diff --git a/app-instance/backend/beaver/skills/learning/__init__.py b/app-instance/backend/beaver/skills/learning/__init__.py
index eb6d616..e664220 100644
--- a/app-instance/backend/beaver/skills/learning/__init__.py
+++ b/app-instance/backend/beaver/skills/learning/__init__.py
@@ -9,6 +9,7 @@ from .missing_skill import (
     MissingSkillSynthesizer,
 )
 from .pipeline import SkillLearningPipelineService
+from .preservation import check_preservation
 from .service import RunReceiptContext, SkillLearningService
 from .synthesizer import SkillDraftSynthesizer
 from .worker import SkillLearningWorker, SkillLearningWorkerConfig, SkillLearningWorkerResult
@@ -23,6 +24,7 @@ __all__ = [
     "MissingSkillSynthesizer",
     "RunReceiptContext",
     "SkillLearningPipelineService",
+    "check_preservation",
     "SkillDraftSynthesizer",
     "SkillLearningService",
     "SkillLearningWorker",
diff --git a/app-instance/backend/beaver/skills/learning/preservation.py b/app-instance/backend/beaver/skills/learning/preservation.py
new file mode 100644
index 0000000..f1c1e75
--- /dev/null
+++ b/app-instance/backend/beaver/skills/learning/preservation.py
@@ -0,0 +1,53 @@
+"""Preservation checks for skill revision drafts."""
+
+from __future__ import annotations
+
+import re
+from typing import Any
+
+
+def check_preservation(*, base_content: str, draft_content: str) -> dict[str, Any]:
+    base_sections = _sections(base_content)
+    draft_sections = _sections(draft_content)
+    preserved: list[str] = []
+    changed: list[str] = []
+    dropped: list[str] = []
+
+    for heading, body in base_sections.items():
+        draft_body = draft_sections.get(heading)
+        if draft_body is None:
+            dropped.append(heading)
+            continue
+        preserved.append(heading)
+        if _normalize(body) != _normalize(draft_body):
+            changed.append(heading)
+
+    risk_level = "high" if dropped else "low"
+    return {
+        "passed": not dropped,
+        "risk_level": risk_level,
+        "preserved_sections": preserved,
+        "changed_sections": changed,
+        "dropped_sections": dropped,
+    }
+
+
+def _sections(content: str) -> dict[str, str]:
+    current = "body"
+    sections: dict[str, list[str]] = {current: []}
+    for line in (content or "").splitlines():
+        match = re.match(r"^#{1,6}\s+(.+?)\s*$", line)
+        if match:
+            current = match.group(1).strip()
+            sections.setdefault(current, [])
+            continue
+        sections.setdefault(current, []).append(line)
+    return {
+        heading: "\n".join(lines).strip()
+        for heading, lines in sections.items()
+        if "\n".join(lines).strip()
+    }
+
+
+def _normalize(value: str) -> str:
+    return re.sub(r"\s+", " ", value or "").strip().lower()
diff --git a/app-instance/backend/tests/unit/test_skill_learning_preservation.py b/app-instance/backend/tests/unit/test_skill_learning_preservation.py
new file mode 100644
index 0000000..f82e330
--- /dev/null
+++ b/app-instance/backend/tests/unit/test_skill_learning_preservation.py
@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+from beaver.skills.learning.preservation import check_preservation
+
+
+def test_preservation_passes_when_base_sections_remain() -> None:
+    base = "# Skill\n\n## Workflow\n\n- Read first.\n\n## Safety\n\n- Do not delete files.\n"
+    draft = "# Skill\n\n## Workflow\n\n- Read first.\n- Then write.\n\n## Safety\n\n- Do not delete files.\n"
+
+    report = check_preservation(base_content=base, draft_content=draft)
+
+    assert report["passed"] is True
+    assert report["risk_level"] == "low"
+    assert "Workflow" in report["preserved_sections"]
+    assert "Safety" in report["preserved_sections"]
+    assert report["dropped_sections"] == []
+
+
+def test_preservation_flags_dropped_section() -> None:
+    base = "# Skill\n\n## Workflow\n\n- Read first.\n\n## Safety\n\n- Do not delete files.\n"
+    draft = "# Skill\n\n## Workflow\n\n- Read first.\n"
+
+    report = check_preservation(base_content=base, draft_content=draft)
+
+    assert report["passed"] is False
+    assert report["risk_level"] == "high"
+    assert "Safety" in report["dropped_sections"]