diff --git a/Directory.Build.props b/Directory.Build.props
index 822cbde..6852b86 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -9,7 +9,7 @@
 	<Product>Simpra</Product>
 	<Company>ALTA Software llc.</Company>
 	<Copyright>Copyright © 2024 ALTA Software llc.</Copyright>
-	<Version>1.0.15</Version>
+	<Version>1.0.16</Version>
   </PropertyGroup>
 
   <PropertyGroup>
diff --git a/README.md b/README.md
index c72354f..aee7fc0 100644
--- a/README.md
+++ b/README.md
@@ -179,6 +179,7 @@ Simpra provides a set of internal functions out-of-the-box:
 - **Strings**
   - `number(str)` – converts to number
   - `date(str)` – converts to date
+  - `substring(str, index, count)` – Retrieves part of a string starting at a **1-based position**.
 ---
 
 ### Extended Membership Operators
diff --git a/src/AltaSoft.Simpra/InternalLanguageFunctions.cs b/src/AltaSoft.Simpra/InternalLanguageFunctions.cs
index d4f6f1d..bcdef88 100644
--- a/src/AltaSoft.Simpra/InternalLanguageFunctions.cs
+++ b/src/AltaSoft.Simpra/InternalLanguageFunctions.cs
@@ -10,6 +10,8 @@ internal static class InternalLanguageFunctions
     // ReSharper disable InconsistentNaming
 #pragma warning disable IDE1006 // Naming Styles
 
+    public static SimpraString substring(SimpraString input, SimpraNumber index) => SimpraString.Substring(input, index);
+    public static SimpraString substring(SimpraString input, SimpraNumber index, SimpraNumber count) => SimpraString.Substring(input, index, count);
     public static SimpraNumber length(SimpraString input) => new(input.Value.Length);
     public static SimpraNumber length(SimpraString[] input) => new(input.Length);
     public static SimpraNumber length(SimpraNumber[] input) => new(input.Length);
@@ -48,7 +50,8 @@ internal static class InternalLanguageFunctions
         nameof(sum),
         nameof(@string),
         nameof(@number),
-        nameof(@date)
+        nameof(@date),
+        nameof(substring)
     ];
 
     internal static bool IsSimpraBuiltInFunction(this string name) => s_builtInFunctions.Contains(name);
diff --git a/src/AltaSoft.Simpra/Types/SimpraString.cs b/src/AltaSoft.Simpra/Types/SimpraString.cs
index 1b9e517..d7ebf6c 100644
--- a/src/AltaSoft.Simpra/Types/SimpraString.cs
+++ b/src/AltaSoft.Simpra/Types/SimpraString.cs
@@ -68,15 +68,26 @@ public static SimpraString Substring(SimpraString value, SimpraNumber index, Sim
         if (!value.HasValue)
             return value;
 
-        var startIndex = (int)index.Value;
+        var startIndex = (int)index.Value - 1; // convert 1-based to 0-based index
         var length = (int)count.Value;
 
-        return startIndex < 0 || length < 0 || startIndex + length > value.Value.Length
-            ? throw new InvalidOperationException("The index and count must be within the bounds of the string")
-            : new SimpraString(value.Value.Substring(startIndex, length));
+        if (startIndex < 0)
+            startIndex = 0;
+
+        if (length < 0)
+            length = 0;
+
+        if (startIndex >= value.Value.Length || length == 0)
+            return new SimpraString(string.Empty);
+
+        if (startIndex + length >= value.Value.Length)
+        {
+            length = value.Value.Length - startIndex;
+        }
+        return new SimpraString(value.Value.Substring(startIndex, length));
     }
 
-    public static SimpraString Substring(SimpraString value, SimpraNumber index) => !value.HasValue ? value : new SimpraString(value.Value.Substring((int)index.Value, 1));
+    public static SimpraString Substring(SimpraString value, SimpraNumber index) => Substring(value, index, 1);
 
     public static SimpraBool Like(SimpraString value, SimpraString pattern)
     {
diff --git a/tests/AltaSoft.Simpra.tests/SimpraExpressionTests.cs b/tests/AltaSoft.Simpra.tests/SimpraExpressionTests.cs
index f270f16..3d99ba6 100644
--- a/tests/AltaSoft.Simpra.tests/SimpraExpressionTests.cs
+++ b/tests/AltaSoft.Simpra.tests/SimpraExpressionTests.cs
@@ -4,6 +4,171 @@ namespace AltaSoft.Simpra.Tests;
 
