diff --git a/src/agentex/lib/core/tracing/obs_ids.py b/src/agentex/lib/core/tracing/obs_ids.py
index 1be803a84..1b308ddca 100644
--- a/src/agentex/lib/core/tracing/obs_ids.py
+++ b/src/agentex/lib/core/tracing/obs_ids.py
@@ -24,7 +24,7 @@
 import os
 from typing import Dict, Optional, Tuple
 
-__all__ = ("get_obs_mode", "obs_correlation")
+__all__ = ("get_obs_mode", "obs_correlation", "sync_ddtrace_to_lgtm")
 
 DD_ONLY = "dd_only"
 DUAL = "dual"
@@ -81,3 +81,31 @@ def obs_correlation() -> Dict[str, str]:
     if not ids:
         return {}
     return {"obs.trace_id": ids[0], "obs.span_id": ids[1]}
+
+
+def sync_ddtrace_to_lgtm() -> None:
+    """In ``dual`` mode, make ddtrace adopt the active OpenTelemetry/LGTM trace
+    context so ddtrace-emitted spans (best-effort Datadog) share the SAME
+    trace_id as the OTel trace. Call at request ingress once the OTel span is
+    active, and after any context boundary (e.g. entering a Temporal activity).
+
+    No-op outside ``dual`` mode, or when OTel/ddtrace is unavailable or no OTel
+    span is active. Never raises.
+    """
+    if get_obs_mode() != DUAL:
+        return
+    try:
+        from opentelemetry import trace
+
+        try:
+            from ddtrace.trace import Context  # ddtrace 3.x
+        except ImportError:  # pragma: no cover - older ddtrace layout
+            from ddtrace.context import Context  # type: ignore[no-redef]
+        from ddtrace import tracer
+    except ImportError:
+        return
+
+    sc = trace.get_current_span().get_span_context()
+    if not (sc and sc.is_valid):
+        return
+    tracer.context_provider.activate(Context(trace_id=sc.trace_id, span_id=sc.span_id))
diff --git a/src/agentex/lib/sdk/fastacp/base/base_acp_server.py b/src/agentex/lib/sdk/fastacp/base/base_acp_server.py
index 7cb04af7a..02cd11532 100644
--- a/src/agentex/lib/sdk/fastacp/base/base_acp_server.py
+++ b/src/agentex/lib/sdk/fastacp/base/base_acp_server.py
@@ -33,6 +33,7 @@
 from agentex.lib.environment_variables import EnvironmentVariables, refreshed_environment_variables
 from agentex.types.task_message_update import TaskMessageUpdate, StreamTaskMessageFull
 from agentex.types.task_message_content import TaskMessageContent
+from agentex.lib.core.tracing.obs_ids import sync_ddtrace_to_lgtm as _sync_ddtrace_to_lgtm
 from agentex.lib.core.tracing.span_queue import shutdown_default_span_queue
 from agentex.lib.core.compat.version_guard import assert_backend_compatible
 from agentex.lib.sdk.fastacp.base.constants import (
@@ -101,6 +102,9 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         # request so business spans created here (and any downstream Temporal
         # workflow started from this request) share the same observability trace.
         token = _attach_incoming_trace_context(scope)
+        # In dual mode, make ddtrace adopt this OTel trace id so its best-effort
+        # Datadog spans share the same trace_id. No-op outside dual mode.
+        _sync_ddtrace_to_lgtm()
         try:
             await self.app(scope, receive, send)
         finally: