diff --git a/README.md b/README.md
index efaa8ed..cb5e1c5 100644
--- a/README.md
+++ b/README.md
@@ -71,8 +71,8 @@ slnx-validator MySolution.slnx
 [FAIL] MySolution.slnx
 
 MySolution.slnx
-  - line 5: [SLNX0013] The element 'Folder' in namespace '...' has invalid child element 'Folder'. List of possible elements expected: 'Project'.
-  - line 12: [SLNX0011] File not found: docs\CONTRIBUTING.md
+  - line 5: [SLNX013] The element 'Folder' in namespace '...' has invalid child element 'Folder'. List of possible elements expected: 'Project'.
+  - line 12: [SLNX011] File not found: docs\CONTRIBUTING.md
 ```
 
 ### Multiple files — mixed results
@@ -86,8 +86,8 @@ slnx-validator src\
 [FAIL] src\Backend.slnx
 
 src\Backend.slnx
-  - line 4: [SLNX0011] File not found: docs\CONTRIBUTING.md
-  - line 8: [SLNX0012] Wildcard patterns are not supported in file paths: docs\*.md
+  - line 4: [SLNX011] File not found: docs\CONTRIBUTING.md
+  - line 8: [SLNX012] Wildcard patterns are not supported in file paths: docs\*.md
 ```
 
 ## What is validated
@@ -116,13 +116,13 @@ The following are **intentionally out of scope** because the toolchain already h
 
 | Code | Name | Description |
 |------|------|-------------|
-| `SLNX0001` | `FileNotFound`            | The input `.slnx` file does not exist. |
-| `SLNX0002` | `InvalidExtension`        | The input file does not have a `.slnx` extension. |
-| `SLNX0003` | `NotATextFile`            | The file is binary and cannot be parsed as XML. |
-| `SLNX0010` | `InvalidXml`              | The file is not valid XML (see [`examples/invalid-not-xml.slnx`](examples/invalid-not-xml.slnx)). |
-| `SLNX0011` | `FileNotFound`            | A file referenced in `<File Path="...">` does not exist on disk. |
-| `SLNX0012` | `InvalidWildcardUsage`    | A `<File Path="...">` contains a wildcard pattern (see [`examples/invalid-wildcard.slnx`](examples/invalid-wildcard.slnx)). |
-| `SLNX0013` | `XsdViolation`            | The XML structure violates the schema, e.g. `<Folder>` inside `<Folder>` (see [`examples/invalid-xsd.slnx`](examples/invalid-xsd.slnx)). |
+| `SLNX001` | `FileNotFound`            | The input `.slnx` file does not exist. |
+| `SLNX002` | `InvalidExtension`        | The input file does not have a `.slnx` extension. |
+| `SLNX003` | `NotATextFile`            | The file is binary and cannot be parsed as XML. |
+| `SLNX010` | `InvalidXml`              | The file is not valid XML (see [`examples/invalid-not-xml.slnx`](examples/invalid-not-xml.slnx)). |
+| `SLNX011` | `ReferencedFileNotFound`  | A file referenced in `<File Path="...">` does not exist on disk. |
+| `SLNX012` | `InvalidWildcardUsage`    | A `<File Path="...">` contains a wildcard pattern (see [`examples/invalid-wildcard.slnx`](examples/invalid-wildcard.slnx)). |
+| `SLNX013` | `XsdViolation`            | The XML structure violates the schema, e.g. `<Folder>` inside `<Folder>` (see [`examples/invalid-xsd.slnx`](examples/invalid-xsd.slnx)). |
 
 ## XSD Schema
 
diff --git a/examples/invalid-not-xml.slnx b/examples/invalid-not-xml.slnx
index afe74b5..0dd67a7 100644
--- a/examples/invalid-not-xml.slnx
+++ b/examples/invalid-not-xml.slnx
@@ -1,2 +1,2 @@
 This file intentionally contains plain text instead of XML,
-to demonstrate the SLNX0010 InvalidXml error.
+to demonstrate the SLNX010 InvalidXml error.
diff --git a/src/SLNX-validator.Core/ValidationResults/ValidationErrorCodeExtensions.cs b/src/SLNX-validator.Core/ValidationResults/ValidationErrorCodeExtensions.cs
index 35d24b0..07dd4cf 100644
--- a/src/SLNX-validator.Core/ValidationResults/ValidationErrorCodeExtensions.cs
+++ b/src/SLNX-validator.Core/ValidationResults/ValidationErrorCodeExtensions.cs
@@ -3,5 +3,5 @@ namespace JulianVerdurmen.SlnxValidator.Core.ValidationResults;
 public static class ValidationErrorCodeExtensions
 {
     public static string ToCode(this ValidationErrorCode code) =>
-        $"SLNX{(int)code:D4}";
+        $"SLNX{(int)code:D3}";
 }
diff --git a/tests/SLNX-validator.Core.Tests/SlnxValidatorTests.cs b/tests/SLNX-validator.Core.Tests/SlnxValidatorTests.cs
index 47d0eae..4f4ac46 100644
--- a/tests/SLNX-validator.Core.Tests/SlnxValidatorTests.cs
+++ b/tests/SLNX-validator.Core.Tests/SlnxValidatorTests.cs
@@ -146,12 +146,12 @@ public async Task ValidateAsync_WildcardInFilePath_ReturnsInvalidWildcardUsageEr
     [Test]
     public async Task ValidationErrorCode_ToCode_ReturnsPrefixedCode()
     {
-        ValidationErrorCode.FileNotFound.ToCode().Should().Be("SLNX0001");
-        ValidationErrorCode.InvalidExtension.ToCode().Should().Be("SLNX0002");
-        ValidationErrorCode.NotATextFile.ToCode().Should().Be("SLNX0003");
-        ValidationErrorCode.InvalidXml.ToCode().Should().Be("SLNX0010");
-        ValidationErrorCode.ReferencedFileNotFound.ToCode().Should().Be("SLNX0011");
-        ValidationErrorCode.InvalidWildcardUsage.ToCode().Should().Be("SLNX0012");
-        ValidationErrorCode.XsdViolation.ToCode().Should().Be("SLNX0013");
+        ValidationErrorCode.FileNotFound.ToCode().Should().Be("SLNX001");
+        ValidationErrorCode.InvalidExtension.ToCode().Should().Be("SLNX002");
+        ValidationErrorCode.NotATextFile.ToCode().Should().Be("SLNX003");
+        ValidationErrorCode.InvalidXml.ToCode().Should().Be("SLNX010");
+        ValidationErrorCode.ReferencedFileNotFound.ToCode().Should().Be("SLNX011");
+        ValidationErrorCode.InvalidWildcardUsage.ToCode().Should().Be("SLNX012");
+        ValidationErrorCode.XsdViolation.ToCode().Should().Be("SLNX013");
     }
 }