groundlens 2026.4.22

Geometric LLM hallucination detection. No second LLM. Deterministic. Auditable.

Geometric LLM hallucination detection. No second LLM. Deterministic. Auditable.

Documentation | Research Papers | Examples | Vision | Contributing

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groundlens detects LLM hallucinations using embedding geometry instead of a second LLM. It computes deterministic, auditable scores from the spatial relationships between questions, responses, and source context in an embedding space. The result is a verification signal you can explain in an audit, reproduce on demand, and run in regulated environments.

Why groundlens?

Problem	How groundlens solves it
Second-LLM judges are non-deterministic and expensive	Single embedding model (all-MiniLM-L6-v2), deterministic output, sub-second latency
Probabilistic scores cannot be audited	Geometric ratios and angular measurements with clear mathematical definitions
Regulatory compliance requires explainability	Every score traces to Euclidean distances and cosine similarities in $\mathbf{R}^n$ (n-dimensional real vector space query/anwser)
One method does not fit all use cases	SGI for RAG/context verification, DGI for context-free chat, evaluate() auto-selects

SGI: Semantic Grounding Index | DGI: Directional Grounding Index

I want to...

Goal	Start here
Verify my RAG pipeline outputs	SGI quick start · RAG verification guide
Score chat responses without context	DGI quick start · DGI deep dive
Evaluate a batch of outputs	Batch evaluation · Batch guide
Wrap my LLM provider with auto-scoring	Provider guard · Providers docs
Integrate with LangChain / CrewAI / etc.	Integrations · Integration docs
Improve accuracy for my domain	Domain calibration · Calibration guide
Comply with the EU AI Act	EU AI Act guide
Understand the math	How it works · Research papers
Understand what it can and cannot detect	Hallucination taxonomy
Check my environment is set up correctly	groundlens doctor
Contribute	CONTRIBUTING.md · CLAUDE.md · AGENTS.md

Installation

pip install groundlens

With LLM provider support:

pip install "groundlens[openai]"       # OpenAI
pip install "groundlens[anthropic]"    # Anthropic
pip install "groundlens[google]"       # Google Generative AI
pip install "groundlens[providers]"    # All providers

With framework integrations:

pip install "groundlens[langchain]"    # LangChain
pip install "groundlens[crewai]"       # CrewAI
pip install "groundlens[semantic-kernel]"  # Semantic Kernel
pip install "groundlens[autogen]"      # AutoGen
pip install "groundlens[all]"          # Everything

Requirements: Python 3.10+, numpy, sentence-transformers.

Quick start

SGI -- with context (RAG verification)

SGI (Semantic Grounding Index) measures whether a response engaged with the provided context or stayed anchored to the question. It requires three inputs.

from groundlens import compute_sgi

result = compute_sgi(
    question="What is the capital of France?",
    context="France is in Western Europe. Its capital is Paris.",
    response="The capital of France is Paris.",
)

print(result.value)       # 1.23 — ratio of distances
print(result.normalized)  # 0.61 — mapped to [0, 1]
print(result.flagged)     # False — above review threshold
print(result.explanation) # "SGI=1.230 — strong context engagement (pass)"

Interpretation: SGI > 1.0 means the response is closer to the context than to the question in embedding space. The response engaged with the source material.

DGI -- without context

DGI (Directional Grounding Index) detects hallucinations without requiring source context. It checks whether the question-to-response displacement vector aligns with the characteristic direction of verified grounded responses.

from groundlens import compute_dgi

result = compute_dgi(
    question="What causes seasons on Earth?",
    response="Seasons are caused by Earth's 23.5-degree axial tilt.",
)

print(result.value)       # 0.42 — cosine similarity to reference direction
print(result.normalized)  # 0.71 — mapped to [0, 1]
print(result.flagged)     # False — above pass threshold (0.30)

Domain calibration improves DGI accuracy from AUROC ~0.8 with a basic calibration to 0.90-0.99 with domain-specific calibration:

from groundlens import compute_dgi

result = compute_dgi(
    question="What is the statute of limitations for breach of contract in California?",
    response="Four years under California Code of Civil Procedure Section 337.",
    reference_csv="legal_calibration_pairs.csv",
)

evaluate() -- auto-select

The evaluate() function picks the right method automatically: SGI when context is provided, DGI when it is not.

