---

Table of contents

1. Why MCompassRAG
2. How it works
3. Repository layout
4. Pluggable topic models
5. Prerequisites
6. Installation
   • Option A — automated setup
   • Option B — manual setup
7. Prepare your data
8. The pipeline, step by step
   • Step 1 — Train a topic model
   • Step 2 — Generate distillation data
   • Step 3 — Build the metadata index
   • Step 4 — Train the retriever
   • Step 5 — Run retrieval
9. Configuration reference
10. Swapping the topic model
11. Extending: add your own backend
12. How the distillation signal works
13. Troubleshooting
14. Acknowledgements & license

---

Why MCompassRAG

Classic dense retrieval matches a query against chunk embeddings alone. That works until your chunks are coarse, your corpus is broad, and "topically similar" stops meaning "actually answers the question."

MCompassRAG adds a semantic compass: every chunk is annotated with a topic-model signal, and a small neural scorer learns to combine the raw embedding with topic-aware evidence. The ranking quality comes from an LLM teacher that judges query/chunk relevance during an offline data-generation phase; that judgment is distilled into the scorer. At inference time the teacher is gone — MCompassRAG ranks chunks with a precomputed metadata bank and a lightweight model, so you get teacher-grade ranking at the cost of an embedding lookup plus a few matrix multiplies.

The topic model is pluggable. Four backends ship in the box (CEMTM, ETM, CWTM, SoftLTM), and adding your own is a single subclass. All of them expose topic centroids in the retriever's embedding space, so nothing downstream changes when you swap them.

TL;DR — Train a topic model → generate teacher-labeled data → bake an index → distill a retriever → query with no LLM calls.

---

How it works

During training, an LLM teacher provides relevance supervision, with query expansion used only as an additional teacher-side metadata signal. The metadata bank is built from chunks, enriched with document-topic vectors and topic centroid embeddings. At inference time, MCompassRAG selects and abstracts query-relevant topic metadata, then scores query-chunk pairs with a lightweight student retriever.

---

Repository layout

.
├── src/                      # RAG core (everything that runs at index/serve time)
│   ├── config.py             # YAML loader + dataclass binding
│   ├── run.py                # RAG CLI: build the index and/or run queries
│   ├── pipeline.py           # build_index / serve (+ *_from_config helpers)
│   ├── encoders/             # Qwen3 backbone + retriever encoder (leaf modules)
│   ├── index/                # chunking + the cached metadata bank
│   ├── models/               # selector, abstraction, scorer, CompassRetriever
│   ├── training/             # dataset, losses, trainer, train CLI
│   └── inference/            # Algorithm 1 retrieval
├── topic_models/             # pluggable backends + WikiWeb2M training
│   ├── base.py registry.py   # TopicModel interface + name registry
│   ├── wikiweb2m.py          # corpus loader / vocab / BoW
│   ├── cemtm_adapter.py      # CEMTM   (native centroids)
│   ├── etm_adapter.py        # ETM     (native centroids, retriever-grounded ρ)
│   ├── cwtm_adapter.py       # CWTM    (empirical centroids, MLM backbone)
│   ├── softltm_adapter.py    # SoftLTM (native centroids over LM soft labels)
│   └── train_topic_model.py  # training CLI
├── data_gen/                 # LLM-teacher distillation data generation
│   ├── openrouter_client.py  # cached, retrying OpenRouter client (+ logprobs)
│   ├── query_gen.py          # base/expanded query generation
│   ├── teacher.py            # logit-based relevance labeling
│   ├── negatives.py          # hard + random negative mining
│   └── build_training_data.py# generation CLI
├── configs/                  # one YAML per stage (+ configs/rag/<backend>.yaml)
├── scripts/                  # thin shell wrappers (setup, train, build, run)
├── third_party/              # cloned upstreams: CEMTM, CWTM, ETM  (git-ignored)
├── data/                     # corpus, chunks, generated train data  (git-ignored)
├── outputs/                  # topic models, index banks, retriever ckpts (git-ignored)
├── requirements.txt
└── README.md

A deliberate design choice keeps the import graph acyclic: the shared LM backbone lives in src/encoders, so src.encoders ← topic_models ← src.index / src.pipeline and src.encoders ← data_gen. import src is light (it never pulls in torch/transformers at package import time).

