Portrait of Esther Xin

Software Engineer & AI Researcher

Esther Xin

LLM data pipelines.
Evaluation infrastructure.

I combine data-pipeline engineering with empirical research on LLM training-data quality, reward and verifier reliability, and scaling-aware evaluation. My research also includes controlled MoE pretraining experiments.

I am a Software Engineer II at Microsoft and bring five years of data-pipeline experience. I have led architecture and deployment of production workflows combining evidence processing, specialist agents, report validation, and human review.

Selected research

Pretraining optimization & scaling-aware evaluation

Tune the Baseline First: Most of Muon's Scaled Weight Decay Gain Is a Tuning Gap, but Its Small Real Advantage Grows With Model Size

At the smallest scale, joint tuning reduced Muon-SW's advantage from 0.035 to 0.0026 BPB—about 90% of the gap disappeared (95% CI: 0.0018–0.0034). The surviving share rose from roughly 10% to 21% across the tested ladder; larger-rung estimates use single-seed sweeps.

Data & method

81 runs across 3 MoE rungs, 1.9M–15M active parameters; source-scaled data and joint LR/WD tuning, with 4 paired seeds at the smallest tuned rung.

Does Chain-of-Thought Reliance Follow a Scaling Law? Larger Models Follow Their Steps More, Independent of Post-Training

On synthetic box-swap tasks, Qwen3 answer reliance on written reasoning rose from 16–18% at 0.6B to 97–100% at 14B. Reliance correlated with step accuracy (r = 0.99); base/post-trained Qwen3 differences were at most 0.03 from 4B upward.

Data & method

25 open-weight models/checkpoints from 4 families, 7,200 evaluation items each; exact-ground-truth tasks and controlled CoT edits, inference only.

RLVR post-training: data quality & reward reliability

What Should We Remove? Evaluating the Reliability and Costs of RLVR Data Decontamination

No tested semantic threshold met the 90% conservative-precision target on calibration. The existing judge-adjudicated policy achieved 98.1% resolved precision and 98.2% candidate-frame recall on held-out corpora—not corpus-wide recall.

Data & method

16 corpora with 1,673,514 corpus-specific items, 6 benchmarks, 8 policies, and 594 blinded dual-reviewed pairs; disjoint calibration/held-out sets.

When a Data Artifact Isn't a Shortcut: Causal Auditing of Synthetic RLVR Corpora

Surface features weakly distinguished answer provenance (AUROC 0.562), but the unmodified-data policy showed no exploitation advantage: gaps were 0.021 vs. 0.027 for the matched control. A detectable artifact was not shown to be exploited under the tested budget.

Data & method

315,499 GooseReason-0.7M answer options, 5 surface features, and 2 policy-training arms with equal dataset sizes and a fixed budget; paraphrase-matched control.

Are Verifier Errors Independent Within a GRPO Group? Evidence from Qwen2.5 Rollouts

Within-group verifier-error correlation was 0.530 (95% CI: 0.500–0.560), implying 1.70 effective samples per group of 8 under an exchangeable-error model. Advantage-sign disagreements affected up to 0.83% of groups; shared prompt difficulty and answer form were not disentangled.

Data & method

24,998 groups of 8 Qwen2.5-1.5B completions across MATH, GSM8K, and DeepMath-103K; correlation estimation and advantage replay across 4 rule-verifier configurations.

Where the Verifier Fails: A Category-Level Audit of Reward Signals in RLVR

Self-validation ranged from 53.8% to 95.2%; two same-library configurations disagreed on 49.9% of pairs. Whitespace/punctuation caused 93.0% of default LaTeX in-contract failures; the reference numeric cascade accepted all tested off-by-one cases at magnitudes ≥104.

Data & method

307,420 verdicts across 4 verifier implementations; certified-equivalent and adversarial answer transformations, with category-level audits separating rejection from execution failure.

Selected engineering

Production systems

Engineering delivery alongside independent research, with an emphasis on evidence, reliability, and human decision boundaries.

Microsoft · Project lead · Shipped

Performance Signoff Multi-Agent System

Led architecture, deployment, and iterative refinement of a production performance-investigation workflow. A router prepares shared evidence, parallel specialist agents investigate performance signals, and a critique stage validates the report before publication. The final signoff decision remains with a human reviewer.

10 → 3 days
Time to resolution
70% reduction
>60%
Previously manual
workflow steps automated
400 cases
Six-month
measurement period
Evidence intake flows to a router and four parallel specialists, then critique and validation, controlled report publication, human review, and an audit record. Feedback returns to routing.
Public-facing architecture overview; internal service names omitted. View full size.

Operational results reported for a 400-case, six-month measurement. Time to resolution and workflow-step automation are distinct measures; neither is presented as a developer-hours saving.

Contact

For conversations about LLM training systems, data quality, and evaluation reliability: