fix(llmobs): prevent duplicate tags in x-datadog-tags on injection

When injecting LLMObs trace context into outbound carriers, handleLLMObsInjection
previously appended propagated keys without checking whether existing x-datadog-tags
already contained entries for those keys (for example, from standard trace tag
propagation or upstream service headers). This resulted in duplicate entries such
as multiple _dd.p.llmobs_ml_app values in x-datadog-tags.

Use stripTagsetEntry before appending resolved LLMObs keys so existing entries
are cleanly replaced without duplication.

Fixes #9714

Author: patrickswedish

packages/dd-trace/src/llmobs/index.js

@@ -163,12 +163,30 @@ function handleLLMObsInjection ({ carrier }) {
   // tags, so it may be undefined here — coalesce before appending.
   const existing = readDatadogTags(carrier)
   let tags = existing || ''
-  if (parentId) tags += `${tags ? ',' : ''}${PROPAGATED_PARENT_ID_KEY}=${parentId}`
-  if (mlApp) tags += `${tags ? ',' : ''}${PROPAGATED_ML_APP_KEY}=${mlApp}`
-  if (sessionId) tags += `${tags ? ',' : ''}${PROPAGATED_SESSION_ID_KEY}=${sessionId}`
-  if (sampleRate != null) tags += `${tags ? ',' : ''}${PROPAGATED_SAMPLE_RATE_KEY}=${sampleRate}`
-  if (samplingDecision != null) tags += `${tags ? ',' : ''}${PROPAGATED_SAMPLING_DECISION_KEY}=${samplingDecision}`
-  if (propagatedTraceId != null) tags += `${tags ? ',' : ''}${PROPAGATED_TRACE_ID_KEY}=${propagatedTraceId}`
+  if (parentId) {
+    tags = stripTagsetEntry(tags, PROPAGATED_PARENT_ID_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_PARENT_ID_KEY}=${parentId}`
+  }
+  if (mlApp) {
+    tags = stripTagsetEntry(tags, PROPAGATED_ML_APP_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_ML_APP_KEY}=${mlApp}`
+  }
+  if (sessionId) {
+    tags = stripTagsetEntry(tags, PROPAGATED_SESSION_ID_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_SESSION_ID_KEY}=${sessionId}`
+  }
+  if (sampleRate != null) {
+    tags = stripTagsetEntry(tags, PROPAGATED_SAMPLE_RATE_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_SAMPLE_RATE_KEY}=${sampleRate}`
+  }
+  if (samplingDecision != null) {
+    tags = stripTagsetEntry(tags, PROPAGATED_SAMPLING_DECISION_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_SAMPLING_DECISION_KEY}=${samplingDecision}`
+  }
+  if (propagatedTraceId != null) {
+    tags = stripTagsetEntry(tags, PROPAGATED_TRACE_ID_KEY)
+    tags += `${tags ? ',' : ''}${PROPAGATED_TRACE_ID_KEY}=${propagatedTraceId}`
+  }
   // When a local agent attribution is resolved, strip any stale upstream pagent entries that
   // `_injectTags` may have already written into the carrier (it propagates all `_dd.p.*` from
   // `_trace.tags`). This ensures the downstream sees a consistent id-only or id+name pair

packages/dd-trace/test/llmobs/index.spec.js

@@ -283,6 +283,49 @@ describe('module', () => {
       )
     })
 
+    it('does not duplicate _dd.p.llmobs_ml_app when already present in x-datadog-tags', () => {
+      llmobsModule.enable({ llmobs: { mlApp: 'test', agentlessEnabled: false } })
+
+      const carrier = {
+        'x-datadog-tags': '_dd.p.tid=69fe014200000000,_dd.p.dm=-0,_dd.p.llmobs_ml_app=test',
+      }
+      injectCh.publish({ carrier })
+
+      assert.strictEqual(
+        carrier['x-datadog-tags'],
+        '_dd.p.tid=69fe014200000000,_dd.p.dm=-0,_dd.p.llmobs_ml_app=test'
+      )
+    })
+
+    it('updates existing LLMObs tags in x-datadog-tags without duplicating keys', () => {
+      llmobsModule.enable({ llmobs: { mlApp: 'test', agentlessEnabled: false } })
+      store.span = {
+        context () {
+          return {
+            toSpanId () {
+              return 'new-parent-id'
+            },
+          }
+        },
+      }
+      LLMObsTagger.tagMap.set(store.span, {
+        [SESSION_ID]: 'new-session',
+        [SAMPLE_RATE]: '0.8',
+        [SAMPLING_DECISION]: '1',
+      })
+
+      const carrier = {
+        'x-datadog-tags':
+          '_dd.p.tid=69fe014200000000,_dd.p.llmobs_parent_id=old-id,_dd.p.llmobs_ml_app=old-app,_dd.p.llmobs_sid=old-session',
+      }
+      injectCh.publish({ carrier })
+
+      assert.strictEqual(
+        carrier['x-datadog-tags'],
+        '_dd.p.tid=69fe014200000000,_dd.p.llmobs_parent_id=new-parent-id,_dd.p.llmobs_ml_app=test,_dd.p.llmobs_sid=new-session,_dd.p.llmobs_sr=0.8,_dd.p.llmobs_sd=1'
+      )
+    })
+
     describe('with DD_TRACE_X_DATADOG_TAGS_MAX_LENGTH=0', () => {
       let config