📌 Overview

Traditional epistemic uncertainty estimation in LLMs relies on Semantic Entropy (SE) (Kuhn et al., 2023; Farquhar et al., Nature 2024). While effective, Semantic Entropy requires clustering $K$ sampled generation paths using pairwise bidirectional NLI entailment classifiers (e.g., DeBERTa-v3-base).

This introduces two severe production bottlenecks:

1. Quadratic Cost: $\binom{K}{2}$ forward passes per query (45 neural evaluations for $K=10$).
2. Serving Latency: Adds $\sim$90 ms of GPU overhead per inference call, making it unusable for high-throughput production serving.

Spanda introduces Exact-Match Normalized Entropy ($R_{sc}$): a zero-parameter, zero-GPU metric that computes uncertainty directly over deterministic lexical clusters.

Across empirical evaluations spanning two orders of magnitude (1.5B to 120B parameters), Spanda matches or exceeds neural Semantic Entropy on structured reasoning while operating ~90,000$\times$ faster ($< 1,\mu\text{s}$ vs. $92.4,\text{ms}$).

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🔬 Key Empirical Discoveries

1. The Coherence Scaling Law

As model capacity increases from 1.5B to 27B parameters, internal reasoning coherence causes correct predictions to naturally converge to identical lexical sequences. On mathematical reasoning (GSM8K), exact-match AUROC scales monotonically:

$$\text{AUROC}{\text{GSM8K}}: \underbrace{0.577}{\text{1.5B}} \longrightarrow \underbrace{0.706}{\text{7B}} \longrightarrow \mathbf{\underbrace{0.889}{\text{27B}}} \quad (p = 1.89 \times 10^{-28})$$

At 7B+ parameters, Spanda achieves the exact same discriminative power as heavy DeBERTa-v3 NLI cross-encoders, rendering the neural clustering step redundant for reasoning.

2. Confident Mode Collapse (Safety Warning)

At the 120B frontier scale on ungrounded factual recall (TriviaQA), the model exhibits Confident Mode Collapse: its parametric memory and RLHF tuning cause it to hallucinate the exact same incorrect answer identically across all $K$ paths. This yields an inverted AUROC of 0.091 ($d = -2.23, p = 8.28 \times 10^{-15}$).

⚠️ Critical Safety Implication: Any system using self-consistency or agreement as a proxy for truth will be systematically deceived by frontier models on ungrounded factual recall. External grounding (RAG) is mandatory in this regime.

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📊 Benchmark Results

Model Scale	Benchmark	Accuracy	Spanda ($R_{sc}$) AUROC	Neural SE AUROC	Latency	GPU Req.
Qwen-1.5B	GSM8K	11.4%	0.577	0.584	$<1,\mu\text{s}$	None
Qwen-1.5B	TriviaQA	32.0%	0.797	0.801	$<1,\mu\text{s}$	None
Mistral-7B	GSM8K	8.2%	0.706	0.705	$<1,\mu\text{s}$	None
Mistral-7B	TriviaQA	45.0%	0.698	0.755	$<1,\mu\text{s}$	None
Qwen-27B	GSM8K	61.2%	0.889	---	$<1,\mu\text{s}$	None
DeBERTa Baseline	N/A	---	---	---	$\sim$92.4 ms	Required

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📐 Mathematical Formulation

Given $K$ sampled final answers ${y_1, \dots, y_K}$ for prompt $x$, deterministic normalization partitions them into $n$ equivalence classes ${C_1, \dots, C_n}$ with empirical probabilities $w_i = \frac{|C_i|}{K}$.

The Normalized Shannon Entropy is: $$H_{\text{norm}} = \begin{cases} 0 & \text{if } n = 1 \ \displaystyle\frac{-\sum_{i=1}^n w_i \ln w_i}{\ln K} & \text{if } n > 1 \end{cases}$$

The combined Spanda Risk Score ($R_{sc}$) balances entropy dispersion with modal dominance ($w_{\max} = \max_i w_i$): $$R_{sc} = \alpha \cdot H_{\text{norm}} + (1 - \alpha) \cdot (1 - w_{\max}), \quad \alpha = 0.5$$

• $R_{sc} = 0$: Complete consensus (model is confident).
• $R_{sc} \to 1$: Maximum epistemic divergence (model is guessing / hallucinating).

