diff --git a/backend/packages/harness/deerflow/agents/thread_state.py b/backend/packages/harness/deerflow/agents/thread_state.py
index 0805cc0f3..8e5690779 100644
--- a/backend/packages/harness/deerflow/agents/thread_state.py
+++ b/backend/packages/harness/deerflow/agents/thread_state.py
@@ -18,6 +18,27 @@ class ViewedImageData(TypedDict):
     mime_type: str
 
 
+def merge_sandbox(existing: SandboxState | None, new: SandboxState | None) -> SandboxState | None:
+    """Reducer for sandbox state - accepts idempotent writes only.
+
+    Multiple sandbox tools can initialize lazily in the same graph step and
+    emit the same sandbox_id via Command(update=...). LangGraph needs an
+    explicit reducer for that shared state key. Different sandbox ids in the
+    same thread indicate a lifecycle/isolation bug, so fail closed instead of
+    choosing one silently.
+    """
+    if new is None:
+        return existing
+    if existing is None:
+        return new
+
+    existing_id = existing.get("sandbox_id")
+    new_id = new.get("sandbox_id")
+    if existing_id == new_id:
+        return existing
+    raise ValueError(f"Conflicting sandbox state updates: {existing_id!r} != {new_id!r}")
+
+
 def merge_artifacts(existing: list[str] | None, new: list[str] | None) -> list[str]:
     """Reducer for artifacts list - merges and deduplicates artifacts."""
     if existing is None:
@@ -85,7 +106,7 @@ def merge_promoted(existing: PromotedTools | None, new: PromotedTools | None) ->
 
 
 class ThreadState(AgentState):
-    sandbox: NotRequired[SandboxState | None]
+    sandbox: Annotated[NotRequired[SandboxState | None], merge_sandbox]
     thread_data: NotRequired[ThreadDataState | None]
     title: NotRequired[str | None]
     artifacts: Annotated[list[str], merge_artifacts]
diff --git a/backend/tests/test_thread_state_reducers.py b/backend/tests/test_thread_state_reducers.py
index bc419c93a..6c79f85fb 100644
--- a/backend/tests/test_thread_state_reducers.py
+++ b/backend/tests/test_thread_state_reducers.py
@@ -7,14 +7,51 @@ overwrites the previously accumulated value.
 
 from typing import get_type_hints
 
+import pytest
+
 from deerflow.agents.thread_state import (
     ThreadState,
     merge_artifacts,
+    merge_sandbox,
     merge_todos,
     merge_viewed_images,
 )
 
 
+class TestMergeSandbox:
+    """Reducer for ThreadState.sandbox - allows idempotent concurrent writes."""
+
+    def test_none_new_preserves_existing(self):
+        existing = {"sandbox_id": "sandbox-1"}
+        assert merge_sandbox(existing, None) == existing
+
+    def test_none_existing_accepts_new(self):
+        new = {"sandbox_id": "sandbox-1"}
+        assert merge_sandbox(None, new) == new
+
+    def test_same_sandbox_id_is_idempotent(self):
+        existing = {"sandbox_id": "sandbox-1"}
+        new = {"sandbox_id": "sandbox-1"}
+        assert merge_sandbox(existing, new) == existing
+
+    def test_both_none_sandbox_id_is_idempotent(self):
+        existing = {"sandbox_id": None}
+        new = {"sandbox_id": None}
+        assert merge_sandbox(existing, new) == existing
+
+    def test_omitted_sandbox_id_is_idempotent(self):
+        """An omitted sandbox_id represents uninitialized sandbox state."""
+        existing = {}
+        new = {}
+        assert merge_sandbox(existing, new) == existing
+
+    def test_conflicting_sandbox_ids_raise(self):
+        existing = {"sandbox_id": "sandbox-1"}
+        new = {"sandbox_id": "sandbox-2"}
+        with pytest.raises(ValueError, match="Conflicting sandbox state updates"):
+            merge_sandbox(existing, new)
+
+
 class TestMergeTodos:
     """Reducer for ThreadState.todos - keeps last non-None value."""
 
@@ -95,3 +132,13 @@ class TestThreadStateAnnotations:
         """Sanity check that existing reducer wiring is preserved."""
         hints = get_type_hints(ThreadState, include_extras=True)
         assert merge_artifacts in hints["artifacts"].__metadata__
+
+    def test_sandbox_field_is_wired_to_merge_sandbox(self):
+        """ThreadState.sandbox must merge idempotent lazy-init updates.
+
+        Without this Annotated binding, concurrent sandbox tools that all
+        persist the same lazily acquired sandbox_id can trigger LangGraph's
+        INVALID_CONCURRENT_GRAPH_UPDATE error.
+        """
+        hints = get_type_hints(ThreadState, include_extras=True)
+        assert merge_sandbox in hints["sandbox"].__metadata__