diff --git a/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager.cs b/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager.cs
index d3d0ad4f40..b21b4ba89c 100644
--- a/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager.cs
+++ b/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager.cs
@@ -31,6 +31,12 @@ public partial class ConfigurationManager<T> : BaseConfigurationManager, IConfig
         private readonly IConfigurationValidator<T> _configValidator;
         private T _currentConfiguration;
 
+        // Tracks the most recent fetch failure for the blocking path. Promoted from a local in
+        // GetConfigurationWithBlockingAsync so the original exception (e.g. an IOException carrying
+        // HttpDocumentRetriever.StatusCode/ResponseContent in its Data dictionary) is preserved across
+        // calls that arrive within the backoff window (_syncAfter > now) and skip the fetch.
+        private Exception _fetchMetadataFailure;
+
         // task states are used to ensure the call to 'update config' (UpdateCurrentConfiguration) is a singleton. Uses Interlocked.CompareExchange.
         // metadata is not being obtained
         private const int ConfigurationRetrieverIdle = 0;
diff --git a/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager_Blocking.cs b/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager_Blocking.cs
index 28492c627b..67db24f04c 100644
--- a/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager_Blocking.cs
+++ b/src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager_Blocking.cs
@@ -23,7 +23,6 @@ public partial class ConfigurationManager<T> where T : class
 
         private async Task<T> GetConfigurationWithBlockingAsync(CancellationToken cancel)
         {
-            Exception _fetchMetadataFailure = null;
             await _refreshLock.WaitAsync(cancel).ConfigureAwait(false);
 
             long startTimestamp = TimeProvider.GetTimestamp();
@@ -52,6 +51,8 @@ private async Task<T> GetConfigurationWithBlockingAsync(CancellationToken cancel
 
                                 UpdateConfiguration(configurationRetrieved.Configuration, configurationRetrieved.RetrievalTime, retrievalContext);
 
+                                _fetchMetadataFailure = null;
+
                                 return _currentConfiguration;
                             }
                         }
@@ -81,6 +82,8 @@ private async Task<T> GetConfigurationWithBlockingAsync(CancellationToken cancel
                             _refreshRequested = false;
 
                         UpdateConfiguration(configuration, TimeProvider.GetUtcNow(), retrievalContext);
+
+                        _fetchMetadataFailure = null;
                     }
                     catch (Exception ex)
                     {
diff --git a/test/Microsoft.IdentityModel.Protocols.OpenIdConnect.Tests/ConfigurationManagerTests.cs b/test/Microsoft.IdentityModel.Protocols.OpenIdConnect.Tests/ConfigurationManagerTests.cs
index 9cd114f139..cb827355fd 100644
--- a/test/Microsoft.IdentityModel.Protocols.OpenIdConnect.Tests/ConfigurationManagerTests.cs
+++ b/test/Microsoft.IdentityModel.Protocols.OpenIdConnect.Tests/ConfigurationManagerTests.cs
@@ -187,6 +187,98 @@ public async Task FetchMetadataFailureTest_Blocking()
             await FetchMetadataFailureTestBody();
         }
 
+        // Reproduces a bug in the blocking path (Switch.Microsoft.IdentityModel.UpdateConfigAsBlocking).
+        // When a fetch fails and a subsequent call arrives within the backoff window (i.e. _syncAfter > now),
+        // GetConfigurationWithBlockingAsync skips the fetch entirely and throws IDX20803 with a null
+        // InnerException because _fetchMetadataFailure is declared as a local variable (reset to null on
+        // every invocation) rather than persisted between calls. This loses the original IOException
+        // (and the HttpDocumentRetriever.StatusCode / ResponseContent it carries in Data), preventing
+        // callers from distinguishing client errors (4xx -> misconfigured tenant) from server errors
+        // (5xx -> transient).
+        [Fact]
+        public async Task FetchMetadataFailure_Blocking_PreservesInnerExceptionDuringBackoffWindow()
+        {
+            AppContext.SetSwitch(AppContextSwitches.UpdateConfigAsBlockingSwitch, true);
+
+            var context = new CompareContext($"{this}.{nameof(FetchMetadataFailure_Blocking_PreservesInnerExceptionDuringBackoffWindow)}");
+
+            var documentRetriever = new HttpDocumentRetriever(
+                HttpResponseMessageUtils.SetupHttpClientThatReturns("OpenIdConnectMetadata.json", HttpStatusCode.NotFound));
+            var configManager = new ConfigurationManager<OpenIdConnectConfiguration>(
+                "https://example.invalid/.well-known/openid-configuration",
+                new OpenIdConnectConfigurationRetriever(),
+                documentRetriever);
+
+            // First call: fetch is attempted and fails. The thrown InvalidOperationException should
+            // wrap the original IOException carrying the HTTP status code in its Data dictionary.
+            Exception firstException = null;
+            try
+            {
+                _ = await configManager.GetConfigurationAsync(CancellationToken.None);
+            }
+            catch (Exception ex)
+            {
+                firstException = ex;
+            }
+
+            if (firstException == null)
+                context.AddDiff("Expected first GetConfigurationAsync call to throw.");
+            else
+            {
+                if (firstException.InnerException == null)
+                    context.AddDiff("Expected first call's InvalidOperationException to wrap the underlying IOException.");
+                else if (!ExceptionChainContainsStatusCode(firstException))
+                    context.AddDiff("Expected first call's exception chain to contain HttpDocumentRetriever.StatusCode in Data.");
+            }
+
+            // Force the backoff window: ensure _syncAfter is in the future so the next call skips the fetch
+            // and goes through the "stale metadata is better than no metadata" path. _currentConfiguration
+            // is still null (bootstrap never succeeded), so the manager re-throws IDX20803.
+            TestUtilities.SetField(configManager, "_syncAfter", DateTimeOffset.UtcNow.AddHours(1));
+
+            Exception secondException = null;
+            try
+            {
+                _ = await configManager.GetConfigurationAsync(CancellationToken.None);
+            }
+            catch (Exception ex)
+            {
+                secondException = ex;
+            }
+
+            if (secondException == null)
+                context.AddDiff("Expected second GetConfigurationAsync call (within backoff window) to throw.");
+            else
+            {
+                // The bug: second call throws IDX20803 with InnerException == null, losing the HTTP
+                // status code that callers use to classify the error (401 vs 503).
+                if (secondException.InnerException == null)
+                {
+                    context.AddDiff(
+                        "BUG: Second call within backoff window threw IDX20803 with a null InnerException. " +
+                        "The original IOException (with HttpDocumentRetriever.StatusCode in Data) was lost " +
+                        "because _fetchMetadataFailure is a local variable in GetConfigurationWithBlockingAsync.");
+                }
+                else if (!ExceptionChainContainsStatusCode(secondException))
+                {
+                    context.AddDiff("Expected second call's exception chain to contain HttpDocumentRetriever.StatusCode in Data.");
+                }
+            }
+
+            TestUtilities.AssertFailIfErrors(context);
+        }
+
+        private static bool ExceptionChainContainsStatusCode(Exception exception)
+        {
+            for (Exception current = exception; current != null; current = current.InnerException)
+            {
+                if (current.Data.Contains(HttpDocumentRetriever.StatusCode))
+                    return true;
+            }
+
+            return false;
+        }
+
         private async ValueTask FetchMetadataFailureTestBody()
         {
             var context = new CompareContext($"{this}.FetchMetadataFailureTest");