Taxonomy Surgery, Cosine = 1.0000, and Making Routing Disappear into Infrastructure

This is part 3 of the Adaptive Model Routing series. Part 1 built an LLM categorizer with Groq — 8 categories, 3 tiers. Part 2 added k-NN embedding lookup in shadow mode, discovered 83% tier accuracy, and found 61% cost savings on paper. This post covers what happened next. When Phase 2 ended, I had a working embedding pool in shadow mode inside crab-bot. The category accuracy was sitting at 78.6%. Not bad — but the breakdown hid something worth looking at. Phase 3: When Validation Tells You a Category Doesn't Need to Exist The leave-one-out accuracy by category told the real story: Category Accuracy Tier casual 94% cheap simple_lookup 91% cheap creative 88% medium coding 92% strong reasoning 89% strong analysis 59% medium research_lookup 61% medium Two categories were basically a coin flip. And they were confusing each other — almost all of analysis's misses landed on research_lookup and vice versa. The obvious move would be to try fixing the categorizer prompt, tuning the LLM, or gathering more labeled data. I was about to go down that road when I noticed the column next to the accuracy: both categories mapped to the same tier . Medium. That changed everything. The question stopped being "why can't the model tell these apart?" and became: "what routing decision are we actually getting wrong?" The answer was zero. A misclassification between analysis and research_lookup produces no routing error. The routing outcome is identical either way. The confusion wasn't a model failure — it was a signal from the embedding space that the boundary between these two categories was artificial. If k-NN can't draw a line between them in 384 dimensions with 1,300 examples, maybe the line doesn't belong there. Decision: merge research_lookup into analysis. -- Re-label 243 rows where category was 'research_lookup' UPDATE routing_log SET category = 'analysis' WHERE category = 'research_lookup' ; The embeddings didn't change. The vectors were already correct — only the label stored al