Chunking & Embeddings

Text splitting, ONNX/static embedding generation, RAG pipeline integration

Locations: crates/xberg/src/chunking/ and crates/xberg/src/embeddings/ (both directories, not single files).

Chunking

ExtractionConfig.chunking: Option<ChunkingConfig> drives it. The standalone entry points are chunking::chunk_text(text, &ChunkingConfig, page_boundaries) -> Result<ChunkingResult> (chunking/core.rs) and chunking::rag::chunk_for_rag(text, &ChunkingConfig) (chunking/rag.rs), which upgrades ChunkerType::Text to Markdown and fills each chunk's heading_path.

ChunkingResult { chunks: Vec<Chunk>, chunk_count: usize }. Chunk carries content, chunk_type, metadata, and the optional vectors embedding, sparse_embedding, late_interaction (types/extraction.rs).

ChunkerType — there is no strategy enum beyond this

Text (default), Markdown, Yaml, Semantic (core/config/processing.rs). Semantic splits at embedding-based topic shifts when an EmbeddingConfig is present, and falls back to a structural-boundary heuristic otherwise — topic_threshold has no effect on the fallback path.

ChunkingConfig fields and their serde wire names

| Field | Wire name (config file) | Default | | --- | --- | --- | | max_characters | max_chars (alias max_characters) | 1000 | | overlap | max_overlap (alias overlap) | 200 | | trim | trim | true | | chunker_type | chunker_type | Text | | preset | preset | none |

The renames are load-bearing: a config file that writes max_characters works only via the alias, and a typo'd key is silently ignored (see config-loading-precedence).

Presets set chunk size AND the embedding model

ChunkingConfig.preset resolves through resolve_preset(), which is #[cfg(feature = "embeddings")]-gated — without that feature it is a no-op and the preset name does nothing. A preset overrides max_characters and overlap and, if no embedding config was given, selects the model.

| Preset | chunk_size | overlap | dims | backend | | --- | --- | --- | --- | --- | | fast | 512 | 50 | 384 | ONNX | | balanced | 1024 | 100 | 768 | ONNX | | quality | 2000 | 200 | 1024 | ONNX | | multilingual | 1024 | 100 | 768 | ONNX | | gte-modernbert-base | 1024 | 100 | 768 | ONNX | | lightweight | 512 | 50 | 256 | static (model2vec) | | arctic-embed-m-v2.0 | 1024 | 100 | 768 | ONNX | | qwen3-embedding-0.6b | 2000 | 200 | 1024 | ONNX |

Source of truth: EMBEDDING_PRESETS in crates/xberg/src/embeddings/mod.rs.

Embeddings

There is no TextEmbeddingManager, no embed_chunks(), no ChunkWithEmbedding, no RagDocument, and no fastembed dependency — do not write code against any of those.

Model selection is EmbeddingModelType, a tagged enum (core/config/processing.rs): Preset { name } (recommended), Custom { … } (HuggingFace ONNX), Llm { … }, Plugin { … }.

Two defaults disagree and both are live: EmbeddingModelType::default() is the gte-modernbert-base preset (what bindings and #[serde(default)] get), while EmbeddingConfig::default() names balanced via default_balanced_embedding_model(). Read the constructor you are actually going through before assuming which model runs.

EmbeddingConfig defaults: normalize = true, batch_size = 32, max_embed_duration_secs = Some(60), max_sequence_length = None (falls back to 512, capped at the model's own model_max_length).

Feature gating

toml
embeddings = ["onnx-runtime", "dep:ndarray", "chunking", "tokio-runtime", "embedding-presets"]

ort-bundled (the default ORT linkage) downloads ONNX Runtime at build time — no system install, no ORT_DYLIB_PATH. That variable matters only under ort-dynamic.

static-embeddings is the pure-Rust model2vec path and the only dense embedder available on no-ort-target (WASM/Android). embedding-presets carries preset metadata alone and is WASM-safe.

Critical Rules

1. Chunk before embedding — vectors are attached per chunk, not per document.
2. A preset without the embeddings feature is inert — resolve_preset() is compiled out.
3. Write serde wire names in config files — max_chars/max_overlap, not the Rust field names.
4. Degrade, don't fail — a build without ORT should skip embeddings, not error.
5. Normalize for cosine similarity — EmbeddingConfig.normalize defaults to true; leave it on.

Related Skills

• extraction-pipeline-patterns — text extraction preceding chunking
• config-loading-precedence — how ChunkingConfig is resolved and why typos are silent
• feature-flag-policy — embeddings vs static-embeddings vs embedding-presets