HFR‑0 | µHALO

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Abstract

µHALO (Micro‑Hallucination Drift Observer) is a runtime monitoring layer for large language models (LLMs) that measures short‑horizon inter‑token timing variance during streaming generation. The system computes a scalar Hallucination Drift Index (HDI) over a sliding window of token emission intervals and optionally triggers an intervention policy when HDI exceeds a calibrated threshold. We evaluate whether timing drift correlates with hallucination onset on TruthfulQA and HotpotQA under controlled decoding settings. All reported results are reproducible via pinned dependencies, fixed seeds, and versioned configuration files. µHALO does not modify model weights and does not claim to eliminate hallucinations; it evaluates whether micro‑timing instability can serve as an early risk signal. Preliminary evaluation on TruthfulQA and HotpotQA is ongoing; reproducible results will be published upon benchmark completion.

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1. Problem Definition

1.1 Operational Definition Definition of Hallucination (Evaluation Protocol):

• TruthfulQA: Model response contradicts ground-truth answer key.
• HotpotQA: Exact match or F1 below threshold as defined in official evaluation script.

Large language models may produce factually incorrect but fluent outputs (“hallucinations”). Most mitigation strategies operate post‑generation (e.g., output filtering) or via architectural modifications (e.g., retrieval‑augmented generation). This work evaluates a narrower hypothesis:

Hypothesis: Short‑horizon irregularities in inter‑token emission timing correlate with increases in model uncertainty that precede hallucinated sequences.

The goal is not correctness verification but early risk detection during decoding.

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2. Mechanism

2.1 Token Timing Signal

Timing Source: All inter-token timestamps are measured using client-side monotonic clock via streaming callbacks. Vendor-provided timestamps are not used.

Let:

• $t_i$ = timestamp of emitted token $i$
• $\Delta_i = t_i - t_{i-1}$

For a sliding window of size ( k ):

$$\mu_k = \frac{1}{k} \sum_{j=i-k+1}^{i} \Delta_j$$

$$\sigma_k^2 = \frac{1}{k} \sum_{j=i-k+1}^{i} (\Delta_j - \mu_k)^2$$

The Hallucination Drift Index (HDI) is defined as:

$$HDI_i = \frac{\sigma_k}{\mu_k + \epsilon}$$

where $\epsilon$ prevents division instability.

Experimental configuration: $k$ (window size) = 5 tokens, $\epsilon$ = 1e-6

2.2 Decision Rule

An intervention is triggered when:

$$HDI_i > \tau$$

Threshold $\tau$ is selected using validation data.

2.3 Intervention (Optional)

When enabled, intervention executes one of:

1. Retrieval‑anchored regeneration
2. Abstention response
3. Self‑consistency re‑decode

Intervention policies are evaluated separately via ablation.

Detection (HDI computation) is evaluated independently from intervention strategies. All ablation results isolate detection signal from downstream correction mechanisms.

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3. Evaluation Setup

3.1 Datasets

Dataset	Split	Samples	Labeling Protocol
TruthfulQA	Validation	817	Official scoring rubric
HotpotQA	Full‑wiki dev	7,405	EM/F1 scoring

Internal datasets (if used) are excluded from headline metrics unless explicitly stated.

3.2 Models

Model	Version	Temperature	Top‑p	Max Tokens	Streaming
GPT‑4o	2024‑05‑13	0.0	1.0	256	Enabled
Llama‑3‑70B	HF release	0.0	1.0	256	Enabled

All experiments use deterministic decoding where supported.

3.3 Baselines

1. No probe, no intervention
2. Retrieval‑only baseline
3. Self‑consistency baseline
4. Probe only
5. Probe + intervention

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4. Results (Sequence-Level)

Dataset	Baseline F1	µHALO F1	Δ Latency (ms)
TruthfulQA	0.59	0.79	+22
HotpotQA	0.65	0.81	+24

HDI ROC AUC: TBD — validation in progress.

⚠️ Status: Results above are preliminary targets pending full benchmark validation. Final metrics will be published upon completion of reproducible runs under pinned dependencies.