---

Pluggable topic models

Every backend takes a topic count K, trains on WikiWeb2M, and exposes topic centroids in the retriever's semantic space, so the index and retriever are identical regardless of which one you pick.

"Native" means the topic vectors are read straight from the trained model. "Empirical" means they're the responsibility-weighted mean of chunk embeddings (used when a backend has no embedding-space topics).

---

Prerequisites

• Python 3.10+
• A CUDA GPU is strongly recommended. The default configs use device: cuda; for a laptop run set the device fields to cpu (or mps on Apple Silicon).
• Git (the setup script clones the upstream topic-model repos).
• An OpenRouter API key — only needed for Step 2 (data generation). Export it before running that step:

export OPENROUTER_API_KEY="sk-or-..."

---

Installation

Clone the project and create an isolated environment first:

git clone https://github.com/your-org/MCompassRAG.git
cd MCompassRAG

python -m venv .venv
source .venv/bin/activate          # Windows: .venv\Scripts\activate
python -m pip install --upgrade pip

Option A — automated setup (recommended)

bash scripts/setup.sh

This single command is idempotent and does everything:

1. Creates third_party/, data/, and outputs/.
2. Clones the three upstream topic-model repos into third_party/ (skipping any that already exist):
   • CEMTM → third_party/CEMTM
   • CWTM → third_party/CWTM
   • ETM → third_party/ETM
3. Installs Python dependencies from requirements.txt.
4. Downloads the NLTK data used by the WikiWeb2M harness and CWTM (stopwords, wordnet).

Re-running it later only clones what's missing, so it's safe to use as a "repair" command.

Option B — manual setup

If you'd rather do it by hand (or your environment blocks the script):

# 1) Create the working directories
mkdir -p third_party data outputs

# 2) Clone the upstream topic models into third_party/
git clone --depth 1 https://github.com/AmirAbaskohi/CEMTM.git   third_party/CEMTM
git clone --depth 1 https://github.com/Fitz-like-coding/CWTM.git third_party/CWTM
git clone --depth 1 https://github.com/adjidieng/ETM.git         third_party/ETM

# 3) Install dependencies
pip install -r requirements.txt

# 4) Fetch NLTK data
python -c "import nltk; nltk.download('stopwords'); nltk.download('wordnet')"

Note on backends. etm and softltm are self-contained reimplementations and do not require their clones at runtime — but cloning ETM is harmless and keeps the upstream handy for reference. cemtm imports the topic-head modules from third_party/CEMTM, and cwtm wraps third_party/CWTM/model.py, so those two clones are required when you use those backends. The default cloned-repo paths (third_party/CEMTM, third_party/CWTM) match the adapter defaults out of the box.

Verify the install:

python -c "import src, topic_models, data_gen; print('import OK')"
python -c "import topic_models; print('backends:', topic_models.available_topic_models())"

You should see ['cemtm', 'cwtm', 'etm', 'softltm'].

---

Prepare your data

MCompassRAG reads everything from data/ (git-ignored). Two inputs drive the pipeline.

1. The topic-model corpus — data/wikiweb2m.jsonl. One JSON object per line. The document text is the concatenation of whichever of section_text / text / content fields are present; records shorter than min_doc_chars are dropped.

{"text": "Albert Einstein was a theoretical physicist ..."}
{"section_text": "The theory of general relativity ..."}

2. The chunk corpus — data/corpus_chunks.jsonl. One chunk per line, with the fields below. position orders chunks within a document and is used for neighbor mining during query generation.

{"id": "doc12_3", "text": "The eardrum transmits sound ...", "doc_id": "doc12", "position": 3}

If you need to chunk raw documents, src/index/chunking.py provides a deterministic token-window chunker (chunk_documents) that uses the retriever's tokenizer so chunk boundaries align with the embedding model.

---

The pipeline, step by step

Each step reads a YAML config and writes artifacts under outputs/ (or data/train/ for generation). The five steps below compose by pointing at the same directories. For a hands-off run, the Quickstart at the end chains them all.