from groundlens import evaluate

# With context -> SGI
score = evaluate(
    question="What is X?",
    response="X is Y.",
    context="According to the manual, X is Y.",
)
assert score.method == "sgi"

# Without context -> DGI
score = evaluate(
    question="What is X?",
    response="X is Y.",
)
assert score.method == "dgi"

Batch evaluation

from groundlens import evaluate_batch

items = [
    {"question": "Q1?", "response": "A1.", "context": "Source."},
    {"question": "Q2?", "response": "A2."},
    {"question": "Q3?", "response": "A3.", "context": "Reference."},
]

results = evaluate_batch(items)
flagged = [r for r in results if r.flagged]
print(f"{len(flagged)}/{len(results)} flagged for review")

CLI

# Check environment health
groundlens doctor

# Single response check
groundlens check \
  --question "What is the capital of France?" \
  --response "The capital of France is Paris." \
  --context "France is in Western Europe. Its capital is Paris."

# Batch CSV evaluation
groundlens evaluate input.csv --output results.csv

# Domain calibration
groundlens calibrate --pairs domain_pairs.csv --output calibration.json

# Run the confabulation benchmark
groundlens benchmark

LLM provider guard

from groundlens.providers.openai import OpenAIProvider

provider = OpenAIProvider(model="gpt-4o")
response = provider.complete(
    prompt="Summarize this document.",
    context="The document text here...",
)

if response.groundlens_score and response.groundlens_score.flagged:
    print("Hallucination risk detected — review recommended.")
else:
    print(response.text)

Taxonomy of LLM hallucinations

Not all hallucinations are the same. Groundlens is built on a geometric taxonomy (arXiv:2602.13224) that classifies hallucinations by their geometric signature in embedding space — which determines whether they are detectable and which scoring method applies.

_{Every text maps to a point on the hypersphere S^d−1. The question q and context c define a geodesic arc. Grounded responses (blue) fall inside the plausibility region 𝒫q. Type I (purple) stays near q — the response ignored the context. Type II (red) deviates far from both q and c — invented content. Type III (pink) lands inside 𝒫q alongside the correct answer — same vocabulary and structure, wrong facts, geometrically indistinguishable.}

Type	What happens	Example	Detection
Type I — Unfaithfulness	Response ignores the provided source and defaults to the question	RAG system returns an answer from memory instead of from the retrieved document	SGI (distance ratio)
Type II — Confabulation	Response invents content outside the topic's vocabulary	Asked about CRISPR gene editing, the model describes protein-folding correction instead	DGI (displacement direction)
Type III — Within-frame error	Response uses the right vocabulary and structure but gets the facts wrong	"The capital of Australia is Canberra" vs. "The capital of Australia is Sydney" — same frame, wrong city	Undetectable by geometry

Why Type III is undetectable: Sentence embeddings encode distributional similarity (vocabulary, syntax, co-occurrence), not truth value. Two responses that share the same words, entities, and syntactic frame land in the same region of embedding space regardless of which one is correct. This is not a limitation of groundlens — it is a property of the distributional hypothesis (Harris, 1954) that constrains every embedding-based method, including NLI (which inverts to AUROC 0.311 on TruthfulQA, actively favoring false answers over truthful ones).

Implications: Groundlens is verification triage — it detects the hallucination types that leave geometric traces (Types I and II), which are the most common and most damaging in production. For Type III errors in high-stakes domains (medical, legal, financial), complement groundlens with claim-level fact-checking tools on the outputs that pass geometric verification. See Complementary Tools for Type III.

Scoring methods

Each scoring method targets a specific hallucination type from the taxonomy above.

