diff --git a/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/Defaults.cs b/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/Defaults.cs
index 777d723b790..380b109471e 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/Defaults.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/Defaults.cs
@@ -30,4 +30,10 @@ public static class Defaults
     /// in the <see cref="IResponseCacheProvider"/>'s cache before they are considered expired and evicted.
     /// </summary>
     public static TimeSpan DefaultTimeToLiveForCacheEntries { get; } = TimeSpan.FromDays(14);
+
+    /// <summary>
+    /// Defines the version number for the reporting format. If and when the serialized format undergoes
+    /// breaking changes, this number will be incremented.
+    /// </summary>
+    internal const int ReportingFormatVersion = 1;
 }
diff --git a/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/ScenarioRunResult.cs b/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/ScenarioRunResult.cs
index e1a4102e42c..e4b9401d926 100644
--- a/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/ScenarioRunResult.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting/CSharp/ScenarioRunResult.cs
@@ -37,6 +37,7 @@ namespace Microsoft.Extensions.AI.Evaluation.Reporting;
 /// The <see cref="Evaluation.EvaluationResult"/> for the <see cref="ScenarioRun"/> corresponding to the
 /// <see cref="ScenarioRunResult"/> being constructed.
 /// </param>
+/// <param name="formatVersion">The version of the format used to persist the current <see cref="ScenarioRunResult"/>.</param>
 [method: JsonConstructor]
 public sealed class ScenarioRunResult(
     string scenarioName,
@@ -45,7 +46,8 @@ public sealed class ScenarioRunResult(
     DateTime creationTime,
     IList<ChatMessage> messages,
     ChatResponse modelResponse,
-    EvaluationResult evaluationResult)
+    EvaluationResult evaluationResult,
+    int? formatVersion = null)
 {
     /// <summary>
     /// Initializes a new instance of the <see cref="ScenarioRunResult"/> class.
@@ -81,6 +83,11 @@ public ScenarioRunResult(
     {
     }
 
+    /// <summary>
+    /// Gets the version of the format used to persist the current <see cref="ScenarioRunResult"/>.
+    /// </summary>
+    public int? FormatVersion { get; } = formatVersion ?? Defaults.ReportingFormatVersion;
+
     /// <summary>
     /// Gets or sets the <see cref="ScenarioRun.ScenarioName"/>.
     /// </summary>
diff --git a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/DistributedCachingChatClient.cs b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/DistributedCachingChatClient.cs
index 67bd64cfa54..c59c78c9cd9 100644
--- a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/DistributedCachingChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/DistributedCachingChatClient.cs
@@ -107,6 +107,11 @@ protected override async Task WriteCacheStreamingAsync(string key, IReadOnlyList
     /// The generated cache key is not guaranteed to be stable across releases of the library.
     /// </para>
     /// </remarks>
-    protected override string GetCacheKey(params ReadOnlySpan<object?> values) =>
-        AIJsonUtilities.HashDataToString(values, _jsonSerializerOptions);
+    protected override string GetCacheKey(params ReadOnlySpan<object?> values)
+    {
+        // Bump the cache version to invalidate existing caches if the serialization format changes in a breaking way.
+        const int CacheVersion = 1;
+
+        return AIJsonUtilities.HashDataToString([CacheVersion, .. values], _jsonSerializerOptions);
+    }
 }
diff --git a/test/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting.Tests/ScenarioRunResultTests.cs b/test/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting.Tests/ScenarioRunResultTests.cs
index 9418a5db359..b6f173abd5a 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting.Tests/ScenarioRunResultTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Evaluation.Reporting.Tests/ScenarioRunResultTests.cs
@@ -38,6 +38,7 @@ public void SerializeScenarioRunResult()
             messages: [new ChatMessage(ChatRole.User, "prompt")],
             modelResponse: new ChatResponse(new ChatMessage(ChatRole.Assistant, "response")),
             evaluationResult: new EvaluationResult(booleanMetric, numericMetric, stringMetric, metricWithNoValue));
+        Assert.Equal(Defaults.ReportingFormatVersion, entry.FormatVersion);
 
         string json = JsonSerializer.Serialize(entry, SerializerContext.Default.ScenarioRunResult);
         ScenarioRunResult? deserialized = JsonSerializer.Deserialize<ScenarioRunResult>(json, SerializerContext.Default.ScenarioRunResult);
@@ -49,6 +50,7 @@ public void SerializeScenarioRunResult()
         Assert.Equal(entry.CreationTime, deserialized.CreationTime);
         Assert.True(entry.Messages.SequenceEqual(deserialized.Messages, ChatMessageComparer.Instance));
         Assert.Equal(entry.ModelResponse, deserialized.ModelResponse, ChatResponseComparer.Instance);
+        Assert.Equal(entry.FormatVersion, deserialized.FormatVersion);
 
         ValidateEquivalence(entry.EvaluationResult, deserialized.EvaluationResult);
     }