DataDog
diff --git a/‎dd-java-agent/agent-bootstrap/build.gradle‎
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diff --git a/‎dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/Agent.java‎
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diff --git a/‎dd-trace-api/build.gradle.kts‎
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diff --git a/‎dd-trace-api/src/main/java/datadog/trace/api/config/FeatureFlaggingConfig.java‎
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diff --git a/‎metadata/supported-configurations.json‎
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diff --git a/‎products/feature-flagging/feature-flagging-agent/src/main/java/com/datadog/featureflag/FeatureFlaggingSystem.java‎
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diff --git a/‎products/feature-flagging/feature-flagging-api/build.gradle.kts‎
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diff --git a/‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/DDEvaluator.java‎
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diff --git a/‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/Provider.java‎
Lines changed: 56 additions & 5 deletions b/‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/Provider.java‎
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diff --git a/‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/SpanEnrichmentGate.java‎
Lines changed: 25 additions & 0 deletions b/‎products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/SpanEnrichmentGate.java‎
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@@ -23,6 +23,7 @@ dependencies {
   api project(':dd-java-agent:agent-debugger:debugger-bootstrap')
   api project(':components:environment')
   api project(':components:json')
+  api project(':products:feature-flagging:feature-flagging-config')
   api project(':products:metrics:metrics-agent')
   api libs.instrument.java
   api libs.slf4j
 
@@ -34,7 +34,6 @@
 import datadog.trace.api.config.CrashTrackingConfig;
 import datadog.trace.api.config.CwsConfig;
 import datadog.trace.api.config.DebuggerConfig;
-import datadog.trace.api.config.FeatureFlaggingConfig;
 import datadog.trace.api.config.GeneralConfig;
 import datadog.trace.api.config.IastConfig;
 import datadog.trace.api.config.JmxFetchConfig;
@@ -44,6 +43,7 @@
 import datadog.trace.api.config.TraceInstrumentationConfig;
 import datadog.trace.api.config.TracerConfig;
 import datadog.trace.api.config.UsmConfig;
+import datadog.trace.api.featureflag.config.FeatureFlaggingConfig;
 import datadog.trace.api.gateway.RequestContextSlot;
 import datadog.trace.api.gateway.SubscriptionService;
 import datadog.trace.api.git.EmbeddedGitInfoBuilder;
 
@@ -38,7 +38,6 @@ extra["excludedClassesCoverage"] = listOf(
   "datadog.trace.api.civisibility.noop.NoOpDDTestSession",
   "datadog.trace.api.civisibility.noop.NoOpDDTestSuite",
   "datadog.trace.api.config.AIGuardConfig",
-  "datadog.trace.api.config.FeatureFlaggingConfig",
   "datadog.trace.api.config.ProfilingConfig",
   "datadog.trace.api.interceptor.MutableSpan",
   "datadog.trace.api.profiling.Profiling",
 
@@ -1505,6 +1505,14 @@
         "aliases": []
       }
     ],
+    "DD_EXPERIMENTAL_FLAGGING_PROVIDER_SPAN_ENRICHMENT_ENABLED": [
+      {
+        "version": "A",
+        "type": "boolean",
+        "default": "false",
+        "aliases": []
+      }
+    ],
     "DD_EXPERIMENTAL_PROPAGATE_PROCESS_TAGS_ENABLED": [
       {
         "version": "B",
 
@@ -11,6 +11,7 @@ public class FeatureFlaggingSystem {
 
   private static volatile RemoteConfigService CONFIG_SERVICE;
   private static volatile ExposureWriter EXPOSURE_WRITER;
+  private static volatile SpanEnrichmentWriter SPAN_ENRICHMENT_WRITER;
 
   private FeatureFlaggingSystem() {}
 
@@ -27,10 +28,22 @@ public static void start(final SharedCommunicationObjects sco) {
     EXPOSURE_WRITER = new ExposureWriterImpl(sco, config);
     EXPOSURE_WRITER.init();
 
+    // APM span enrichment: agent-side listener for flag-evaluation seam events. Uses the process-
+    // wide singleton so a subsystem restart reuses the one already-registered trace interceptor
+    // (which the tracer cannot remove) instead of registering a second, rejected one. Cheap: it
+    // only accumulates once the provider's gate-on capture hook dispatches events, and registers
+    // its interceptor lazily on the first such event.
+    SPAN_ENRICHMENT_WRITER = SpanEnrichmentWriter.getInstance();
+    SPAN_ENRICHMENT_WRITER.init();
+
     LOGGER.debug("Feature Flagging system started");
   }
 
   public static void stop() {
+    if (SPAN_ENRICHMENT_WRITER != null) {
+      SPAN_ENRICHMENT_WRITER.close();
+      SPAN_ENRICHMENT_WRITER = null;
+    }
     if (EXPOSURE_WRITER != null) {
       EXPOSURE_WRITER.close();
       EXPOSURE_WRITER = null;
 