 public class SimpraExpressionTests
 {
+    [Theory]
+    // basic / within bounds
+    [InlineData("USD", 1, 3, "USD")]   // from U
+    [InlineData("USD", 1, 2, "US")]
+    [InlineData("USD", 2, 2, "SD")]   // from S
+    [InlineData("USD", 3, 1, "D")]    // from D
+
+    // length clamping at the end
+    [InlineData("USD", 1, 10003, "USD")]
+    [InlineData("USD", 3, 10, "D")]
+
+    // start beyond end
+    [InlineData("USD", 1000, 1, "")]
+    [InlineData("USD", 4, 1, "")]     // beyond "USD"
+
+    // start exactly at end
+    [InlineData("USD", 4, 0, "")]
+    [InlineData("USD", 4, 10, "")]
+
+    // zero length
+    [InlineData("USD", 1, 0, "")]
+    [InlineData("USD", 2, 0, "")]
+    [InlineData("USD", 3, 0, "")]
+
+    // negative/zero start → clamp to 1
+    [InlineData("USD", 0, 1, "U")]
+    [InlineData("USD", -5, 2, "US")]
+    [InlineData("USD", -2, 100, "USD")]
+
+    // negative length → empty
+    [InlineData("USD", 1, -1, "")]
+    [InlineData("USD", 3, -10, "")]
+    [InlineData("USD", -2, -10, "")]
+
+    // empty input
+    [InlineData("", 1, 5, "")]
+    [InlineData("", 10, 1, "")]
+    [InlineData("", -3, 2, "")]
+    [InlineData("", 1, 0, "")]
+
+    // whitespace
+    [InlineData(" ", 1, 1, " ")]
+    [InlineData("   ", 2, 2, "  ")]
+    [InlineData("   ", 2, 100, "  ")]
+
+    // non-ASCII (safe checks with multi-byte chars)
+    [InlineData("თბილისი", 1, 2, "თბ")]
+    [InlineData("თბილისი", 3, 3, "ილი")]
+    [InlineData("თბილისი", 11, 5, "")]
+    [InlineData("თბილისი", -3, 100, "თბილისი")]
+
+    public void BuiltInFunction_Substring_EdgeCases(string input, int start, int length, string expected)
+    {
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = input;
+        var expressionCode = $"return substring(Ccy,{start},{length})";
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), expressionCode);
+
+        Assert.Equal(expected, result);
+    }
+
+    [Fact]
+    public void BuiltInFunction_Substring_ShouldMatchExamplesFromPrompt()
+    {
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = "USD";
+
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,0,3)");
+        Assert.Equal("USD", result);
+
+        result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,0,10003)");
+        Assert.Equal("USD", result);
+
+        result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,1000,1)");
+        Assert.Equal("", result);
+
+        result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,1000,0)");
+        Assert.Equal("", result);
+
+        result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,1,0)");
+        Assert.Equal("", result);
+    }
+    [Theory]
+    // basic in-bounds
+    [InlineData("USD", 1, "U")]
+    [InlineData("USD", 2, "S")]
+    [InlineData("USD", 3, "D")]
+
+    // at/after end => empty
+    [InlineData("USD", 4, "")]
+    [InlineData("USD", 1000, "")]
+    [InlineData("U", 2, "")]
+    [InlineData("", 1, "")]
+    [InlineData("", 5, "")]
+
+    // zero/negative start → clamp to 1
+    [InlineData("USD", 0, "U")]
+    [InlineData("USD", -1, "U")]
+    [InlineData("USD", -5, "U")]
+    [InlineData("", -3, "")]
+
+    // whitespace
+    [InlineData(" X ", 1, " ")]
+    [InlineData(" X ", 2, "X")]
+    [InlineData(" X ", 3, " ")]
+
+    // non-ASCII (single UTF-16 code units)
+    [InlineData("თბილისი", 1, "თ")]
+    [InlineData("თბილისი", 3, "ი")]
+    [InlineData("თბილისი", 6, "ს")]
+    [InlineData("თბილისი", 7, "ი")]
+    [InlineData("თბილისი", 8, "")]
+
+    // very large index
+    [InlineData("USD", int.MaxValue, "")]
+    public void BuiltInFunction_Substring_StartOnly_EdgeCases(string input, int start, string expected)
+    {
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = input;
+        var expressionCode = $"return substring(Ccy,{start})";
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), expressionCode);
+
+        Assert.Equal(expected, result);
+    }
+
+    [Fact]
+    public void BuiltInFunction_Substring_StartOnly_NullSource_ShouldBeEmpty()
+    {
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = null;
+
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,1)");
+        Assert.Equal(null, result);
+    }
+
+    [Fact]
+    public void BuiltInFunction_Substring_NullSource_ShouldBeNull()
+    {
+        // If your DSL defines a behavior for nulls, keep this.
+        // If it should throw instead, change to Assert.Throws.
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = null!;
+
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), "return substring(Ccy,0,3)");
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void BuiltInFunction_Substring_ShouldReturnCorrectSubstringOfLength1()
+    {
+        const string expressionCode = "return substring(Ccy,1)";
+
+        var simpra = new Simpra();
+        var model = GetTestModel();
+        model.Ccy = "USD";
+
+        var result = simpra.Execute<string, TestModel, IFunctions>(model, new TestFunctions(), expressionCode);
+        Assert.Equal("U", result);
+    }
+
     [Fact]
     public void InvalidSimpraSyntax_ShouldThrowException_WhenIncorrectAndSignIsUsedAndReturnStatement()
     {