How to fix Error 4001 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4001 — “Client Sends spresetconnection While There Is Still Pending Requests”

Why This Error Happens

This error occurs when the Tabular Data Stream (TDS) protocol detects that a client-side connection pool attempted to reset and reuse a connection via sp_reset_connection before the server finished processing an outstanding batch or result set from a prior command. This typically results from improper connection handling in application code, such as failing to fully read or cancel a DataReader before returning the connection to the pool, or from MARS (Multiple Active Result Sets) misconfiguration. It can also surface after driver mismatches between the client library (ODBC, OLE DB, or ADO.NET) and SQL Server 2025, which enforces stricter TDS state validation than previous versions.

Step-by-Step Resolution Guide

Step 1: Audit Application Connection Handling

  • Review application code for any SqlDataReader, SqlCommand, or similar objects that are not explicitly closed or disposed before the connection is returned to the pool.
  • Ensure every DataReader is wrapped in a using block or explicitly calls .Close()/.Dispose().
  • Check for asynchronous calls (ExecuteReaderAsync, ExecuteNonQueryAsync) that are not properly awaited, as this is the most common cause of orphaned pending requests.

Step 2: Enable and Verify MARS Configuration

  • Open the connection string used by the application and confirm whether MultipleActiveResultSets=True is required for your workload.
  • If MARS is enabled unintentionally, disable it unless the application explicitly handles multiple concurrent result sets, since MARS increases the risk of this race condition.
  • Update the connection string in web.config, app.config, or the relevant application settings file, for example:
    Server=myServer;Database=myDB;MultipleActiveResultSets=False;TrustServerCertificate=True;

Step 3: Update SQL Server Native Client / Driver Stack

  • On Windows Server 2025 / Windows 11, confirm the client is using the latest Microsoft ODBC Driver 18+ or Microsoft.Data.SqlClient (not the legacy System.Data.SqlClient).
  • Navigate to Control Panel > Programs > Programs and Features to check installed driver versions, or run:
    Get-OdbcDriver | Where-Object {$_.Name -like "*SQL Server*"}
  • Reinstall or update via the official Microsoft Download Center package matching SQL Server 2025 compatibility requirements.

Step 4: Adjust Connection Pooling Settings

  • In the application’s connection string, reduce Max Pool Size temporarily and set Pooling=True with a lower Connection Lifetime to force more frequent, cleaner connection resets.
  • For IIS-hosted applications, recycle the Application Pool via IIS Manager > Application Pools > Recycle after configuration changes to clear any stale pooled connections.

Step 5: Enable Extended Events Tracing for Confirmation

  • Open SQL Server Management Studio (SSMS) and navigate to Management > Extended Events > Sessions.
  • Create a new session capturing the sqlserver.error_reported event, filtered on Error Code 4001, to capture the exact query/session triggering the reset conflict.
  • Correlate the captured session_id with the application thread to pinpoint the exact faulty code path.

Alternative or Verification Steps

  • After applying fixes, monitor the SQL Server Error Log (Management > SQL Server Logs in SSMS) for 24–48 hours to confirm Error 4001 no longer appears.
  • If the issue persists, as a temporary workaround, disable connection pooling entirely (Pooling=False in the connection string) to isolate whether the pool itself is the root cause versus an application logic defect.