@@ -44,9 +44,12 @@ dependencies {
   api("dev.openfeature:sdk:1.20.1")
 
   compileOnly(project(":products:feature-flagging:feature-flagging-bootstrap"))
+  compileOnly(project(":products:feature-flagging:feature-flagging-config"))
+  compileOnly(project(":utils:config-utils"))
   compileOnly("io.opentelemetry:opentelemetry-api:1.47.0")
 
   testImplementation(project(":products:feature-flagging:feature-flagging-bootstrap"))
+  testImplementation(project(":utils:config-utils"))
   testImplementation("io.opentelemetry:opentelemetry-api:1.47.0")
   testImplementation(libs.bundles.junit5)
   testImplementation(libs.bundles.mockito)
 
@@ -46,6 +46,16 @@ class DDEvaluator implements Evaluator, FeatureFlaggingGateway.ConfigListener {
   private static final Set<Class<?>> SUPPORTED_RESOLUTION_TYPES =
       new HashSet<>(asList(String.class, Boolean.class, Integer.class, Double.class, Value.class));
 
+  // Evaluation-metadata keys consumed by the span-enrichment capture hook (see
+  // SpanEnrichmentHook). Emitted only when the span-enrichment gate is on.
+  static final String METADATA_SPLIT_SERIAL_ID = "__dd_split_serial_id";
+  static final String METADATA_DO_LOG = "__dd_do_log";
+
+  // Read once: when off, the __dd_* span-enrichment metadata is not attached to evaluations, so an
+  // enabled provider pays nothing extra unless span enrichment is also enabled. The gate does not
+  // change at runtime, and this class is loaded lazily (well after startup) so config is ready.
+  private static final boolean SPAN_ENRICHMENT_ENABLED = SpanEnrichmentGate.isEnabled();
+
   private final Runnable configCallback;
   private final AtomicReference<ServerConfiguration> configuration = new AtomicReference<>();
   private final CountDownLatch initializationLatch = new CountDownLatch(1);
@@ -392,6 +402,17 @@ private static <T> ProviderEvaluation<T> resolveVariant(
             .addString("flagKey", flag.key)
             .addString("variationType", flag.variationType.name())
             .addString("allocationKey", allocation.key);
+    // Surface the UFC split's serial id and the allocation's doLog flag for APM span enrichment —
+    // only when span enrichment is on, so a provider without enrichment pays nothing extra.
+    // __dd_split_serial_id is omitted when the split carries no serial id; __dd_do_log is always
+    // present (when enrichment is on) so the span-enrichment hook can decide whether to record the
+    // subject.
+    if (SPAN_ENRICHMENT_ENABLED) {
+      if (split.serialId != null) {
+        metadataBuilder.addInteger(METADATA_SPLIT_SERIAL_ID, split.serialId);
+      }
+      metadataBuilder.addBoolean(METADATA_DO_LOG, allocation.doLog != null && allocation.doLog);
+    }
     final ProviderEvaluation<T> result =
         ProviderEvaluation.<T>builder()
             .value(mappedValue)
 
@@ -16,6 +16,7 @@
 import dev.openfeature.sdk.exceptions.OpenFeatureError;
 import dev.openfeature.sdk.exceptions.ProviderNotReadyError;
 import java.lang.reflect.Constructor;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
@@ -28,13 +29,19 @@ public class Provider extends EventProvider implements Metadata {
   private static final Logger log = LoggerFactory.getLogger(Provider.class);
   static final String METADATA = "datadog-openfeature-provider";
   private static final String EVALUATOR_IMPL = "datadog.trace.api.openfeature.DDEvaluator";
+
   private static final Options DEFAULT_OPTIONS = new Options().initTimeout(30, SECONDS);
   private volatile Evaluator evaluator;
   private final Options options;
   private final AtomicReference<InitializationState> initializationState =
       new AtomicReference<>(InitializationState.NOT_STARTED);
   private final FlagEvalMetrics flagEvalMetrics;
   private final FlagEvalHook flagEvalHook;
+  // Span enrichment: null unless the gate is on, so the feature has no idle overhead when off.
+  private final SpanEnrichmentHook spanEnrichmentHook;
+  // Precomputed hook list returned by getProviderHooks() on every evaluation. Immutable and built
+  // once so gate-off evaluation allocates nothing on this hot path.
+  private final List<Hook> providerHooks;
 
   public Provider() {
     this(DEFAULT_OPTIONS, null);
@@ -45,6 +52,17 @@ public Provider(final Options options) {
   }
 
