[API] Add support for W3C randomness flag

src/OpenTelemetry.Api/CHANGELOG.md

@@ -10,6 +10,9 @@ Notes](../../RELEASENOTES.md).
   Add support for using environment variables as context propagation carriers.
   ([#7174](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7174))
 
+* Update `TraceContextPropagator` to support the W3C randomness flag.
+  ([#7301](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7301))
+
 ## 1.15.3
 
 Released 2026-Apr-21

src/OpenTelemetry.Api/Context/Propagation/TraceContextPropagator.cs

@@ -113,7 +113,8 @@ public override void Inject<T>(PropagationContext context, T carrier, Action<T,
         var traceparent = string.Create(55, context.ActivityContext, WriteTraceParentIntoSpan);
 #else
         var traceparent = string.Concat("00-", context.ActivityContext.TraceId.ToHexString(), "-", context.ActivityContext.SpanId.ToHexString());
-        traceparent = string.Concat(traceparent, (context.ActivityContext.TraceFlags & ActivityTraceFlags.Recorded) != 0 ? "-01" : "-00");
+        var traceFlags = FormatActivityTraceFlags(context.ActivityContext.TraceFlags);
+        traceparent = string.Concat(traceparent, traceFlags);
 #endif
 
         setter(carrier, TraceParent, traceparent);
@@ -131,6 +132,78 @@ public override void Inject<T>(PropagationContext context, T carrier, Action<T,
                 }
             }
         }
+
+#if NET
+        static void WriteTraceParentIntoSpan(Span<char> destination, ActivityContext context)
+        {
+            "00-".CopyTo(destination);
+
+            context.TraceId.ToHexString().CopyTo(destination.Slice(3));
+
+            destination[35] = '-';
+
+            context.SpanId.ToHexString().CopyTo(destination.Slice(36));
+
+            // https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/3867
+            // will change this code to use ActivityTraceFlags.RandomTraceId instead of 2.
+            // If new enum values are added in the future the Fallback path will ensure
+            // that the handling is functionally correct, but the condition should be updated
+            // to include the new value(s) for better readability and performance where possible.
+            if (context.TraceFlags == ActivityTraceFlags.Recorded)
+            {
+                "-01".CopyTo(destination.Slice(52));
+            }
+            else if (context.TraceFlags == (ActivityTraceFlags)2)
+            {
+                "-02".CopyTo(destination.Slice(52));
+            }
+            else if (context.TraceFlags == (ActivityTraceFlags.Recorded | (ActivityTraceFlags)2))
+            {
+                "-03".CopyTo(destination.Slice(52));
+            }
+            else
+            {
+                var flags = (byte)context.TraceFlags;
+                destination[52] = '-';
+                destination[53] = GetHexChar(flags >> 4);
+                destination[54] = GetHexChar(flags & 0xF);
+            }
+        }
+#else
+        static string FormatActivityTraceFlags(ActivityTraceFlags flags)
+        {
+            // https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/3867
+            // will change this code to use ActivityTraceFlags.RandomTraceId instead of 2.
+            // If new enum values are added in the future the Fallback path will ensure
+            // that the handling is functionally correct, but the switch should be updated
+            // to include the new value(s) for better readability and performance where possible.
+            return flags switch
+            {
+                ActivityTraceFlags.None => "-00",
+                ActivityTraceFlags.Recorded => "-01",
+                (ActivityTraceFlags)2 => "-02",
+                ActivityTraceFlags.Recorded | (ActivityTraceFlags)2 => "-03",
+                _ => Fallback((byte)flags),
+            };
+
+            static string Fallback(byte flags)
+            {
+                Span<char> buffer = stackalloc char[3];
+
+                buffer[0] = '-';
+                buffer[1] = GetHexChar(flags >> 4);
+                buffer[2] = GetHexChar(flags & 0xF);
+
+                return buffer.ToString();
+            }
+        }
+#endif
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        static char GetHexChar(int value)
+        {
+            return (char)(value + (value < 10 ? '0' : 'a' - 10));
+        }
     }
 
     internal static bool TryExtractTraceparent(string traceparent, out ActivityTraceId traceId, out ActivitySpanId spanId, out ActivityTraceFlags traceOptions)
@@ -208,6 +281,13 @@ internal static bool TryExtractTraceparent(string traceparent, out ActivityTrace
             traceOptions |= ActivityTraceFlags.Recorded;
         }
 
