---
license: mit
tags:
- langchain
- rag
- bayesian
- probabilistic
- uncertainty
- retrieval
---

# Bayesian LangChain RAG

A corrected, Hugging Face-ready implementation of uncertainty-aware retrieval augmented generation built on **LangChain Core**.

This repository replaces the earlier heuristic "tool succeeded = high likelihood" and beam-style scoring with an explicit generalized-Bayesian retrieval posterior. For a query `q` and candidate document `d`,

`p(d | q) ∝ p(d) (similarity(q,d) + eps)^tau`

where the prior `p(d)` is built from a Beta source-reliability mean, source authority, and freshness metadata. The similarity term is deliberately described as a **generalized-Bayes/Gibbs likelihood**, not as a literal language-model likelihood.

## What was corrected

- Removed notebook/Colab shell commands and hard-coded paths from library code.
- Removed the non-Bayesian `p_langchain` beam scorer from the Bayesian runtime.
- Replaced deterministic identical-particle behavior with an importance-corrected SMC utility for workflows that need particles.
- Added proposal-density correction `log target - log q` to SMC weights.
- Fixed all-`-inf` log-weight normalization.
- Preserved final posterior weights by forbidding final-step resampling.
- Added a real `BaseRetriever` implementation and LangChain `Runnable` entry point.
- RAG now reasons over retrieved evidence quality rather than rewarding any successful tool call.
- Added explicit abstention for missing evidence and unresolved near-equal conflicts.
- Added citation-grounded answer synthesis and a reproducible end-to-end RAG benchmark.

## Install

```bash
pip install -e .
```

No GPU is required. The default evaluation generator is extractive and grounded. Any LangChain-compatible model can be supplied for generative answering.

## Quick start

```python
from bayesian_langchain import DocumentRecord, BayesianIndex, BayesianRAG

docs = [
    DocumentRecord(
        id="official",
        title="Current portal policy",
        text="The current HR portal is PeopleHub.",
        source="official",
        metadata={"reliability_alpha": 18, "reliability_beta": 2, "authority": 0.98, "freshness": 1.0},
    ),
    DocumentRecord(
        id="legacy",
        title="HR portal guide",
        text="The HR portal is WorkSphere.",
        source="legacy-cache",
        metadata={"reliability_alpha": 6, "reliability_beta": 6, "authority": 0.45, "freshness": 0.18},
    ),
]

rag = BayesianRAG(BayesianIndex(docs))
print(rag.invoke("What is the current HR portal?"))
```

`BayesianRAG.as_runnable()` returns a standard LangChain `RunnableLambda`.

## RAG validation

Run:

```bash
PYTHONPATH=src:. python evaluation/rag_benchmark.py
```

The committed result is in `artifacts/rag_benchmark_results.json`.

| Suite | System | Recall@1 | MRR | Answer accuracy | Abstention accuracy | End-to-end accuracy |
|---|---|---:|---:|---:|---:|---:|
| Clean RAG (8 QA) | Bayesian | 1.000 | 1.000 | 1.000 | n/a | 1.000 |
| Clean RAG (8 QA) | Plain TF-IDF | 1.000 | 1.000 | 1.000 | n/a | 1.000 |
| Conflict/missing RAG (18 QA) | Bayesian | 1.000 | 1.000 | 1.000 | 1.000 | 1.000 |
| Conflict/missing RAG (18 QA) | Plain TF-IDF | 0.250 | 0.611 | 0.250 | 0.333 | 0.278 |

The conflict suite deliberately contains stale high-overlap documents, current authoritative documents, questions with no supporting evidence, and equal-authority contradictions. The results are **controlled repository tests**, not a claim of universal RAG accuracy.

### Framework-level LangChain RAG test

The repository also contains `evaluation/framework_rag_benchmark.py`, which routes every case through the real LangChain path:

`BayesianRetriever(BaseRetriever) -> BayesianRAG.as_runnable() -> grounded generation/abstention`.

It was validated against **langchain-core 1.6.11**. Results: **8/8 clean cases** and **18/18 conflict/missing-evidence cases** passed end-to-end. The committed output is `artifacts/framework_rag_benchmark_results.json`.

Run it with:

```bash
PYTHONPATH=src:. python evaluation/framework_rag_benchmark.py
```

## Layout

- `src/bayesian_langchain/core.py` — posterior retrieval, abstention, grounded answerer
- `src/bayesian_langchain/retriever.py` — LangChain `BaseRetriever`
- `src/bayesian_langchain/rag.py` — end-to-end LangChain RAG and Runnable
- `src/bayesian_langchain/smc.py` — corrected importance-weighted SMC utility
- `evaluation/rag_benchmark.py` — controlled retrieval/grounding benchmark
- `evaluation/framework_rag_benchmark.py` — real LangChain `BaseRetriever`/Runnable RAG benchmark
- `tests/` — regression and integration tests
- `VALIDATION.md` — exact framework/version validation record