Judging billions of trace tokens a day with a frontier LLM was financially untenable 💸 Here's how fine-tuning a small open model matched the accuracy at 10-100x lower cost. Title: Building a 100x Cheaper Trace Judge with Fireworks URL: 💸 Overview LangChain Labs partnered with Fireworks to build a "Perceived Error" detector for agent traces. It detects cases where users felt the assistant made a mistake or needed correction — using a small open model. ❓ Challenges Solved LangSmith processes billions of tokens daily across production traces. ・Evaluating them with frontier LLMs is prohibitively expensive at scale ・The question: how to cost-effectively mine important signals from every trace while maintaining frontier performance? 💡 Methodology & Proposed Approach ・Supervised fine-tuning (SFT) with LoRA on the open-source Qwen-3.5-35B, on Fireworks infrastructure ・Training data from two production datasets: chat-langchain (technical Q&A, 707 examples) and Fleet (no-code agent, 727 examples) ・It learns "Perceived Error," doing the judging without relying on huge frontier models 📊 Experimental Results ・Accuracy: the fine-tuned Qwen matched or exceeded frontier models (96.1% on chat-langchain, 90.8% cross-domain on Fleet) ・Cost: 10-100x cheaper than frontier models depending on trace volume ・Transferability: the model trained on chat-langchain beat all frontier models on Fleet without retraining #LLMEvaluation# #FineTuning#