@@ -29,6 +29,7 @@ interface CommonTags {
 
   String ENV = TAG_PREFIX + "env";
   String SERVICE = TAG_PREFIX + "service";
+  String TRACE_ID = TAG_PREFIX + "trace_id";
   String PARENT_ID = TAG_PREFIX + "parent_id";
   String SESSION_ID = TAG_PREFIX + LLMObsTags.SESSION_ID;
   String AGENT_VERSION = TAG_PREFIX + LLMObsTags.AGENT_VERSION;
 
@@ -116,9 +116,9 @@ protected void doAfterStart(@Nonnull AgentSpan span) {
       span.setTag(CommonTags.INTEGRATION, INTEGRATION);
 
       // Resolve the LLMObs parent context, gated on trace-id consistency: a stale context
-      // from a different trace (e.g. async boundary leakage) must not contribute parent_id,
-      // session_id, agent_version, agent attribution, or a sampling verdict to this span.
-      // Matches DDLLMObsSpan's manual-span gate. One flag drives every inherited value, so a
+      // from a different trace (e.g. async boundary leakage) must not contribute trace_id, ml_app,
+      // parent_id, session_id, agent_version, agent attribution, or a sampling verdict to this
+      // span. Matches DDLLMObsSpan's manual-span gate. One flag drives every inherited value, so a
       // new propagated tag cannot accidentally ship with a weaker gate of its own.
       AgentSpanContext parent = LLMObsContext.current();
       boolean inheritable = parent != null && parent.getTraceId().equals(span.getTraceId());
@@ -129,6 +129,17 @@ protected void doAfterStart(@Nonnull AgentSpan span) {
       if (inheritable) {
         parentSpanId = String.valueOf(parent.getSpanId());
 
+        // Stay in the parent's LLMObs trace and application. The parent may have adopted both
+        // from another service, so the APM trace id and the service default can differ from them.
+        String traceId = LLMObsContext.currentTraceId();
+        if (traceId != null && !traceId.isEmpty()) {
+          span.setTag(CommonTags.TRACE_ID, traceId);
+        }
+        String mlApp = LLMObsContext.currentMlApp();
+        if (mlApp != null && !mlApp.isEmpty()) {
+          span.setTag(CommonTags.ML_APP, mlApp);
+        }
+
         // Inherit session_id from the active LLMObs parent (e.g. a manual workflow span).
         // Matches dd-trace-py / dd-trace-js, where auto-instrumented LLM spans inherit
         // session_id from the workflow root via context propagation. Without this, the
@@ -161,18 +172,20 @@ protected void doAfterStart(@Nonnull AgentSpan span) {
       }
       span.setTag(CommonTags.PARENT_ID, parentSpanId);
 
-      // Compute the sampling decision if none was inherited (no LLMObs parent), which makes this
-      // span the root of its own LLMObs trace. Unlike the tags above, this cannot be skipped when
-      // there is nothing to inherit: an unstamped span is retained at any configured rate.
-      if (samplingDecision == null || sampleRate == null) {
+      // Roll a decision only when this span is the root of its own LLMObs trace. A parent with no
+      // decision continued a trace whose caller sent none, so this span leaves both tags unset
+      // too, as DDLLMObsSpan does; a local roll here could drop the span while its parent is kept.
+      if (!inheritable) {
         sampleRate = sampler.formattedRate();
         samplingDecision =
             sampler.sample(span.getTraceId().toLong())
                 ? LLMObsContext.SAMPLING_DECISION_SAMPLED
                 : LLMObsContext.SAMPLING_DECISION_DROPPED;
       }
-      span.setTag(CommonTags.SAMPLING_DECISION, samplingDecision);
-      span.setTag(CommonTags.SAMPLE_RATE, sampleRate);
+      if (samplingDecision != null && sampleRate != null) {
+        span.setTag(CommonTags.SAMPLING_DECISION, samplingDecision);
+        span.setTag(CommonTags.SAMPLE_RATE, sampleRate);
+      }
     }
     super.doAfterStart(span);
   }
 
@@ -1,6 +1,7 @@
 package datadog.trace.instrumentation.openai_java;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
@@ -15,6 +16,7 @@
 import com.sun.net.httpserver.HttpServer;
 import datadog.context.ContextScope;
 import datadog.trace.agent.test.AbstractInstrumentationTest;
+import datadog.trace.api.Config;
 import datadog.trace.api.llmobs.GenAiApmTags;
 import datadog.trace.api.llmobs.LLMObsContext;
 import datadog.trace.bootstrap.instrumentation.api.AgentSpan;
@@ -267,6 +269,105 @@ void openAiRequestSpanComputesItsOwnSamplingDecisionWhenNoLlmObsContext() throws
     assertEquals("1", openAiSpan.getTag("_ml_obs_tag.sample_rate"));
   }
 
+  @Test
+  void openAiRequestSpanStaysInTheParentsAdoptedLlmObsTrace() throws Exception {
+    // A parent that continued a trace from another service carries that service's LLMObs trace id,
+    // which is not the local APM trace id. The LLM span must report the parent's id, or it lands
+    // in a different LLMObs trace from its parent.
+    String adoptedTraceId = "6ab41f6e00000000fea4f6b7295bd5e9";
+
+    AgentSpan parentSpan = AgentTracer.startSpan("test", "parent");
+    try (ContextScope ignored1 = AgentTracer.activateSpan(parentSpan)) {
+      try (ContextScope ignored2 =
+          LLMObsContext.attach(
+              parentSpan.spanContext(),
+              adoptedTraceId,
+              null,
+              null,
+              null,
+              "1",
+              LLMObsContext.SAMPLING_DECISION_SAMPLED,
+              null,
+              null)) {
+        openAiClient.chat().completions().create(buildMinimalChatParams());
+      }
+    } finally {
+      parentSpan.finish();
+    }
+
+    writer.waitForTraces(1);
+    DDSpan openAiSpan = findSpanByOperationName(writer, "openai.request");
+    assertNotNull(openAiSpan, "openai.request span should have been created");
+    assertNotEquals(adoptedTraceId, openAiSpan.getTraceId().toHexString());
+    assertEquals(adoptedTraceId, openAiSpan.getTag("_ml_obs_tag.trace_id"));
+  }
+
+  @Test
+  void openAiRequestSpanInheritsMlAppFromActiveContext() throws Exception {
+    String expectedMlApp = "upstream-app";
+
+    AgentSpan parentSpan = AgentTracer.startSpan("test", "parent");
+    try (ContextScope ignored1 = AgentTracer.activateSpan(parentSpan)) {
+      try (ContextScope ignored2 =
+          LLMObsContext.attach(
+              parentSpan.spanContext(),
+              null,
+              expectedMlApp,
+              null,
+              null,
+              "1",
+              LLMObsContext.SAMPLING_DECISION_SAMPLED,
+              null,
+              null)) {
+        openAiClient.chat().completions().create(buildMinimalChatParams());
+      }
+    } finally {
+      parentSpan.finish();
+    }
+
+    writer.waitForTraces(1);
+    DDSpan openAiSpan = findSpanByOperationName(writer, "openai.request");
+    assertNotNull(openAiSpan, "openai.request span should have been created");
+    assertEquals(expectedMlApp, openAiSpan.getTag("_ml_obs_tag.ml_app"));
+  }
+
+  @Test
+  void openAiRequestSpanUsesServiceDefaultsWithoutLlmObsContext() throws Exception {
+    openAiClient.chat().completions().create(buildMinimalChatParams());
+
+    // No parent: no trace_id tag, so the mapper seeds it from the APM trace id, and ml_app is the
+    // configured default.
+    writer.waitForTraces(1);
+    DDSpan openAiSpan = findSpanByOperationName(writer, "openai.request");
+    assertNotNull(openAiSpan, "openai.request span should have been created");
+    assertNull(openAiSpan.getTag("_ml_obs_tag.trace_id"));
+    assertEquals(Config.get().getLlmObsMlApp(), openAiSpan.getTag("_ml_obs_tag.ml_app"));
+  }
+
+  @Test
+  void openAiRequestSpanRecordsNoSamplingDecisionWhenItsParentHasNone() throws Exception {
+    // The parent continued a trace whose caller sent no verdict, so it has none. The LLM span must
+    // not roll its own: below a rate of 1 that could drop the span while its parent is kept.
+    AgentSpan parentSpan = AgentTracer.startSpan("test", "parent");
+    try (ContextScope ignored1 = AgentTracer.activateSpan(parentSpan)) {
+      try (ContextScope ignored2 =
+          LLMObsContext.attach(
+              parentSpan.spanContext(), null, null, null, null, null, null, null, null)) {
+        openAiClient.chat().completions().create(buildMinimalChatParams());
+      }
+    } finally {
+      parentSpan.finish();
+    }
+
+    writer.waitForTraces(1);
+    DDSpan openAiSpan = findSpanByOperationName(writer, "openai.request");
+    assertNotNull(openAiSpan, "openai.request span should have been created");
+    assertEquals(
+        String.valueOf(parentSpan.getSpanId()), openAiSpan.getTag("_ml_obs_tag.parent_id"));
+    assertNull(openAiSpan.getTag("_ml_obs_tag.sampling_decision"));
+    assertNull(openAiSpan.getTag("_ml_obs_tag.sample_rate"));
+  }
+
   @Test
   void openAiRequestSpanInheritsNothingFromStaleCrossTraceContext() throws Exception {
     // Simulates a stale LLMObsContext leaked across an async boundary: the context is attached,