diff --git a/src/Cellm.Tests/Unit/AddIn/ArgumentParserTests.cs b/src/Cellm.Tests/Unit/AddIn/ArgumentParserTests.cs
index acae2fe..927ea75 100644
--- a/src/Cellm.Tests/Unit/AddIn/ArgumentParserTests.cs
+++ b/src/Cellm.Tests/Unit/AddIn/ArgumentParserTests.cs
@@ -1,4 +1,8 @@
+using System.Runtime.CompilerServices;
 using Cellm.AddIn;
+using Cellm.AddIn.Exceptions;
+using ExcelDna.Integration;
+using Microsoft.Extensions.Configuration;
 using Xunit;
 using CellmRange = Cellm.AddIn.Range;
 
@@ -6,6 +10,84 @@ namespace Cellm.Tests.Unit.AddIn;
 
 public class ArgumentParserTests
 {
+    private static ArgumentParser CreateParser()
+    {
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["CellmAddInConfiguration:DefaultProvider"] = "OpenAi",
+                ["CellmAddInConfiguration:DefaultTemperature"] = "0",
+                ["OpenAiConfiguration:DefaultModel"] = "gpt-4.1",
+            })
+            .Build();
+
+        return new ArgumentParser(config);
+    }
+
+    // ExcelReference constructor calls into Excel, so we create an uninitialized instance for unit tests
+    private static ExcelReference CreateExcelReference() =>
+        (ExcelReference)RuntimeHelpers.GetUninitializedObject(typeof(ExcelReference));
+
+    #region Parse Old Syntax Tests
+
+    [Fact]
+    public void Parse_OldArgumentOrder_CellsThenString_ThrowsCellmException()
+    {
+        var parser = CreateParser();
+
+        var ex = Assert.Throws<CellmException>(() =>
+            parser
+                .AddInstructions(CreateExcelReference())
+                .AddCells(new object[] { "Extract keywords", ExcelMissing.Value })
+                .Parse());
+
+        Assert.Contains("argument order changed", ex.Message);
+    }
+
+    [Fact]
+    public void Parse_OldArgumentOrder_CellsThenStringThenTemperature_ThrowsCellmException()
+    {
+        var parser = CreateParser();
+
+        var ex = Assert.Throws<CellmException>(() =>
+            parser
+                .AddInstructions(CreateExcelReference())
+                .AddCells(new object[] { "Extract keywords", 0.7 })
+                .Parse());
+
+        Assert.Contains("argument order changed", ex.Message);
+    }
+
+    [Fact]
+    public void Parse_OldArgumentOrder_CellsThenTemperature_ThrowsCellmException()
+    {
+        var parser = CreateParser();
+
+        var ex = Assert.Throws<CellmException>(() =>
+            parser
+                .AddInstructions(CreateExcelReference())
+                .AddCells(new object[] { 0.7, ExcelMissing.Value })
+                .Parse());
+
+        Assert.Contains("argument order changed", ex.Message);
+    }
+
+    [Fact]
+    public void Parse_OldTemperatureArgument_StringThenTemperature_ThrowsCellmException()
+    {
+        var parser = CreateParser();
+
+        var ex = Assert.Throws<CellmException>(() =>
+            parser
+                .AddInstructions("Extract keywords")
+                .AddCells(new object[] { 0.7, ExcelMissing.Value })
+                .Parse());
+
+        Assert.Contains("temperature argument was removed", ex.Message);
+    }
+
+    #endregion
+
     #region GetColumnName Tests
 
     [Theory]
diff --git a/src/Cellm/AddIn/ArgumentParser.cs b/src/Cellm/AddIn/ArgumentParser.cs
index d35dd4b..edc05bd 100644
--- a/src/Cellm/AddIn/ArgumentParser.cs
+++ b/src/Cellm/AddIn/ArgumentParser.cs
@@ -80,10 +80,16 @@ internal Arguments Parse()
         {
             // =PROMPT("Hello world")
             (string instructions, []) => new Arguments(provider, model, [], instructions, temperature, _outputShape),
+            // Old temperature argument: =PROMPT("instruction", 0.7), =PROMPT("instruction", A1:B2, 0.7), etc.
+            (string, object[] ranges) when ranges.Any(r => r is double) =>
+                throw new CellmException("The temperature argument was removed in v0.5. Please use PROMPT(\"instruction\", A1:B2) and configure temperature via the ribbon UI."),
             // =PROMPT("Hello world", A1, B2, ...)
             (string instructions, object[] ranges) => new Arguments(provider, model, ParseRanges(ranges), instructions, temperature, _outputShape),
             // =PROMPT(A1:B2)
             (ExcelReference instructions, []) => new Arguments(provider, model, [], new Range(instructions.RowFirst, instructions.ColumnFirst, instructions.GetValue()), temperature, _outputShape),
+            // Old argument order: =PROMPT(A1:B2, "instruction"), =PROMPT(A1:B2, "instruction", 0.7), =PROMPT(A1:B2, 0.7), etc.
+            (ExcelReference, object[] ranges) when ranges.Any(r => r is string or double) =>
+                throw new CellmException("The argument order changed in v0.5. Please use PROMPT(\"instruction\", A1:B2) instead of PROMPT(A1:B2, \"instruction\"). Temperature is now configured via the ribbon UI."),
             // =PROMPT(A1:B2, C1, D2, ...)
             (ExcelReference instructions, object[] ranges) => new Arguments(provider, model, ParseRanges(ranges), new Range(instructions.RowFirst, instructions.ColumnFirst, instructions.GetValue()), temperature, _outputShape),
             // Short-circuit if instructions contain an Excel error