diff --git a/src/ExcelMcp.Core/Commands/PowerQuery/IPowerQueryCommands.cs b/src/ExcelMcp.Core/Commands/PowerQuery/IPowerQueryCommands.cs
index c032e08a..73928430 100644
--- a/src/ExcelMcp.Core/Commands/PowerQuery/IPowerQueryCommands.cs
+++ b/src/ExcelMcp.Core/Commands/PowerQuery/IPowerQueryCommands.cs
@@ -23,11 +23,11 @@ namespace Sbroenne.ExcelMcp.Core.Commands;
 ///
 /// TARGET CELL: targetCellAddress places tables without clearing sheet.
 /// TIMEOUT: 5 min auto-timeout for refresh/load. For network queries, use timeout=120 or higher.
-/// timeout=0 is INVALID - must be greater than zero.
+/// timeout=0 or omitted uses the 5 min default.
 /// </summary>
 [ServiceCategory("powerquery", "PowerQuery")]
 [McpTool("powerquery", Title = "Power Query Operations", Destructive = true, Category = "query",
-    Description = "Power Query M code and data loading. TEST-FIRST WORKFLOW: 1. evaluate (test M code without persisting) 2. create/update (store validated query) 3. refresh/load-to (load data to destination). IF CREATE FAILS: Use evaluate for detailed M engine error. DATETIME: Always include Table.TransformColumnTypes() for explicit column types. DESTINATIONS: worksheet (default), data-model (for DAX), both, connection-only. M-CODE: Auto-formatted via powerqueryformatter.com. TARGET CELL: targetCellAddress places tables without clearing sheet. TIMEOUT: 5 min auto-timeout. For network queries, use timeout=120 or higher. timeout=0 is INVALID.")]
+    Description = "Power Query M code and data loading. TEST-FIRST WORKFLOW: 1. evaluate (test M code without persisting) 2. create/update (store validated query) 3. refresh/load-to (load data to destination). IF CREATE FAILS: Use evaluate for detailed M engine error. DATETIME: Always include Table.TransformColumnTypes() for explicit column types. DESTINATIONS: worksheet (default), data-model (for DAX), both, connection-only. M-CODE: Auto-formatted via powerqueryformatter.com. TARGET CELL: targetCellAddress places tables without clearing sheet. TIMEOUT: 5 min auto-timeout. For network queries, use timeout=120 or higher. timeout=0 or omitted uses the 5 min default.")]
 public interface IPowerQueryCommands
 {
     /// <summary>
diff --git a/src/ExcelMcp.Core/Commands/PowerQuery/PowerQueryCommands.Refresh.cs b/src/ExcelMcp.Core/Commands/PowerQuery/PowerQueryCommands.Refresh.cs
index bcb77348..dc8b9190 100644
--- a/src/ExcelMcp.Core/Commands/PowerQuery/PowerQueryCommands.Refresh.cs
+++ b/src/ExcelMcp.Core/Commands/PowerQuery/PowerQueryCommands.Refresh.cs
@@ -29,7 +29,7 @@ public PowerQueryRefreshResult Refresh(IExcelBatch batch, string queryName, Time
 
         if (timeout <= TimeSpan.Zero)
         {
-            throw new ArgumentOutOfRangeException(nameof(timeout), "Timeout must be greater than zero.");
+            timeout = TimeSpan.FromMinutes(5); // Default timeout when not specified
         }
 
         using var timeoutCts = new CancellationTokenSource(timeout);
diff --git a/tests/ExcelMcp.Core.Tests/Integration/Commands/PowerQuery/PowerQueryCommandsTests.RefreshErrors.cs b/tests/ExcelMcp.Core.Tests/Integration/Commands/PowerQuery/PowerQueryCommandsTests.RefreshErrors.cs
index 71355357..fe0bf656 100644
--- a/tests/ExcelMcp.Core.Tests/Integration/Commands/PowerQuery/PowerQueryCommandsTests.RefreshErrors.cs
+++ b/tests/ExcelMcp.Core.Tests/Integration/Commands/PowerQuery/PowerQueryCommandsTests.RefreshErrors.cs
@@ -233,6 +233,68 @@ public void Refresh_ValidDataModelQuery_Succeeds()
         Assert.False(result.HasErrors);
     }
 
+    /// <summary>
+    /// Regression test: Refresh with TimeSpan.Zero timeout should use the default 5-minute timeout
+    /// instead of throwing ArgumentOutOfRangeException.
+    ///
+    /// BUG: The source generator produces `args.Timeout ?? default(TimeSpan)` = TimeSpan.Zero
+    /// when no timeout is supplied (CLI without --timeout, MCP without timeout parameter).
+    /// The Core method used to throw ArgumentOutOfRangeException("Timeout must be greater than zero.").
+    /// FIX: TimeSpan.Zero now falls back to 5-minute default.
+    /// </summary>
+    [Fact]
+    public void Refresh_ZeroTimeout_UsesDefaultAndSucceeds()
+    {
+        // Arrange - Create a valid worksheet query
+        var testExcelFile = _fixture.CreateTestFile();
+        var queryName = "ZeroTimeoutQuery";
+
+        var validMCode = @"let
+    Source = #table({""Name""}, {{""Test""}})
+in
+    Source";
+
+        using var batch = ExcelSession.BeginBatch(testExcelFile);
+
+        _powerQueryCommands.Create(batch, queryName, validMCode, PowerQueryLoadMode.LoadToTable);
+        batch.Save();
+
+        // Act - Refresh with TimeSpan.Zero (the exact value generated when timeout is omitted)
+        var result = _powerQueryCommands.Refresh(batch, queryName, TimeSpan.Zero);
+
+        // Assert - Should succeed, not throw ArgumentOutOfRangeException
+        Assert.True(result.Success, $"Refresh failed: {result.ErrorMessage}");
+        Assert.False(result.HasErrors);
+    }
+
+    /// <summary>
+    /// Regression test: Refresh with negative timeout should use the default 5-minute timeout.
+    /// </summary>
+    [Fact]
+    public void Refresh_NegativeTimeout_UsesDefaultAndSucceeds()
+    {
+        // Arrange - Create a valid worksheet query
+        var testExcelFile = _fixture.CreateTestFile();
+        var queryName = "NegativeTimeoutQuery";
+
+        var validMCode = @"let
+    Source = #table({""Name""}, {{""Test""}})
+in
+    Source";
+
+        using var batch = ExcelSession.BeginBatch(testExcelFile);
+
+        _powerQueryCommands.Create(batch, queryName, validMCode, PowerQueryLoadMode.LoadToTable);
+        batch.Save();
+
+        // Act - Refresh with negative timeout
+        var result = _powerQueryCommands.Refresh(batch, queryName, TimeSpan.FromSeconds(-1));
+
+        // Assert - Should succeed, not throw ArgumentOutOfRangeException
+        Assert.True(result.Success, $"Refresh failed: {result.ErrorMessage}");
+        Assert.False(result.HasErrors);
+    }
+
     /// <summary>
     /// Verifies that refresh throws when query is connection-only (no QueryTable or DataModel connection).
     /// Connection-only queries have no mechanism to refresh - they're only query definitions.