SGI (Semantic Grounding Index) — detects Type I

When context is available, SGI measures whether the response engaged with the source or stayed anchored to the question:

SGI = dist(phi(response), phi(question)) / dist(phi(response), phi(context))

Score	Interpretation
SGI > 1.20	Strong context engagement (pass)
0.95 < SGI < 1.20	Partial engagement (review recommended)
SGI < 0.95	Weak engagement (flagged — possible Type I)

DGI (Directional Grounding Index) — detects Type II

When no context is available, DGI checks whether the question-to-response displacement aligns with a learned "grounded direction":

delta = phi(response) - phi(question)
DGI = dot(delta / ||delta||, mu_hat)

Score	Interpretation
DGI > 0.30 $^1$	Aligns with grounded patterns (pass)
0.00 < DGI < 0.30	Weak alignment (flagged — possible Type II)
DGI < 0.00	Opposes grounded direction (high risk)

$^1$ This score corresponds to a general calibration. In domain-specific calibrations the score can vary.

Providers and integrations

Component	Install extra	Description
OpenAI	openai	Wraps openai SDK with automatic scoring
Anthropic	anthropic	Wraps anthropic SDK with automatic scoring
Google	google	Wraps google-generativeai with automatic scoring
LangChain	langchain	Evaluator + callback handler
CrewAI	crewai	Tool for agent pipelines
Semantic Kernel	semantic-kernel	Function calling filter
AutoGen	autogen	Agent chat checker

Domain calibration

Generic DGI uses a bundled reference direction that achieves AUROC ~0.8 with a basic calibration. For production use, a domain-specific calibration can be applied (a minimum of 200 queries recommended):

from groundlens import calibrate

result = calibrate(csv_path="my_domain_pairs.csv")
print(f"Concentration: {result.concentration:.2f}")
result.save("calibration.json")

Domain-specific calibration typically reaches AUROC 0.90-0.99. The confabulation benchmark (arXiv:2603.13259) reports DGI AUROC 0.958 with domain calibration.

Architecture

┌─────────────────────────────────────────────┐
│            Public API (evaluate)            │
├──────────────────┬──────────────────────────┤
│   SGI (sgi.py)   │      DGI (dgi.py)        │
├──────────────────┴──────────────────────────┤
│        _internal (geometry, embeddings)     │
├─────────────────────────────────────────────┤
│  sentence-transformers (all-MiniLM-L6-v2)   │
└─────────────────────────────────────────────┘
         ▲                           ▲
         │                           │
   ┌─────┴──────┐            ┌───────┴──────┐
   │ Providers  │            │ Integrations │
   │ (OpenAI,   │            │ (LangChain,  │
   │  Anthropic,│            │  CrewAI,     │
   │  Google)   │            │  SK, AutoGen │
   └────────────┘            └──────────────┘

See AGENTS.md for detailed file-by-file documentation. See CLAUDE.md for AI-assisted development guidelines.

Research

groundlens implements the methods described in three research papers:

1. Semantic Grounding Index (SGI) Marin, J. (2025). Semantic Grounding Index for LLM Hallucination Detection. arXiv:2512.13771

2. Directional Grounding Index (DGI) Marin, J. (2026). A Geometric Taxonomy of Hallucinations in Large Language Models. arXiv:2602.13224

3. Mechanistic Interpretability Marin, J. (2026). Rotational Dynamics of Factual Constraint Processing in Large Language Models. arXiv:2603.13259

4. Hallucination Benchmark https://github.com/groundlens-dev/grounding-benchmark/blob/4abf98ec5d2f846850a44f713115323659c2a793/paper/A_Methodology_for_Building_Human_Confabulated_Hallucination_Benchmarks.pdf

Security

See SECURITY.md for vulnerability reporting, scope, and response timelines.

Contributing

See CONTRIBUTING.md for development setup, code standards, and PR process.

# Quick start for contributors
git clone https://github.com/groundlens-dev/groundlens.git
cd groundlens
pip install -e ".[dev]"
pre-commit install
groundlens doctor     # verify your environment
pytest tests/unit/    # run fast tests

About

Groundlens is built and maintained by Javier Marin -- an engineer who has reinvented himself more times than most people change jobs. The math comes from engineering, the skepticism from regulated industries, and the stubbornness from experience. Read the origin story.

License

MIT -- Javier Marin ([e-mail])