async ValueTask EnsureStartedAsync(AgentsApp app)
{
    if (started) return;
    try { await app.StartAsync(); started = true; }
    catch (InvalidOperationException) { started = true; /* already running */ }
}

Try / catch

try { await app.StartAsync(); }
catch (InvalidOperationException) { /* already running; safe to continue publishing */ }

Prevention

Choose one startup strategy: either explicit StartAsync once, or rely on PublishMessageAsync auto-start.
Centralize app startup in composition root to avoid multiple callers starting.
Avoid calling StartAsync from concurrent initialization paths.

When it happens

Trigger: Calling StartAsync twice without an intervening ShutdownAsync; calling PublishMessageAsync (which auto-starts the host when _runningCount==0) and then explicitly calling StartAsync; a hosted-service/DI lifecycle starting the app and application code starting it again.

Common situations: Double initialization in ASP.NET host startup; mixing the auto-start path of PublishMessageAsync with an explicit StartAsync; retries that re-enter startup.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/58ea32ed388cbee3. Report an issue: GitHub.