diff --git a/astrbot/core/knowledge_base/chunking/markdown.py b/astrbot/core/knowledge_base/chunking/markdown.py
index 9ace43110d..51e3d99d2e 100644
--- a/astrbot/core/knowledge_base/chunking/markdown.py
+++ b/astrbot/core/knowledge_base/chunking/markdown.py
@@ -129,11 +129,14 @@ async def _sections_to_chunks(
                 # 扣除前缀长度，确保添加前缀后不超过 chunk_size
                 prefix_len = self._estimate_prefix_length(heading_path)
                 effective_chunk_size = max(chunk_size // 4, chunk_size - prefix_len)
+                effective_overlap = self._scale_overlap(
+                    chunk_overlap, chunk_size, effective_chunk_size
+                )
 
                 sub_chunks = await self._fallback_chunker.chunk(
                     section_text,
                     chunk_size=effective_chunk_size,
-                    chunk_overlap=chunk_overlap,
+                    chunk_overlap=effective_overlap,
                 )
                 for i, sub_chunk in enumerate(sub_chunks):
                     chunk_text = self._apply_heading_context(
@@ -143,6 +146,22 @@ async def _sections_to_chunks(
 
         return raw_chunks
 
+    @staticmethod
+    def _scale_overlap(
+        chunk_overlap: int, chunk_size: int, effective_chunk_size: int
+    ) -> int:
+        """按正文预算等比缩放重叠长度。
+
+        标题前缀会压缩子块可用的 chunk_size，而外部传入的 chunk_overlap 是针对
+        完整 chunk_size 配置的。原样传递会让合法配置在内部变成
+        overlap >= effective_chunk_size，被递归分块器拒绝。保持
+        overlap / chunk_size 的比例，并确保结果严格小于 effective_chunk_size。
+        """
+        if chunk_size <= 0 or effective_chunk_size >= chunk_size:
+            return chunk_overlap
+        scaled = chunk_overlap * effective_chunk_size // chunk_size
+        return max(0, min(scaled, effective_chunk_size - 1))
+
     def _build_context_prefix(self, heading_path: list[str]) -> str:
         """构建标题路径前缀"""
         if self.include_heading_context and heading_path:
diff --git a/tests/test_markdown_chunker.py b/tests/test_markdown_chunker.py
new file mode 100644
index 0000000000..0b7bfcb746
--- /dev/null
+++ b/tests/test_markdown_chunker.py
@@ -0,0 +1,64 @@
+import pytest
+
+from astrbot.core.knowledge_base.chunking.markdown import MarkdownChunker
+
+# 600 characters of body made of unique markers (w000 ... w119), so a lost
+# body segment is detected even though overlapping chunks duplicate text.
+BODY_MARKERS = [f"w{i:03d}" for i in range(120)]
+BODY = " ".join(BODY_MARKERS) + " "
+LONG_HEADING_DOC = "# " + "A" * 200 + "\n\n## Child\n" + BODY
+
+
+def _assert_body_preserved(chunks: list[str]) -> None:
+    joined = "\n".join(chunks)
+    missing = [marker for marker in BODY_MARKERS if marker not in joined]
+    assert not missing, f"body markers lost during chunking: {missing}"
+
+
+@pytest.mark.asyncio
+async def test_long_heading_prefix_keeps_valid_overlap_configuration():
+    """A valid (chunk_size, chunk_overlap) pair must stay valid for the
+    recursive fallback even when the heading prefix shrinks the body budget."""
+    chunker = MarkdownChunker(chunk_size=256, chunk_overlap=100)
+
+    chunks = await chunker.chunk(LONG_HEADING_DOC)
+
+    assert chunks
+    _assert_body_preserved(chunks)
+
+
+@pytest.mark.asyncio
+async def test_overlap_override_is_scaled_too():
+    chunker = MarkdownChunker(chunk_size=1024, chunk_overlap=50)
+
+    chunks = await chunker.chunk(LONG_HEADING_DOC, chunk_size=256, chunk_overlap=100)
+
+    assert chunks
+    _assert_body_preserved(chunks)
+
+
+@pytest.mark.asyncio
+async def test_without_heading_context_behaviour_is_unchanged():
+    chunker = MarkdownChunker(
+        chunk_size=256, chunk_overlap=100, include_heading_context=False
+    )
+
+    chunks = await chunker.chunk(LONG_HEADING_DOC)
+
+    assert chunks
+    _assert_body_preserved(chunks)
+    assert all(len(chunk) <= 256 for chunk in chunks)
+
+
+@pytest.mark.parametrize(
+    ("overlap", "size", "effective", "expected"),
+    [
+        (100, 256, 64, 25),  # proportional to the reduced budget
+        (100, 256, 256, 100),  # no reduction, untouched
+        (100, 256, 300, 100),  # larger budget, untouched
+        (255, 256, 64, 63),  # never reaches the effective size
+        (0, 256, 64, 0),
+    ],
+)
+def test_scale_overlap(overlap, size, effective, expected):
+    assert MarkdownChunker._scale_overlap(overlap, size, effective) == expected