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⚡ Installation

Spanda is lightweight and requires zero third-party dependencies (pure Python standard library).

pip install spanda

Or install from source:

git clone https://github.com/Adarshent/Spnda.git
cd Spnda
pip install -e .

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🚀 Quick Start

1. Basic Uncertainty Quantification

from spanda import compute_rsc

# High-consensus query (Model is confident)
samples_confident = ["Paris", "paris.", "Paris", "Paris", "Paris"]
res_conf = compute_rsc(samples_confident)

print(f"R_sc Score: {res_conf['rsc']}")  # 0.0
print(f"Dominant Answer: {res_conf['dominant_answer']}")  # 'Paris'

# Uncertain / guessing query (Model is hallucinating)
samples_uncertain = ["Berlin", "Rome", "Madrid", "London", "Paris"]
res_unc = compute_rsc(samples_uncertain)

print(f"R_sc Score: {res_unc['rsc']}")  # 0.9 (High risk!)

2. Hallucination Detection Guardrail

from spanda import detect_hallucination

samples = ["42", "42", "24", "17", "99"]
guard = detect_hallucination(samples, threshold=0.35)

if guard["is_uncertain"]:
    print(f"🚨 Hallucination Warning (R_sc = {guard['rsc']}). Routing to RAG / Human Review.")
else:
    print(f"✅ Safe output: {guard['dominant_answer']}")

3. High-Throughput Batch Processing

from spanda import batch_compute_rsc

batch = [
    ["Answer A", "Answer A", "Answer A"],
    ["Choice 1", "Choice 2", "Choice 3"]
]

results = batch_compute_rsc(batch)
for r in results:
    print(r["rsc"], r["dominant_answer"])

4. Enterprise Cascaded Guardrail (RAG & Autonomous Agents)

For mission-critical production pipelines, Spanda provides a 2-Tier Cascaded Guardrail that combines sub-millisecond consensus filtering with context grounding and tool-call safety:

from spanda import CascadedGuardrail

guard = CascadedGuardrail(
    uncertainty_threshold=0.3,
    grounding_threshold=0.15
)

# 1. RAG Query with Mode Collapse Protection
rag_context = "Documentation: The production cluster runs in us-east-1."
unanimous_hallucination = ["eu-west-3 Paris", "eu-west-3 Paris", "eu-west-3 Paris"]

receipt = guard.evaluate(unanimous_hallucination, context=rag_context)
print(receipt.decision)       # 'MODE_COLLAPSE_RISK'
print(receipt.is_safe)        # False (Unanimous agreement, but 0% grounded in source!)
print(receipt.tier_executed)  # Tier 2
print(receipt.latency_ms)     # < 0.05 ms

# 2. Agent Tool Call Argument Verification (e.g. preventing bad 'rm')
tool_calls = [
    {"command": "rm -rf /var/cache"},
    {"command": "rm -rf /var/log"},  # Conflict detected across parallel paths!
]
agent_receipt = guard.evaluate_tool_calls(tool_calls)
print(agent_receipt.decision) # 'TOOL_ARG_MISMATCH' (Execution blocked!)

# 3. Export SOC2 Audit Receipt
import json
print(json.dumps(receipt.to_dict(), indent=2))

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🛡️ Operational Envelope

Use Case / Architecture	Recommendation	Rationale
Math, Code & Structured QA (7B–70B)	✅ Recommended	Coherence Scaling Law ensures exact-match matches neural SE at 0 cost.
High-Throughput Production APIs	✅ Recommended	90,000x latency reduction without GPU requirements.
Free-form Paraphrase QA (<7B)	⚠️ Use Neural SE	Small models produce inconsistent surface phrasing.
Ungrounded Facts on Frontier Models (>100B)	❌ Do Not Use Alone	Subject to Confident Mode Collapse; must combine with retrieval (RAG).

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🧪 Testing

Run the test suite:

python3 -m unittest discover tests

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📄 Citation

If you use Spanda in your research or production systems, please cite:

@article{nayak2026spanda,
  title={Spanda: Zero-Cost Lexical Entropy Matches Neural Semantic Uncertainty---Until Frontier Models Break It},
  author={Nayak, Bhupen},
  journal={arXiv preprint},
  year={2026},
  doi={10.5281/zenodo.22233648},
  url={https://doi.org/10.5281/zenodo.22233648}
}

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📜 License

This project is licensed under the MIT License - see the LICENSE file for details.