Segment 52

This sub-session focused on diagnosing why the DFlash drafter for Qwen3.6-27B was plateauing at much lower acceptance than expected. First, the user built a comprehensive evaluation harness on CT129 that extracted hidden states from fresh coding prompts and ran drafter inference side-by-side with the z-lab reference model, revealing a 4x gap (τ≈3.0 vs τ≈12.4). Tracing the root cause led to an architectural mismatch: the fc projection used only 4 of 5 target layers, omitting layer 61 which carries the richest next-token information. A deeper investigation into training logs and code comparison against the official speculators repository uncovered three critical bugs: noise was corrupting the target logits by being applied to the combined hidden state tensor before extracting the last layer; the fc included the target layer, creating a shortcut where the same information appeared in both conditioning and loss target; and the loss function used soft KL divergence with streak weighting instead of the paper's hard cross-entropy, diluting gradients. The user abandoned the current run, implemented all three fixes (split hidden states so noise only applies to fc input, revert fc to 4-layer input matching official architecture, switch loss to pure hard CE with gamma=7.0), and launched v5 training run which is now running with early metrics showing comparable accuracy.

Build evaluation harness for DFlash drafter on CT129Compare drafter performance against z-lab reference modelTrace root cause of performance gap to fc layer count mismatchAbandon current training run and commit scripts before fixesInvestigate hidden state numerical differences between torch and flaDiscover noise corrupting target logits bugDiscover fc shortcut including target layer bugDiscover loss function mismatch (soft KL vs hard CE)Implement split hidden states fix for noiseRevert fc to 4-layer input matching official architectureSwitch loss to pure hard cross-entropy with gamma=7.0Launch v5 training run with all fixes

The Three-Bug Exorcism: How a 4x Performance Gap Was Traced, Diagnosed, and Fixed in DFlash Drafter Training 4055 words

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