+        if ((optionsLowByte & 2) == 2)
+        {
+            // https://github.com/open-telemetry/opentelemetry-dotnet/pull/6899
+            // will change this to use ActivityTraceFlags.RandomTraceId instead.
+            traceOptions |= (ActivityTraceFlags)2;
+        }
+
         if ((!bestAttempt) && (traceparent.Length != VersionAndTraceIdAndSpanIdLength + OptionsLength))
         {
             return false;
@@ -248,12 +328,10 @@ private static bool TryExtractTracestate(IEnumerable<string>? tracestateCollecti
             }
 
             hasTraceState = true;
-            if (list.Count == 1)
-            {
-                return TryExtractSingleTracestate(list[0], out tracestateResult);
-            }
 
-            return TryExtractMultipleTracestate(list, out tracestateResult);
+            return list.Count == 1 ?
+                TryExtractSingleTracestate(list[0], out tracestateResult) :
+                TryExtractMultipleTracestate(list, out tracestateResult);
         }
 
         if (tracestateCollection is IReadOnlyList<string> readOnlyList)
@@ -264,12 +342,11 @@ private static bool TryExtractTracestate(IEnumerable<string>? tracestateCollecti
             }
 
             hasTraceState = true;
-            if (readOnlyList.Count == 1)
-            {
-                return TryExtractSingleTracestate(readOnlyList[0], out tracestateResult);
-            }
 
-            return TryExtractMultipleTracestate(readOnlyList, out tracestateResult);
+            return
+                readOnlyList.Count == 1 ?
+                TryExtractSingleTracestate(readOnlyList[0], out tracestateResult) :
+                TryExtractMultipleTracestate(readOnlyList, out tracestateResult);
         }
 
         using var enumerator = tracestateCollection.GetEnumerator();
@@ -280,12 +357,10 @@ private static bool TryExtractTracestate(IEnumerable<string>? tracestateCollecti
 
         hasTraceState = true;
         var singleTraceState = enumerator.Current;
-        if (!enumerator.MoveNext())
-        {
-            return TryExtractSingleTracestate(singleTraceState, out tracestateResult);
-        }
 
-        return TryExtractMultipleTracestate(EnumerateFrom(singleTraceState, enumerator), out tracestateResult);
+        return enumerator.MoveNext() ?
+            TryExtractMultipleTracestate(EnumerateFrom(singleTraceState, enumerator), out tracestateResult) :
+            TryExtractSingleTracestate(singleTraceState, out tracestateResult);
     }
 
     private static IEnumerable<string> EnumerateFrom(string first, IEnumerator<string> enumerator)
@@ -721,22 +796,4 @@ private static bool ValidateValue(ReadOnlySpan<char> value)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static bool IsAsciiLetterOrDigitLower(char c)
         => char.IsAsciiDigit(c) || char.IsAsciiLetterLower(c);
-
-#if NET
-    private static void WriteTraceParentIntoSpan(Span<char> destination, ActivityContext context)
-    {
-        "00-".CopyTo(destination);
-        context.TraceId.ToHexString().CopyTo(destination.Slice(3));
-        destination[35] = '-';
-        context.SpanId.ToHexString().CopyTo(destination.Slice(36));
-        if ((context.TraceFlags & ActivityTraceFlags.Recorded) != 0)
-        {
-            "-01".CopyTo(destination.Slice(52));
-        }
-        else
-        {
-            "-00".CopyTo(destination.Slice(52));
-        }
-    }
-#endif
 }

src/OpenTelemetry/CHANGELOG.md

@@ -14,6 +14,9 @@ Notes](../../RELEASENOTES.md).
   once per collection cycle.
   ([#7188](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7188))
 
+* Update `OpenTelemetrySdkEventSource` to support the W3C randomness flag.
+  ([#7301](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7301))
+
 ## 1.15.3
 