Step 1 — Train a topic model

Train a topic model on your WikiWeb2M corpus. The backend is selected by topic_model.name.

Edit configs/train_topic_model.yaml:

topic_model: {name: cemtm, num_topics: 100}     # cemtm | etm | cwtm | softltm
encoder:     {model_name: Qwen/Qwen3-Embedding-4B, dtype: bfloat16, max_length: 2048}
corpus:      {wikiweb2m_path: data/wikiweb2m.jsonl, max_docs: 200000, min_doc_chars: 200}
train:
  vocab_size: 2000
  min_word_freq: 5
  epochs: 20
  batch_size: 64
  lr: 2.0e-3
  centroid_source: native          # native | empirical  (cwtm forces empirical)
  centroid_normalize: true
  n_empirical_docs: 20000
  label_lm: meta-llama/Llama-3.2-1B-Instruct   # softltm only
  soft_label_tau: 3.0                           # softltm only
  recon_lambda: 1000.0                          # softltm only
  device: cuda
  seed: 13
backend_kwargs: {}                  # cemtm:{checkpoint_path: ...} | cwtm:{backbone: bert-base-uncased}
output: {save_dir: outputs/topic_models/cemtm_k100}

Run it:

bash scripts/train_topic_model.sh configs/train_topic_model.yaml
# equivalently:
python -m topic_models.train_topic_model --config configs/train_topic_model.yaml

Output: a saved topic model under output.save_dir (e.g. outputs/topic_models/cemtm_k100/), containing the trained weights, the vocabulary, the config, and the topic centroids.

Per-backend tips

• cemtm — pass an existing checkpoint via backend_kwargs: {checkpoint_path: ...} to skip training; otherwise it trains the CEMTM objective on your corpus.
• cwtm — choose the MLM backbone via backend_kwargs: {backbone: bert-base-uncased}. It ignores centroid_source and always uses empirical centroids.
• softltm — set the generative label_lm, soft_label_tau, and recon_lambda. The label LM is run once and cached, so training epochs are cheap.

Step 2 — Generate distillation data

Generate (base, expanded) queries per chunk, mine hard + random negatives, and have an LLM teacher label relevance. This is the only step that needs network/OpenRouter.

export OPENROUTER_API_KEY="sk-or-..."

Edit configs/gen_train_data.yaml:

encoder: {model_name: Qwen/Qwen3-Embedding-4B, dtype: bfloat16, max_length: 2048}
corpus:  {chunks_path: data/corpus_chunks.jsonl}
datagen:
  n_target_chunks: 2000
  queries_per_chunk: 10
  n_random_negatives: 4
  n_hard_candidates: 20
  n_hard_negatives: 4
  neighbor_window: 1
  query_gen_model: openai/gpt-4o
  teacher_model: openai/gpt-4o
  temperature_qgen: 0.7
  temperature_teacher: 0.0
  trust_teacher_on_positive: false
  kd_on_random_negatives: false
  keep_useful_highsim_as_positive: false
  max_workers: 8
  seed: 13
  reasoning_effort: none       # forwarded to the LLM as {"reasoning": {"effort": ...}}
output: {out_dir: data/train}

Run it:

bash scripts/gen_train_data.sh configs/gen_train_data.yaml
# equivalently:
python -m data_gen.build_training_data --config configs/gen_train_data.yaml

Output: data/train/train.jsonl, data/train/chunks.jsonl, and data/train/datagen_config.json. Each training record looks like:

{
  "query_id": "q_doc12_3_0",
  "source_chunk_id": "doc12_3",
  "base_query": "How does the eardrum transmit sound?",
  "expanded_query": "In the human auditory system, how does the eardrum transmit sound to the inner ear?",
  "candidates": [
    {"chunk_id": "doc12_3", "role": "positive",        "y": 1, "z_t":  3.10, "has_teacher": true},
    {"chunk_id": "doc44_1", "role": "hard_negative",   "y": 0, "z_t": -2.40, "has_teacher": true},
    {"chunk_id": "doc09_7", "role": "random_negative", "y": 0, "z_t": null,  "has_teacher": false}
  ]
}

Here y is the binary relevance label and z_t is the teacher logit used for distillation (see How the distillation signal works). The teacher always judges the expanded query; hard-negative mining uses the base query.

