feat(indexer): hierarchical chunking for large sections

- Section-split first for structured notes
- Large sections (>max_section_chars) broken via sliding-window
- Small sections stay intact with heading preserved
- Adds max_section_chars config (default 4000)
- 2 new TDD tests for hierarchical chunking

AGENTS.md

@@ -42,6 +42,11 @@ Plugin `package.json` MUST have:
 
 User config at `~/.obsidian-rag/config.json` or `./obsidian-rag/` dev config.
 
+Key indexing fields:
+- `indexing.chunk_size` — sliding window chunk size (default 500)
+- `indexing.chunk_overlap` — overlap between chunks (default 100)
+- `indexing.max_section_chars` — max chars per section before hierarchical split (default 4000)
+
 Key security fields:
 - `security.require_confirmation_for` — list of categories (e.g. `["health", "financial_debt"]`). Empty list disables guard.
 - `security.auto_approve_sensitive` — `true` bypasses sensitive content prompts.
@@ -49,7 +54,11 @@ Key security fields:
 
 ## Ollama Context Length
 
-`python/obsidian_rag/embedder.py` truncates chunks at `MAX_CHUNK_CHARS = 8000` before embedding. If Ollama 500 error returns, increase this value or reduce `indexing.chunk_size` in config.
+`python/obsidian_rag/embedder.py` truncates chunks at `MAX_CHUNK_CHARS = 8000` before embedding. If Ollama 500 error returns, increase `max_section_chars` (to reduce section sizes) or reduce `chunk_size` in config.
+
+## Hierarchical Chunking
+
+Structured notes (date-named files) use section-split first, then sliding-window within sections that exceed `max_section_chars`. Small sections stay intact; large sections are broken into sub-chunks with the parent section heading preserved.
 
 ## Sensitive Content Guard
 

openclaw.plugin.json

@@ -65,6 +65,11 @@
             "type": "integer",
             "minimum": 0
           },
+          "max_section_chars": {
+            "type": "integer",
+            "minimum": 1,
+            "description": "Max chars per section before splitting into sub-chunks. Default 4000."
+          },
           "file_patterns": {
             "type": "array",
             "items": {

python/obsidian_rag/chunker.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import re
-import unicodedata
 import hashlib
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -181,9 +180,7 @@ def chunk_file(
     Uses section-split for structured notes (journal entries with date filenames),
     sliding window for everything else.
     """
-    import uuid
 
-    vault_path = Path(config.vault_path)
     rel_path = filepath if filepath.is_absolute() else filepath
     source_file = str(rel_path)
     source_directory = rel_path.parts[0] if rel_path.parts else ""
@@ -201,7 +198,6 @@ def chunk_file(
     chunks: list[Chunk] = []
 
     if is_structured_note(filepath):
-        # Section-split for journal/daily notes
         sections = split_by_sections(body, metadata)
         total = len(sections)
 
@@ -211,13 +207,31 @@ def chunk_file(
             section_tags = extract_tags(section_text)
             combined_tags = list(dict.fromkeys([*tags, *section_tags]))
 
-            chunk_text = section_text
+            section_heading = f"#{section}" if section else None
+            if len(section_text) > config.indexing.max_section_chars:
+                sub_chunks = sliding_window_chunks(section_text, chunk_size, overlap)
+                sub_total = len(sub_chunks)
+                for sub_idx, sub_text in enumerate(sub_chunks):
                     chunk = Chunk(
-                chunk_id=f"{chunk_id_prefix}{_stable_chunk_id(content_hash, idx)}",
-                text=chunk_text,
+                        chunk_id=f"{chunk_id_prefix}{_stable_chunk_id(content_hash, idx)}_{sub_idx}",
+                        text=sub_text,
                         source_file=source_file,
                         source_directory=source_directory,
-                section=f"#{section}" if section else None,
+                        section=section_heading,
+                        date=date,
+                        tags=combined_tags,
+                        chunk_index=sub_idx,
+                        total_chunks=sub_total,
+                        modified_at=modified_at,
+                    )
+                    chunks.append(chunk)
+            else:
+                chunk = Chunk(
+                    chunk_id=f"{chunk_id_prefix}{_stable_chunk_id(content_hash, idx)}",
+                    text=section_text,
+                    source_file=source_file,
+                    source_directory=source_directory,
+                    section=section_heading,
                     date=date,
                     tags=combined_tags,
                     chunk_index=idx,

python/obsidian_rag/config.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import json
-import os
 from enum import Enum
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -32,6 +31,7 @@ class VectorStoreConfig:
 class IndexingConfig:
     chunk_size: int = 500
     chunk_overlap: int = 100
+    max_section_chars: int = 4000
     file_patterns: list[str] = field(default_factory=lambda: ["*.md"])
     deny_dirs: list[str] = field(
         default_factory=lambda: [

python/tests/unit/test_chunker.py

@@ -206,6 +206,7 @@ def _mock_config(tmp_path: Path) -> MagicMock:
     cfg.vault_path = str(tmp_path)
     cfg.indexing.chunk_size = 500
     cfg.indexing.chunk_overlap = 100
+    cfg.indexing.max_section_chars = 4000
     cfg.indexing.file_patterns = ["*.md"]
     cfg.indexing.deny_dirs = [".obsidian", ".trash", "zzz-Archive", ".git"]
     cfg.indexing.allow_dirs = []
@@ -248,3 +249,41 @@ def test_chunk_file_unstructured(tmp_path: Path):
     assert len(chunks) > 1
     assert all(c.section is None for c in chunks)
     assert chunks[0].chunk_index == 0
+
+
+def test_large_section_split_into_sub_chunks(tmp_path: Path):
+    """Large section (exceeding max_section_chars) is split via sliding window."""
+    vault = tmp_path / "Notes"
+    vault.mkdir()
+    fpath = vault / "2024-03-15-Podcast.md"
+    large_content = "word " * 3000  # ~15000 chars, exceeds MAX_SECTION_CHARS
+    fpath.write_text(f"# Episode Notes\n\n{large_content}")
+
+    cfg = _mock_config(tmp_path)
+    cfg.indexing.max_section_chars = 4000
+    chunks = chunk_file(fpath, fpath.read_text(), "2024-03-15T10:00:00Z", cfg)
+
+    # Large section should be split into multiple sub-chunks
+    assert len(chunks) > 1
+    # Each sub-chunk should preserve the section heading
+    for chunk in chunks:
+        assert chunk.section == "#Episode Notes", (
+            f"Expected #Episode Notes, got {chunk.section}"
+        )
+
+
+def test_small_section_kept_intact(tmp_path: Path):
+    """Small section (under max_section_chars) remains a single chunk."""
+    vault = tmp_path / "Notes"
+    vault.mkdir()
+    fpath = vault / "2024-03-15-Short.md"
+    fpath.write_text("# Notes\n\nShort content here.")
+
+    cfg = _mock_config(tmp_path)
+    cfg.indexing.max_section_chars = 4000
+    chunks = chunk_file(fpath, fpath.read_text(), "2024-03-15T10:00:00Z", cfg)
+
+    # Small section → single chunk
+    assert len(chunks) == 1
+    assert chunks[0].section == "#Notes"
+    assert chunks[0].text.strip().endswith("Short content here.")