diff --git a/backend/packages/harness/deerflow/agents/memory/prompt.py b/backend/packages/harness/deerflow/agents/memory/prompt.py
index e0c04b77e..47b35e2ae 100644
--- a/backend/packages/harness/deerflow/agents/memory/prompt.py
+++ b/backend/packages/harness/deerflow/agents/memory/prompt.py
@@ -246,6 +246,10 @@ def format_memory_for_injection(memory_data: dict[str, Any], max_tokens: int = 2
         if earlier.get("summary"):
             history_sections.append(f"Earlier: {earlier['summary']}")
 
+        background = history_data.get("longTermBackground", {})
+        if background.get("summary"):
+            history_sections.append(f"Background: {background['summary']}")
+
         if history_sections:
             sections.append("History:\n" + "\n".join(f"- {s}" for s in history_sections))
 
diff --git a/backend/tests/test_memory_prompt_injection.py b/backend/tests/test_memory_prompt_injection.py
index d33b69a92..7c3ad85c4 100644
--- a/backend/tests/test_memory_prompt_injection.py
+++ b/backend/tests/test_memory_prompt_injection.py
@@ -154,3 +154,22 @@ def test_format_memory_renders_correction_without_source_error_normally() -> Non
 
     assert "Use make dev for local development." in result
     assert "avoid:" not in result
+
+
+def test_format_memory_includes_long_term_background() -> None:
+    """longTermBackground in history must be injected into the prompt."""
+    memory_data = {
+        "user": {},
+        "history": {
+            "recentMonths": {"summary": "Recent activity summary"},
+            "earlierContext": {"summary": "Earlier context summary"},
+            "longTermBackground": {"summary": "Core expertise in distributed systems"},
+        },
+        "facts": [],
+    }
+
+    result = format_memory_for_injection(memory_data, max_tokens=2000)
+
+    assert "Background: Core expertise in distributed systems" in result
+    assert "Recent: Recent activity summary" in result
+    assert "Earlier: Earlier context summary" in result