diff --git a/app-instance/backend/beaver/skills/learning/service.py b/app-instance/backend/beaver/skills/learning/service.py
index 3262280..50c9634 100644
--- a/app-instance/backend/beaver/skills/learning/service.py
+++ b/app-instance/backend/beaver/skills/learning/service.py
@@ -205,7 +205,13 @@ class SkillLearningService:
             )
         if candidate.kind == "merge_skills":
             target_name = self._suggest_skill_name(candidate, packet)
-            payload = await self.synthesizer.synthesize_merge(candidate, packet, provider, model)
+            payload = await self.synthesizer.synthesize_merge(
+                candidate,
+                packet,
+                provider,
+                model,
+                base_skill=self._merged_base_skill_snapshot(candidate.related_skill_names),
+            )
             return self.draft_service.create_merge_draft(
                 skill_name=target_name,
                 base_version=None,
@@ -217,7 +223,13 @@ class SkillLearningService:
             )
         target_skill = candidate.related_skill_names[0]
         base_version = candidate.evidence.get("skill_version")
-        payload = await self.synthesizer.synthesize_revision(candidate, packet, provider, model)
+        payload = await self.synthesizer.synthesize_revision(
+            candidate,
+            packet,
+            provider,
+            model,
+            base_skill=self._base_skill_snapshot(target_skill, base_version),
+        )
         return self.draft_service.create_revision_draft(
             skill_name=target_skill,
             base_version=base_version,
@@ -228,6 +240,46 @@ class SkillLearningService:
             evidence_refs=[{"run_id": item} for item in candidate.source_run_ids],
         )
 
+    def _base_skill_snapshot(self, skill_name: str, version: str | None) -> dict[str, Any] | None:
+        loaded = self.draft_service.store.read_published_skill(skill_name, version)
+        if loaded is None:
+            return None
+        return {
+            "skill_name": loaded.version.skill_name,
+            "version": loaded.version.version,
+            "frontmatter": dict(loaded.version.frontmatter),
+            "content": loaded.content,
+            "summary": loaded.version.summary,
+            "tool_hints": list(loaded.version.tool_hints),
+        }
+
+    def _merged_base_skill_snapshot(self, skill_names: list[str]) -> dict[str, Any] | None:
+        snapshots = [
+            snapshot
+            for name in skill_names
+            if (snapshot := self._base_skill_snapshot(name, None)) is not None
+        ]
+        if not snapshots:
+            return None
+        return {
+            "skill_name": "merge:" + ",".join(str(item["skill_name"]) for item in snapshots),
+            "version": "mixed",
+            "frontmatter": {"merged_skills": [item["frontmatter"] for item in snapshots]},
+            "content": "\n\n".join(
+                f"<!-- base skill: {item['skill_name']} {item['version']} -->\n{item['content']}"
+                for item in snapshots
+            ),
+            "summary": "\n".join(str(item["summary"]) for item in snapshots if item.get("summary")),
+            "tool_hints": list(
+                dict.fromkeys(
+                    tool
+                    for item in snapshots
+                    for tool in item.get("tool_hints", [])
+                    if str(tool).strip()
+                )
+            ),
+        }
+
     def rescore_skill_versions(self) -> list[SkillPerformanceSnapshot]:
         snapshots: list[SkillPerformanceSnapshot] = []
         grouped: dict[tuple[str, str], list[SkillEffectRecord]] = {}
diff --git a/app-instance/backend/beaver/skills/learning/synthesizer.py b/app-instance/backend/beaver/skills/learning/synthesizer.py
index 353fc0b..ca95b88 100644
--- a/app-instance/backend/beaver/skills/learning/synthesizer.py
+++ b/app-instance/backend/beaver/skills/learning/synthesizer.py
@@ -17,8 +17,9 @@ class SkillDraftSynthesizer:
         evidence_packet: EvidencePacket,
         provider: LLMProvider,
         model: str,
+        base_skill: dict[str, Any] | None = None,
     ) -> dict[str, Any]:
-        return await self._synthesize(candidate, evidence_packet, provider, model, "revise")
+        return await self._synthesize(candidate, evidence_packet, provider, model, "revise", base_skill=base_skill)
 
