A coding agent's conversation runs 100,000 tokens. Every single message, the model re-reads all of it before answering. Providers cache that work and charge up to 90% less the second time - but only for the model that did the reading. Model routers pick a different model for each message to save money. The moment one switches, the new model has read nothing, and you pay full price for all 100,000 tokens again. A single switch can cost more than the turn it was trying to optimize. No router on the market tracks this, so they all treat switching as free. Inertia makes switching cost something. A cache ledger in MongoDB Atlas records which model has read how much of each conversation, so the router can ask the real question: is a smarter model worth re-reading 100,000 tokens, given how many messages are left? Atlas Vector Search remembers how similar questions actually turned out, so it learns which ones truly need the expensive model instead of guessing.
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