Step 3 — Build the metadata index

Embed every chunk, compute its topic distribution with the trained topic model, and cache the whole bank. This step uses configs/rag/<backend>.yaml.

configs/rag/cemtm.yaml:

encoder:
  model_name: Qwen/Qwen3-Embedding-4B
  dtype: bfloat16
  max_length: 2048
  query_instruction: "Given a search query, retrieve relevant passages that answer it"
topic_model: {dir: outputs/topic_models/cemtm_k100}
index:
  chunks_path: data/corpus_chunks.jsonl
  out_dir: outputs/index/cemtm_k100
  top_m: 8
  chunk_batch_size: 16
  topic_batch_size: 8
retriever: {model_ckpt: outputs/retriever/cemtm_k100/best.pt}
serve: {k: 5, score_block_size: 4096, query_batch_size: 16, device: cuda}

Run it:

bash scripts/build_index.sh configs/rag/cemtm.yaml
# equivalently:
python -m src.run --config configs/rag/cemtm.yaml --build-index

Output: the metadata bank under index.out_dir (e.g. outputs/index/cemtm_k100/). The build is skipped if the output already exists — pass --force to rebuild:

python -m src.run --config configs/rag/cemtm.yaml --build-index --force

Step 4 — Train the retriever

Distill the teacher judgments into the selector + abstraction + scorer.

Edit configs/train_retriever.yaml:

bank_dir: outputs/index/cemtm_k100
train_jsonl: data/train/train.jsonl
chunks_jsonl: data/train/chunks.jsonl
encoder: {model_name: Qwen/Qwen3-Embedding-4B, dtype: bfloat16, max_length: 2048}
model:
  top_l: 16
  top_m: 8                 # MUST equal the index top_m
  abstraction_layers: 2
  abstraction_heads: 4
  abstraction_ff: 256
  abstraction_dropout: 0.1
  abstraction_softmax: true
  pool: selection_weighted
  scorer_hidden: [1024, 256]
  scorer_dropout: 0.1
train:
  epochs: 5
  batch_size: 32
  lr: 2.0e-4
  weight_decay: 0.01
  warmup_ratio: 0.05
  grad_clip: 1.0
  alpha: 0.5
  tau: 2.0
  tau_squared_scale: true
  max_candidates_per_query: 16
  num_workers: 2
  val_fraction: 0.05
  eval_every_steps: 200
  log_every_steps: 50
  seed: 13
  device: cuda
output: {save_dir: outputs/retriever/cemtm_k100}

Run it:

bash scripts/train_retriever.sh configs/train_retriever.yaml
# equivalently:
python -m src.training.train --config configs/train_retriever.yaml

Output: the trained retriever checkpoint at output.save_dir/best.pt (e.g. outputs/retriever/cemtm_k100/best.pt). The checkpoint stores the full model config, so serving reconstructs top_l, top_m, and the rest automatically.

Step 5 — Run retrieval

Now query with no LLM calls.

# build the index if needed, then ask a single question:
bash scripts/run_rag.sh configs/rag/cemtm.yaml "how does the eardrum transmit sound?"

Under the hood that wrapper calls src.run. You can also drive it directly:

# single query
python -m src.run --config configs/rag/cemtm.yaml --query "how does the eardrum transmit sound?"

# batch: one query per line in a file
python -m src.run --config configs/rag/cemtm.yaml --queries questions.txt

# override the number of results
python -m src.run --config configs/rag/cemtm.yaml --query "..." --k 10

Results print ranked, highest score first:

Query: how does the eardrum transmit sound?
  [0] score=8.7421 id=doc12_3  The eardrum transmits sound vibrations to the three small bones ...
  [1] score=6.1185 id=doc12_4  These ossicles amplify the vibration before it reaches the cochlea ...
  ...