     async def synthesize_new_skill(
         self,
@@ -27,7 +28,7 @@ class SkillDraftSynthesizer:
         provider: LLMProvider,
         model: str,
     ) -> dict[str, Any]:
-        return await self._synthesize(candidate, evidence_packet, provider, model, "new")
+        return await self._synthesize(candidate, evidence_packet, provider, model, "new", base_skill=None)
 
     async def synthesize_merge(
         self,
@@ -35,8 +36,9 @@ class SkillDraftSynthesizer:
         evidence_packet: EvidencePacket,
         provider: LLMProvider,
         model: str,
+        base_skill: dict[str, Any] | None = None,
     ) -> dict[str, Any]:
-        return await self._synthesize(candidate, evidence_packet, provider, model, "merge")
+        return await self._synthesize(candidate, evidence_packet, provider, model, "merge", base_skill=base_skill)
 
     async def _synthesize(
         self,
@@ -45,15 +47,18 @@ class SkillDraftSynthesizer:
         provider: LLMProvider,
         model: str,
         action: str,
+        *,
+        base_skill: dict[str, Any] | None,
     ) -> dict[str, Any]:
-        prompt = self._build_prompt(candidate, evidence_packet, action)
+        prompt = self._build_prompt(candidate, evidence_packet, action, base_skill=base_skill)
         response = await provider.chat(
             messages=[
                 {
                     "role": "system",
                     "content": (
                         "You synthesize Beaver skill drafts from execution evidence. "
-                        "Return only JSON with keys: frontmatter, content, change_reason."
+                        "Return only JSON with keys: frontmatter, content, change_reason, "
+                        "preserved_sections, changed_sections, dropped_sections."
                     ),
                 },
                 {"role": "user", "content": prompt},
@@ -69,11 +74,30 @@ class SkillDraftSynthesizer:
         return self._fallback_payload(candidate, evidence_packet, action)
 
     @staticmethod
-    def _build_prompt(candidate: SkillLearningCandidate, evidence_packet: EvidencePacket, action: str) -> str:
+    def _build_prompt(
+        candidate: SkillLearningCandidate,
+        evidence_packet: EvidencePacket,
+        action: str,
+        base_skill: dict[str, Any] | None = None,
+    ) -> str:
         tool_names = _coerce_string_list(evidence_packet.metadata.get("tool_names"))
         tool_section = ", ".join(tool_names) if tool_names else "none observed"
         selected_tool_names = _coerce_string_list(evidence_packet.metadata.get("selected_tool_names"))
         selected_tool_section = ", ".join(selected_tool_names) if selected_tool_names else "none recorded"
+        base_section = ""
+        if base_skill:
+            base_section = (
+                "\n\nBase skill snapshot:\n"
+                f"- skill_name: {base_skill.get('skill_name')}\n"
+                f"- version: {base_skill.get('version')}\n"
+                f"- frontmatter: {json.dumps(base_skill.get('frontmatter') or {}, ensure_ascii=False, sort_keys=True)}\n"
+                f"- tool_hints: {base_skill.get('tool_hints') or []}\n"
+                f"- summary: {base_skill.get('summary') or ''}\n"
+                "Base skill content:\n"
+                f"{base_skill.get('content') or ''}\n"
+                "Preserve existing instructions unless the evidence requires a change. "
+                "If any section is changed or dropped, explain it in changed_sections or dropped_sections."
+            )
         return (
             f"Action: {action}\n"
             f"Candidate kind: {candidate.kind}\n"
@@ -83,11 +107,13 @@ class SkillDraftSynthesizer:
             f"Run-selected tool names: {selected_tool_section}\n"
             f"Task summaries:\n- " + "\n- ".join(evidence_packet.task_summaries)
             + "\n\nSession excerpts:\n" + "\n\n".join(evidence_packet.session_excerpts)
+            + base_section
             + "\n\nReturn JSON only. The frontmatter object must include:"
             + "\n- description: a concise skill description"
             + "\n- tools: an explicit JSON array of exact tool names this skill needs. "
             + "Prefer called tool names when the workflow depends on them; use run-selected tool names only when clearly required. "
             + "Use [] only when no tool is required."
+            + "\nThe JSON may include preserved_sections, changed_sections, and dropped_sections arrays."
         )
 