   Provider(final Options options, final Evaluator evaluator) {
+    this(options, evaluator, null);
+  }
+
+  /**
+   * @param spanEnrichmentEnabledOverride when non-null, forces the span-enrichment gate (test
+   *     seam); when null, the gate is read via {@link SpanEnrichmentGate}.
+   */
+  Provider(
+      final Options options,
+      final Evaluator evaluator,
+      final Boolean spanEnrichmentEnabledOverride) {
     this.options = options;
     this.evaluator = evaluator;
     FlagEvalMetrics metrics = null;
@@ -58,6 +76,34 @@ public Provider(final Options options) {
     }
     this.flagEvalMetrics = metrics;
     this.flagEvalHook = hook;
+
+    // Span enrichment is wired ONLY when the gate is on — off means no capture hook and no idle
+    // per-evaluation overhead.
+    final boolean spanEnrichmentEnabled =
+        spanEnrichmentEnabledOverride != null
+            ? spanEnrichmentEnabledOverride
+            : SpanEnrichmentGate.isEnabled();
+    this.spanEnrichmentHook = spanEnrichmentEnabled ? new SpanEnrichmentHook() : null;
+
+    // Precompute the immutable hook list once so getProviderHooks() (called on every evaluation)
+    // allocates nothing, including when the gate is off.
+    final List<Hook> hooks = new ArrayList<>(2);
+    if (flagEvalHook != null) {
+      hooks.add(flagEvalHook);
+    }
+    if (spanEnrichmentHook != null) {
+      hooks.add(spanEnrichmentHook);
+    }
+    this.providerHooks =
+        hooks.isEmpty() ? Collections.emptyList() : Collections.unmodifiableList(hooks);
+
+    // Announce the span-enrichment state at startup (matches the reference implementation).
+    // "enabled" only when the gate is on (the capture hook was constructed), otherwise "disabled".
+    if (spanEnrichmentHook != null) {
+      log.info("{} span enrichment enabled", METADATA);
+    } else {
+      log.info("{} span enrichment disabled", METADATA);
+    }
   }
 
   @Override
@@ -167,22 +213,27 @@ private Evaluator buildEvaluator() throws Exception {
 
   @Override
   public List<Hook> getProviderHooks() {
-    if (flagEvalHook == null) {
-      return Collections.emptyList();
-    }
-    return Collections.singletonList(flagEvalHook);
+    return providerHooks;
   }
 
   @Override
   public void shutdown() {
     if (flagEvalMetrics != null) {
       flagEvalMetrics.shutdown();
     }
+    // Span enrichment needs no provider-close cleanup here: the capture hook holds no tracer state.
+    // The agent-side write tier owns the interceptor and per-trace state and is torn down with the
+    // feature-flagging subsystem, not per provider.
     if (evaluator != null) {
       evaluator.shutdown();
     }
   }
 
+  // Visible for tests: expose whether span enrichment is wired (gate-on) without leaking the impl.
+  SpanEnrichmentHook spanEnrichmentHook() {
+    return spanEnrichmentHook;
+  }
+
   @Override
   public Metadata getMetadata() {
     return this;
@@ -223,7 +274,7 @@ public ProviderEvaluation<Value> getObjectEvaluation(
     return evaluator.evaluate(Value.class, key, defaultValue, ctx);
   }
 
-  @SuppressForbidden // Class#forName(String) used to lazy load internal-api dependencies
+  @SuppressForbidden // Class#forName(String) used to lazy-load the evaluator implementation
   protected Class<?> loadEvaluatorClass() throws ClassNotFoundException {
     return Class.forName(EVALUATOR_IMPL);
   }
 
@@ -0,0 +1,25 @@
+package datadog.trace.api.openfeature;
+
+import datadog.trace.api.featureflag.config.FeatureFlaggingConfig;
+import datadog.trace.bootstrap.config.provider.ConfigProvider;
+
+/**
+ * Single source for reading the experimental span-enrichment gate, with full {@link ConfigProvider}
+ * precedence (system property > stable config > env var {@code
+ * DD_EXPERIMENTAL_FLAGGING_PROVIDER_SPAN_ENRICHMENT_ENABLED}). OFF by default; distinct from the
+ * provider-enabled gate. Shared so {@link Provider} (per construction) and {@link DDEvaluator}
+ * (once at class load) read it the same way.
+ */
+final class SpanEnrichmentGate {
+
+  private SpanEnrichmentGate() {}
+
+  static boolean isEnabled() {
+    try {
+      return ConfigProvider.getInstance()
+          .getBoolean(FeatureFlaggingConfig.EXPERIMENTAL_SPAN_ENRICHMENT_ENABLED, false);
+    } catch (final Throwable t) {
+      return false; // never let config reading break construction
+    }
+  }
+}