How to fix Error 3957 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 3957 occurs when a transaction starts running under snapshot isolation before the ALLOW_SNAPSHOT_ISOLATION database option is turned ON, or when the option was toggled mid-session. SQL Server captures the isolation state of the database at the moment a transaction begins; if the setting changes afterward (or was never enabled), any snapshot-based read fails because the transaction’s version store context is invalid. This is typically caused by a misconfigured database option, a race condition during a schema/config deployment, or an application connecting before the DBA has finished enabling the feature.

Step-by-Step Resolution Guide

Step 1: Verify Current Snapshot Isolation Setting

  • Open SQL Server Management Studio (SSMS) or Azure Data Studio and connect to the SQL Server 2025 instance.
  • Run the following query to check the current state:
  SELECT name, snapshot_isolation_state_desc 
  FROM sys.databases 
  WHERE name = 'YourDatabaseName';
  • If the result shows OFF, this confirms the root cause.

Step 2: Enable Snapshot Isolation on the Database

  • Ensure no active transactions are blocking the ALTER DATABASE command (snapshot changes require no other connections mid-transition).
  • Run the following T-SQL command with elevated permissions (sysadmin or db_owner role required):
  ALTER DATABASE YourDatabaseName 
  SET ALLOW_SNAPSHOT_ISOLATION ON;
  • On Windows Server 2025, confirm the SQL Server service account has the necessary file system permissions to write to C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\DATA\ for tempdb version store growth, as snapshot isolation relies heavily on tempdb.

Step 3: Restart Application Connections Cleanly

  • Terminate and restart any application connection pools (IIS, .NET Core services, etc.) that initiated transactions before the setting was applied.
  • In IIS on Windows Server 2025, recycle the application pool via:
  iisreset /noforce
  • This ensures new connections pick up the updated isolation state rather than reusing stale pooled connections with the old transaction context.

Step 4: Check for Conflicting ALTER DATABASE Timing

  • Review deployment scripts or CI/CD pipelines (e.g., Azure DevOps, GitHub Actions) to ensure ALTER DATABASE ... SET ALLOW_SNAPSHOT_ISOLATION ON executes and fully commits before the application layer starts issuing queries.
  • Add a wait/poll step in deployment scripts checking sys.databases.snapshot_isolation_state equals 1 before proceeding.

Alternative or Verification Steps

  • To verify the fix, run a test transaction using SET TRANSACTION ISOLATION LEVEL SNAPSHOT; followed by a SELECT statement against the affected table — it should execute without raising Error 3957.
  • If the error persists under high concurrency, consider enabling READ_COMMITTED_SNAPSHOT instead (via ALTER DATABASE YourDatabaseName SET READ_COMMITTED_SNAPSHOT ON;) as it avoids explicit isolation-level timing conflicts for most standard application workloads.