diff --git a/backend/FwLite/FwDataMiniLcmBridge/FwDataFactory.cs b/backend/FwLite/FwDataMiniLcmBridge/FwDataFactory.cs
index 3691bede09..8dff6914bc 100644
--- a/backend/FwLite/FwDataMiniLcmBridge/FwDataFactory.cs
+++ b/backend/FwLite/FwDataMiniLcmBridge/FwDataFactory.cs
@@ -20,6 +20,10 @@ public class FwDataFactory(
     IOptions<FwDataBridgeConfig> config) : IDisposable, IHostedService
 {
     private bool _shuttingDown = false;
+
+    // Sliding window before an idle LcmCache is evicted and disposed. PreventEviction refreshes inside this window.
+    private static readonly TimeSpan CacheSlidingExpiration = TimeSpan.FromMinutes(30);
+
     public FwDataFactory(ILogger<FwDataMiniLcmApi> fwdataLogger,
         IMemoryCache cache,
         ILogger<FwDataFactory> logger,
@@ -51,7 +55,7 @@ private LcmCache GetProjectServiceCached(FwDataProject project)
         var projectService = cache.GetOrCreate(key,
                 entry =>
                 {
-                    entry.SlidingExpiration = TimeSpan.FromMinutes(30);
+                    entry.SlidingExpiration = CacheSlidingExpiration;
                     entry.RegisterPostEvictionCallback(OnLcmProjectCacheEviction, (logger, _projectCacheKeys));
                     logger.LogInformation("Loading project {ProjectFileName}", project.FileName);
                     var projectService = projectLoader.LoadCache(project);
@@ -131,6 +135,24 @@ public IAsyncDisposable DeferCloseAsync(FwDataProject project)
         return Defer.Async(() => CloseProjectAsync(project));
     }
 
+    /// <summary>
+    /// Keeps the project's LcmCache from being evicted until disposed, by periodically resetting its sliding expiration.
+    /// Use around long-running work that holds one api instance past the expiration window (e.g. a sync paused in a debugger).
+    /// </summary>
+    public IDisposable PreventEviction(FwDataProject project)
+    {
+        var key = CacheKey(project);
+        // Refresh now in case little of the window remains, then every half-window so a tick can't be missed.
+        var period = CacheSlidingExpiration / 2;
+        return new Timer(_ =>
+        {
+            // Best-effort: the shared cache can be disposed during shutdown while a sync is still finishing,
+            // and an unhandled throw on this timer thread would take the process down.
+            try { cache.TryGetValue(key, out _); }
+            catch (ObjectDisposedException) { }
+        }, null, TimeSpan.Zero, period);
+    }
+
     public Task StartAsync(CancellationToken cancellationToken)
     {
         return Task.CompletedTask;
diff --git a/backend/FwLite/FwLiteProjectSync/CrdtFwdataProjectSyncService.cs b/backend/FwLite/FwLiteProjectSync/CrdtFwdataProjectSyncService.cs
index e9d9d352ca..fdf163b716 100644
--- a/backend/FwLite/FwLiteProjectSync/CrdtFwdataProjectSyncService.cs
+++ b/backend/FwLite/FwLiteProjectSync/CrdtFwdataProjectSyncService.cs
@@ -1,4 +1,5 @@
 using System.Diagnostics;
+using FwDataMiniLcmBridge;
 using FwDataMiniLcmBridge.Api;
 using LcmCrdt;
 using LexCore.Sync;
@@ -12,7 +13,8 @@ namespace FwLiteProjectSync;
 public class CrdtFwdataProjectSyncService(MiniLcmImport miniLcmImport,
     ILogger<CrdtFwdataProjectSyncService> logger,
     MiniLcmApiValidationWrapperFactory validationWrapperFactory,
-    CrdtProjectsService crdtProjectsService)
+    CrdtProjectsService crdtProjectsService,
+    FwDataFactory fwDataFactory)
 {
     public record DryRunSyncResult(
         int CrdtChanges,
@@ -53,6 +55,9 @@ private async Task<SyncResult> SyncOrImportInternal(IMiniLcmApi crdtApi, IMiniLc
             throw new InvalidOperationException($"Project id mismatch, CRDT Id: {crdt.ProjectData.FwProjectId}, FWData Id: {fwdata.ProjectId}");
         }
 
+        // A sync that outlives the LcmCache's sliding expiration (e.g. paused in a debugger) would otherwise have it disposed mid-sync.
+        using var keepFwdataAlive = fwDataFactory.PreventEviction(fwdata.Project);
+
         // Project snapshot logic/handling is done outside of this class so that Sync vs Import is explicit.
         // We still choose to explicitly verify a consistent state to avoid accidental misuse.
         var hasSyncedSuccessfully = ProjectSnapshotService.HasSyncedSuccessfully(fwdata.Project);
diff --git a/backend/Testing/FwHeadless/Services/SyncWorkerTestHarness.cs b/backend/Testing/FwHeadless/Services/SyncWorkerTestHarness.cs
index 0f4567fdfc..2f6205b511 100644
--- a/backend/Testing/FwHeadless/Services/SyncWorkerTestHarness.cs
+++ b/backend/Testing/FwHeadless/Services/SyncWorkerTestHarness.cs
@@ -253,6 +253,7 @@ private ServiceProvider BuildServiceProvider(
             null!,
             NullLogger<CrdtFwdataProjectSyncService>.Instance,
             null!,
+            null!,
             null!);
 
         syncService