 Released 2026-Apr-21

src/OpenTelemetry/Internal/OpenTelemetrySdkEventSource.cs

@@ -6,6 +6,7 @@
 using System.Diagnostics.CodeAnalysis;
 #endif
 using System.Diagnostics.Tracing;
+using System.Globalization;
 using Microsoft.Extensions.Configuration;
 
 namespace OpenTelemetry.Internal;
@@ -69,7 +70,8 @@ public void ActivityStarted(Activity activity)
             // correct sampling flags
             // https://github.com/dotnet/runtime/issues/61857
             var activityId = string.Concat("00-", activity.TraceId.ToHexString(), "-", activity.SpanId.ToHexString());
-            activityId = string.Concat(activityId, activity.ActivityTraceFlags.HasFlag(ActivityTraceFlags.Recorded) ? "-01" : "-00");
+            var traceFlags = ((byte)activity.ActivityTraceFlags).ToString("x2", CultureInfo.InvariantCulture);
+            activityId = string.Concat(activityId, "-", traceFlags);
             this.ActivityStarted(activity.DisplayName, activityId);
         }
     }
@@ -355,7 +357,7 @@ protected override void OnEventWritten(EventWrittenEventArgs e)
             }
 
             var message = e.Message != null && e.Payload != null && e.Payload.Count > 0
-                ? string.Format(System.Globalization.CultureInfo.CurrentCulture, e.Message, [.. e.Payload])
+                ? string.Format(CultureInfo.CurrentCulture, e.Message, [.. e.Payload])
                 : e.Message;
 
             Debug.WriteLine($"{e.EventSource.Name} - Level: [{e.Level}], EventId: [{e.EventId}], EventName: [{e.EventName}], Message: [{message}]");

test/OpenTelemetry.Api.Tests/Context/Propagation/EnvironmentVariableCarrierTests.cs

@@ -209,13 +209,21 @@ public void Set_TargetsEnvironmentCopyWithoutMutatingProcessEnvironment()
         }
     }
 
-    [Fact]
-    public void TraceContextPropagator_RoundTripsThroughEnvironmentVariableCarrier()
+    [Theory]
+    [InlineData(ActivityTraceFlags.None, "00")]
+    [InlineData(ActivityTraceFlags.Recorded, "01")]
+    //// https://github.com/open-telemetry/opentelemetry-dotnet/pull/6899
+    //// will change this to use ActivityTraceFlags.RandomTraceId instead.
+    [InlineData((ActivityTraceFlags)2, "02")]
+    [InlineData((ActivityTraceFlags)2 | ActivityTraceFlags.Recorded, "03")]
+    public void TraceContextPropagator_RoundTripsThroughEnvironmentVariableCarrier(
+        ActivityTraceFlags flags,
+        string expectedSuffix)
     {
         var activityContext = new ActivityContext(
             ActivityTraceId.CreateFromString("0af7651916cd43dd8448eb211c80319c"),
             ActivitySpanId.CreateFromString("b9c7c989f97918e1"),
-            ActivityTraceFlags.Recorded,
+            flags,
             "key1=value1,key2=value2");
 
         var carrier = new Dictionary<string, string?>(StringComparer.Ordinal);
@@ -226,7 +234,7 @@ public void TraceContextPropagator_RoundTripsThroughEnvironmentVariableCarrier()
 
         var extracted = propagator.Extract(default, EnvironmentVariableCarrier.Capture(carrier), EnvironmentVariableCarrier.Get);
 
-        Assert.Equal("00-0af7651916cd43dd8448eb211c80319c-b9c7c989f97918e1-01", carrier["TRACEPARENT"]);
+        Assert.Equal($"00-0af7651916cd43dd8448eb211c80319c-b9c7c989f97918e1-{expectedSuffix}", carrier["TRACEPARENT"]);
         Assert.Equal("key1=value1,key2=value2", carrier["TRACESTATE"]);
         Assert.Equal(activityContext.TraceId, extracted.ActivityContext.TraceId);
         Assert.Equal(activityContext.SpanId, extracted.ActivityContext.SpanId);

test/OpenTelemetry.Instrumentation.W3cTraceContext.Tests/Dockerfile

@@ -37,7 +37,7 @@ RUN tdnf install -y python3-pip \
   && ln -sf /usr/bin/python3 /usr/bin/python \
   && python3 -m pip install --requirement requirements.txt --require-hashes --break-system-packages \
   && tdnf clean all
-ENV SPEC_LEVEL=1
+ENV SPEC_LEVEL=2
 ENV STRICT_LEVEL=1
 
 ENTRYPOINT ["dotnet", "vstest", "OpenTelemetry.Instrumentation.W3cTraceContext.Tests.dll", "--logger:console;verbosity=detailed"]