diff --git a/src/AI/tests/Essentials.AI.UnitTests/TestHelpers/DataStreamsHelper.cs b/src/AI/tests/Essentials.AI.UnitTests/TestHelpers/DataStreamsHelper.cs
index 36da3a6332cc..148af442b1c2 100644
--- a/src/AI/tests/Essentials.AI.UnitTests/TestHelpers/DataStreamsHelper.cs
+++ b/src/AI/tests/Essentials.AI.UnitTests/TestHelpers/DataStreamsHelper.cs
@@ -19,7 +19,27 @@ public static string GetTxtItinerary(string fileName)
 	public static string[] GetFileLines(string fileName)
 	{
 		var path = GetFile(fileName);
-		return File.ReadAllLines(path);
+		return ReadAllLinesShared(path);
+	}
+
+	// Reads all lines from a file while allowing other readers/writers to keep the
+	// file open concurrently. Test classes run in parallel and multiple theories read
+	// the same shared TestData files, which can otherwise race and throw an IOException
+	// ("The process cannot access the file because it is being used by another process")
+	// on Windows. Opening with FileShare.ReadWrite makes the shared reads deterministic.
+	public static string[] ReadAllLinesShared(string path)
+	{
+		using var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+		using var reader = new StreamReader(stream);
+
+		var lines = new List<string>();
+		string? line;
+		while ((line = reader.ReadLine()) is not null)
+		{
+			lines.Add(line);
+		}
+
+		return lines.ToArray();
 	}
 
 	public static IEnumerable<object[]> JsonlItineraries
diff --git a/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamChunkerTests/Integration.cs b/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamChunkerTests/Integration.cs
index f366913eefd8..0e4d02e1e505 100644
--- a/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamChunkerTests/Integration.cs
+++ b/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamChunkerTests/Integration.cs
@@ -20,7 +20,7 @@ public void Process_FromJsonlFile_ProducesValidJsonMatchingFinalLine(string file
 
 			// Arrange
 			var chunker = new JsonStreamChunker();
-			var lines = File.ReadAllLines(filePath);
+			var lines = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			// Act
 			var chunks = new List<string>();
diff --git a/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamingRoundtripTests/FileBasedTests.cs b/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamingRoundtripTests/FileBasedTests.cs
index c5b43a9a60ae..226a68e7beec 100644
--- a/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamingRoundtripTests/FileBasedTests.cs
+++ b/src/AI/tests/Essentials.AI.UnitTests/Tests/JsonStreamingRoundtripTests/FileBasedTests.cs
@@ -19,7 +19,7 @@ public void Roundtrip_FromJsonlFile_DeserializerProducesEquivalentResult(string
 			_ = fileName; // We are not using the parameter here
 
 			// Arrange
-			var lines = File.ReadAllLines(filePath);
+			var lines = DataStreamsHelper.ReadAllLinesShared(filePath);
 			var finalLine = lines[^1];
 
 			// Act - pass each line through chunker and deserializer
@@ -48,7 +48,7 @@ public void Roundtrip_FromJsonlFile_EachLineDeserializesProgressively(string fil
 			_ = fileName; // We are not using the parameter here
 
 			// This test validates that each intermediate line produces a valid partial object
-			var lines = File.ReadAllLines(filePath);
+			var lines = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			var models = ProcessLines<Itinerary>(lines);
 
diff --git a/src/AI/tests/Essentials.AI.UnitTests/Tests/StreamingJsonDeserializerTests/FileBasedTests.cs b/src/AI/tests/Essentials.AI.UnitTests/Tests/StreamingJsonDeserializerTests/FileBasedTests.cs
index b60de7522523..7d3a69a2ca21 100644
--- a/src/AI/tests/Essentials.AI.UnitTests/Tests/StreamingJsonDeserializerTests/FileBasedTests.cs
+++ b/src/AI/tests/Essentials.AI.UnitTests/Tests/StreamingJsonDeserializerTests/FileBasedTests.cs
@@ -18,7 +18,7 @@ public void ProcessChunk_TxtFile_DeserializesProgressively(string fileName, stri
 		{
 			_ = fileName; // Used for test display name
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			Itinerary? lastItinerary = null;
 
@@ -41,7 +41,7 @@ public void ProcessChunk_TxtFile_FinalResultMatchesDirectDeserialization(string
 		{
 			_ = fileName; // Used for test display name
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			Itinerary? finalItinerary = null;
 
@@ -65,7 +65,7 @@ public void ProcessChunk_TxtFile_FinalResultMatchesDirectDeserialization(string
 		public void ProcessChunk_TxtFile_ShouldProgressDaysArray(string fileName, string filePath)
 		{
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			var daysProgression = new List<(int ChunkNumber, int? DaysCount, int? ActivitiesInFirstDay)>();
 
@@ -98,7 +98,7 @@ public void ProcessChunk_TxtFile_ShouldProgressDaysArray(string fileName, string
 		public void ProcessChunk_TxtFile_ShouldProgressTitleField(string fileName, string filePath)
 		{
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			var titleProgression = new List<string?>();
 
@@ -129,7 +129,7 @@ public void ProcessChunk_TxtFile_EachChunkProducesNonEmptyObject(string fileName
 		{
 			_ = fileName; // Used for test display name
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			foreach (var chunk in chunks)
 			{
@@ -152,7 +152,7 @@ public void ProcessChunk_TxtFile_UpdateRate(string fileName, string filePath)
 			// partial property values, or unchanged content - 70% is a reasonable threshold.
 
 			var deserializer = new StreamingJsonDeserializer<Itinerary>(DeserializationOptions);
-			var chunks = File.ReadAllLines(filePath);
+			var chunks = DataStreamsHelper.ReadAllLinesShared(filePath);
 
 			string? previousJson = null;
 			int totalChunks = 0;