Sequential — when stages depend
Use when: output of stage N is input to stage N+1. Predictable latency, easy to debug. Downside: no parallelism, and one slow stage blocks the whole chain.
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Parallel — when stages don't depend
Use when: N specialists can work on the same input independently. Latency = slowest branch. Downside: N times the cost, harder failure modes.
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Loop — when quality needs iteration
Use when: 'draft, critique, revise' improves quality. Unpredictable latency, always cap iterations. Downside: hard to reason about cost.