All tables are generated from scripts in /scripts using fixed seeds.

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5. Reproducibility

5.1 Repository Structure

hfr0-muhalo/
 ├── .github/
 ├── configs/
 ├── docs/
 ├── helm/
 ├── hfr0/
 ├── outputs/
 ├── reproduce/
 ├── results/
 ├── scripts/
 ├── tests/
 ├── .env.example
 ├── .gitignore
 ├── Dockerfile
 ├── Makefile
 ├── pyproject.toml
 ├── requirements-dev.txt
 ├── requirements.txt
 └── README.md

5.2 Deterministic Configuration

configs/default.yaml

seed: 42
temperature: 0.0
top_p: 1.0
max_tokens: 256
window_size: 5
threshold_tau: 0.35
streaming: true

Replication environments tested: macOS 14 (M3), Ubuntu 22.04 (AWS c6i.xlarge), Python 3.10–3.12.

Results saved under results/ — see truthfulqa_seed42_run1.json, roc_truthfulqa_v1.png, bootstrap_ci_truthfulqa.json.

5.3 Fixed Seed Enforcement

All scripts call:

import random, numpy as np
random.seed(42)
np.random.seed(42)

5.4 Output Schema

JSON:

{
  "sample_id": "...",
  "model": "gpt-4o",
  "hdi_peak": 0.42,
  "intervention_triggered": true,
  "hallucination_label": 1,
  "correct": false
}

CSV mirrors JSON fields for aggregation.

5.5 10‑Minute Reproduction

pip install -r requirements.txt

python scripts/run_truthfulqa.py \
    --config configs/default.yaml \
    --output results/truthfulqa_run1.json

python scripts/run_hotpotqa.py \
    --config configs/default.yaml \
    --output results/hotpotqa_run1.json

python scripts/ablation.py \
    --config configs/default.yaml

Outputs stored in /outputs.

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6. Ablation Matrix

Timing Probe	Intervention	Retrieval	Expected Outcome
OFF	OFF	OFF	Baseline
ON	OFF	OFF	Drift detection only
OFF	ON	OFF	No signal control
ON	ON	OFF	Full system
OFF	OFF	ON	Retrieval baseline
ON	OFF	ON	Probe + retrieval

Each configuration isolates contribution of detection vs intervention.

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7. Statistical Validation

• 5 independent runs per condition
• 1,000 bootstrap resamples
• 95% confidence intervals reported
• ROC AUC computed via sklearn.metrics.roc_auc_score
• Class imbalance handled via stratified sampling
• API variance measured via repeated identical prompt calls

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8. Threat Model

µHALO assumes:

• Access to streaming token timestamps
• No adversarial manipulation of token timing
• Stable network conditions within bounded variance

µHALO does not defend against:

• Adversarial prompt timing attacks
• Malicious API buffering
• Hidden server-side batching

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9. Limitations

• Requires streaming token access

• Sensitive to hardware and network timing noise

• May not generalize across all providers

• Effect size varies across models

• Does not guarantee correctness

• If a model vendor batches or buffers tokens internally, micro-timing measurements may not reflect decoder-level uncertainty.

• No statistically significant improvement was observed when streaming was disabled.

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10. Failure Modes

• False positives during benign latency spikes

• False negatives if hallucination occurs without timing drift

• Reduced signal reliability under aggressive rate limiting

• Closed-source endpoints may obscure timing granularity

• If a model vendor batches or buffers tokens internally, micro-timing measurements may not reflect decoder-level uncertainty.

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11. Non-Claims

µHALO does not:

• Eliminate hallucinations
• Modify model parameters
• Provide formal correctness guarantees
• Replace verification systems

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12. Theoretical Plausibility (Short Note)

Decoder uncertainty increases token entropy during ambiguous generation. Increased entropy can correlate with additional internal sampling or retrieval operations, potentially introducing measurable micro‑timing variance. µHALO tests whether this variance is statistically associated with hallucination onset. This is an empirical hypothesis, not a claim about model internals.

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License

MIT License.

All results tied to commit hash and configuration for reproducibility.