Prevent Mechanism.TerminalKey from being serialized

Author: bitsandfoxes

src/Sentry/Internal/MainExceptionProcessor.cs

@@ -122,6 +122,11 @@ value is string stringValue &&
                     sentryEvent.Contexts[key[ExceptionDataContextKey.Length..]] = value;
                     keysToRemove.Add(key);
                 }
+                else if (key.Equals(Mechanism.TerminalKey, StringComparison.OrdinalIgnoreCase))
+                {
+                    // Terminal is SDK-internal only, used for session tracking. Don't send to Sentry.
+                    // Skip it - will be removed during serialization in Mechanism.WriteTo()
+                }
                 else if (key.StartsWith(ExceptionDataKeyPrefix, StringComparison.OrdinalIgnoreCase))
                 {
                     sentryEvent.SetExtra($"Exception[{i}][{key}]", value);
@@ -202,6 +207,13 @@ private static Mechanism GetMechanism(Exception exception, int id, int? parentId
         // Add HResult to mechanism data before adding exception data, so that it can be overridden.
         mechanism.Data["HResult"] = $"0x{exception.HResult:X8}";
 
+        // Copy Terminal flag for internal processing, but remove it from Exception.Data
+        if (exception.Data[Mechanism.TerminalKey] is bool terminal)
+        {
+            mechanism.Data[Mechanism.TerminalKey] = terminal;
+            exception.Data.Remove(Mechanism.TerminalKey);
+        }
+
         // Copy remaining exception data to mechanism data.
         foreach (var key in exception.Data.Keys.OfType<string>())
         {

src/Sentry/Protocol/Mechanism.cs

@@ -32,8 +32,10 @@ public sealed class Mechanism : ISentryJsonSerializable
     /// <summary>
     /// Key found inside of <c>Exception.Data</c> describing whether the exception is considered terminal.
     /// </summary>
-    /// <remarks> It's not prefixed with 'Sentry' so the MainExceptionProcessor does not remove it. </remarks>
-    public static readonly string TerminalKey = "Terminal";
+    /// <remarks>
+    /// This is an SDK-internal flag used for session tracking and is not sent to Sentry servers.
+    /// </remarks>
+    public static readonly string TerminalKey = "Sentry:Terminal";
 
     internal Dictionary<string, object>? InternalData { get; private set; }
 
@@ -143,7 +145,14 @@ public void WriteTo(Utf8JsonWriter writer, IDiagnosticLogger? logger)
         writer.WriteBooleanIfTrue("is_exception_group", IsExceptionGroup);
         writer.WriteNumberIfNotNull("exception_id", ExceptionId);
         writer.WriteNumberIfNotNull("parent_id", ParentId);
-        writer.WriteDictionaryIfNotEmpty("data", InternalData!, logger);
+
+        // Filter out Terminal flag from Data before serialization (SDK-internal only)
+        var dataToSerialize = InternalData?.Count > 0
+            ? InternalData.Where(kvp => !kvp.Key.Equals(TerminalKey, StringComparison.OrdinalIgnoreCase))
+                          .ToDictionary(kvp => kvp.Key, kvp => kvp.Value)
+            : null;
+        writer.WriteDictionaryIfNotEmpty("data", dataToSerialize!, logger);
+
         writer.WriteDictionaryIfNotEmpty("meta", InternalMeta!, logger);
 
         writer.WriteEndObject();

test/Sentry.Tests/Internals/MainExceptionProcessorTests.cs

@@ -110,7 +110,7 @@ public void Process_ExceptionWith_TerminalTrue_StoresInMechanismData()
         var evt = new SentryEvent();
         var exp = new Exception();
 
-        exp.Data.Add(Mechanism.TerminalKey, true);
+        exp.SetSentryMechanism("TestException", terminal: true);
 
         sut.Process(exp, evt);
 
@@ -127,7 +127,7 @@ public void Process_ExceptionWith_TerminalFalse_StoresInMechanismData()
         var evt = new SentryEvent();
         var exp = new Exception();
 
-        exp.Data.Add(Mechanism.TerminalKey, false);
+        exp.SetSentryMechanism("TestException", terminal: false);
 
         sut.Process(exp, evt);
 

test/Sentry.Tests/Protocol/SentryEventTests.cs

@@ -210,7 +210,6 @@ public void GetExceptionType_UnhandledTerminalException_ReturnsUnhandled()
     [Fact]
     public void GetExceptionType_UnhandledTerminalExceptionViaSentryExceptions_ReturnsUnhandled()
     {
-        // Terminal = null or true means default behavior (terminal)
         var evt = new SentryEvent
         {
             SentryExceptions = [new SentryException { Mechanism = new Mechanism { Handled = false } }]