diff --git a/scaffold/deepdive/phase3/synthesis.py b/scaffold/deepdive/phase3/synthesis.py
new file mode 100644
index 0000000..b3a46f1
--- /dev/null
+++ b/scaffold/deepdive/phase3/synthesis.py
@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+"""Deep Dive Phase 3: Synthesis Engine — STUB
+
+Takes filtered items, generates structured briefing via LLM.
+
+Parent: the-nexus#830
+"""
+
+# TODO: Implement synthesis pipeline
+# - Load scored items from data/deepdive/scored/
+# - Format LLM prompt with briefing template
+# - Call Hermes/Gemma 4 for generation
+# - Parse output into structured briefing
+# - Save to data/deepdive/briefings/
+
+def generate_briefing(items, template):
+    raise NotImplementedError("Phase 3 stub — implement LLM synthesis")
+
+if __name__ == "__main__":
+    print("Phase 3: Synthesis Engine — NOT YET IMPLEMENTED")