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All four variants cut sequential decode steps by guessing several tokens at once: Medusa uses extra heads, EAGLE predicts hidden states, lookahead runs Jacobi over n-grams, vanilla uses a draft model.
Imagine dictating a letter to a typist who can only write one word at a time, then waits for you to say the next. Slow. Speculative decoding adds an apprentice who guesses the next few words ahead, and you just check the guesses in one glance, keeping the correct ones. The variants differ in who guesses. A separate junior typist is vanilla speculative decoding. Extra hands on your own typist that scribble several guesses at once is Medusa. A typist who anticipates your train of thought, not just your words, is EAGLE. And lookahead is the typist replaying common phrases they have already typed and checking which ones fit. Either way, you verify many words per glance instead of one.
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example.
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4 min: why decode is memory bound, the four drafters (separate model, Medusa heads, EAGLE hidden states, lookahead n-grams), exact verification, and when batch size kills the win.
| Variant | Drafter | Needs training | Needs second model |
|---|---|---|---|
| Vanilla speculative | Separate small model | No (reuse a small model) | Yes |
| Medusa | Extra heads on target | Yes (heads) | No |
| EAGLE / EAGLE-2 | Hidden-state predictor | Yes (feature head) | No |
| Lookahead | Jacobi n-gram pool | No | No |
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What an interviewer would ask next. Try answering before peeking at the approach.
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Assuming all four use a separate draft model. Only vanilla does. Medusa and EAGLE bolt onto the target, and lookahead has no draft model and no training at all.
The night-before-the-interview bullets. Scan these on the way to the call.
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