     @staticmethod
@@ -111,6 +137,9 @@ class SkillDraftSynthesizer:
             "frontmatter": frontmatter,
             "content": content_value.strip(),
             "change_reason": str(payload.get("change_reason") or ""),
+            "preserved_sections": _coerce_string_list(payload.get("preserved_sections")),
+            "changed_sections": _coerce_string_list(payload.get("changed_sections")),
+            "dropped_sections": _coerce_string_list(payload.get("dropped_sections")),
         }
 
     @staticmethod
@@ -124,6 +153,9 @@ class SkillDraftSynthesizer:
             "frontmatter": frontmatter,
             "content": str(payload.get("content") or "").strip(),
             "change_reason": str(payload.get("change_reason") or ""),
+            "preserved_sections": _coerce_string_list(payload.get("preserved_sections")),
+            "changed_sections": _coerce_string_list(payload.get("changed_sections")),
+            "dropped_sections": _coerce_string_list(payload.get("dropped_sections")),
         }
 
     @staticmethod
@@ -138,6 +170,9 @@ class SkillDraftSynthesizer:
             },
             "content": f"# {title}\n\n## Evidence\n\n{content}\n",
             "change_reason": candidate.reason or f"Fallback {action} synthesis.",
+            "preserved_sections": [],
+            "changed_sections": [],
+            "dropped_sections": [],
         }
 
 
diff --git a/app-instance/backend/tests/unit/test_skill_learning_synthesizer_preservation.py b/app-instance/backend/tests/unit/test_skill_learning_synthesizer_preservation.py
new file mode 100644
index 0000000..41a9f75
--- /dev/null
+++ b/app-instance/backend/tests/unit/test_skill_learning_synthesizer_preservation.py
@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from beaver.memory.skills import SkillLearningCandidate
+from beaver.skills.learning.evidence import EvidencePacket
+from beaver.skills.learning.synthesizer import SkillDraftSynthesizer
+
+
+def test_revision_prompt_includes_base_skill_snapshot() -> None:
+    candidate = SkillLearningCandidate(
+        candidate_id="candidate-1",
+        kind="revise_skill",
+        source_run_ids=["run-1"],
+        source_session_ids=["session-1"],
+        related_skill_names=["debug-skill"],
+        reason="Improve debugging flow.",
+    )
+    packet = EvidencePacket(
+        run_ids=["run-1"],
+        session_ids=["session-1"],
+        task_summaries=["debug a failing test"],
+        session_excerpts=["assistant: fixed it"],
+    )
+    prompt = SkillDraftSynthesizer._build_prompt(
+        candidate,
+        packet,
+        "revise",
+        base_skill={
+            "skill_name": "debug-skill",
+            "version": "v0001",
+            "frontmatter": {"description": "Debug tests", "tools": ["read_file"]},
+            "content": "# Debug Skill\n\n## Safety\n\nDo not delete files.",
+            "summary": "Debug tests safely.",
+            "tool_hints": ["read_file"],
+        },
+    )
+
+    assert "Base skill snapshot" in prompt
+    assert "# Debug Skill" in prompt
+    assert "Do not delete files." in prompt
+    assert "preserved_sections" in prompt
+    assert "dropped_sections" in prompt