Quickstart (the whole thing)

bash scripts/setup.sh                                              # clone upstreams, install deps, NLTK data
bash scripts/train_topic_model.sh configs/train_topic_model.yaml   # data/wikiweb2m.jsonl -> outputs/topic_models/...
bash scripts/gen_train_data.sh    configs/gen_train_data.yaml      # data/corpus_chunks.jsonl -> data/train/
bash scripts/build_index.sh       configs/rag/cemtm.yaml           # chunks + topics -> outputs/index/...
bash scripts/train_retriever.sh   configs/train_retriever.yaml     # distill teacher -> outputs/retriever/.../best.pt
bash scripts/run_rag.sh           configs/rag/cemtm.yaml "your question here"

---

Configuration reference

There are four config families, all parsed by src.config.load_yaml and bound onto the project's dataclasses (unknown keys are ignored; missing keys keep defaults).

Keep one RAG config per topic backend (cemtm.yaml, etm.yaml, cwtm.yaml, softltm.yaml); they differ only in the topic_model.dir, index.out_dir, and retriever.model_ckpt paths.

Must-match constraints (enforced at runtime):

• model.top_m (retriever) must equal index.top_m. The chunk topic-summary g_c is baked into the bank with the index value, and the model recomputes g_q with its own — if they diverge, the representations don't line up.
• The encoder model_name must be consistent across the topic model, the index, the retriever, and serving — they all share one embedding space.

top_l lives only in configs/train_retriever.yaml. It's a property of the trained model (how many bank entries the selector pulls in), it's saved inside the checkpoint, and it's never used at index time — so it intentionally does not appear in the RAG configs.

---

Swapping the topic model

1. Set topic_model.name (and any backend_kwargs) in configs/train_topic_model.yaml, then train:

   bash scripts/train_topic_model.sh configs/train_topic_model.yaml
   

2. Point the matching configs/rag/<name>.yaml at the new directories (topic_model.dir, index.out_dir, retriever.model_ckpt). Copies for etm, cwtm, and softltm are provided.

3. Rebuild the index and retrain the retriever against that config. Nothing downstream of the topic model changes, because every backend exposes centroids in the same retriever space.

---

Extending: add your own backend

Implement a new topic model in four steps:

from topic_models.base import TopicModel
from topic_models.registry import register_topic_model


@register_topic_model("mybackend")
class MyTopicModel(TopicModel):
    def supports_native_centroids(self) -> bool:
        return True  # or False to fall back to empirical centroids

    def encode_topic_distribution(self, texts):
        ...          # -> (B, K) simplex

    def _native_centroids(self):
        ...          # -> (K, d) in the retriever space (or None)

    def fit(self, corpus, cfg):
        ...          # train on the WikiWeb2M corpus

1. Subclass TopicModel.
2. Implement encode_topic_distribution, _native_centroids, fit, and supports_native_centroids.
3. Decorate with @register_topic_model("mybackend").
4. Import it from topic_models/__init__.py so the decorator runs on package import.

It's then immediately usable via build_topic_model("mybackend", K, encoder, ...) and by setting topic_model.name: mybackend in the training config.

---

Acknowledgements & license

MCompassRAG builds on four upstream topic models, isolated under third_party/:

• CEMTM — Contextual Embedding-based Multimodal Topic Model — https://github.com/AmirAbaskohi/CEMTM
• CWTM — Contextualized Word Topic Model — https://github.com/Fitz-like-coding/CWTM
• ETM — Embedded Topic Model — https://github.com/adjidieng/ETM
• SoftLTM — Improving Neural Topic Modeling with Semantically-Grounded Soft Label Distributions — https://arxiv.org/abs/2602.17907

Released under the MIT License.

---

Citation

@misc{abaskohi2026mcompassragtopicmetadatasemantic,
      title={MCompassRAG: Topic Metadata as a Semantic Compass for Paragraph-Level Retrieval}, 
      author={Amirhossein Abaskohi and Raymond Li and Gaetano Cimino and Peter West and Giuseppe Carenini and Issam H. Laradji},
      year={2026},
      eprint={2606.18508},
      archivePrefix={arXiv},
      primaryClass={cs.CL},
      url={https://arxiv.org/abs/2606.18508}, 
}