Model Selection
AI Engineer path · lesson 92 of 124Run model choice as a four-stage funnel — hard gates, public evidence, your own eval, then economics — weigh a hosted API against open weights, watch benchmark contamination inflate a score, and find the volume where self-hosting starts to pay.
Key insight
Re-run the funnel, do not re-open the decision. When a new model lands, the shortlist stage is cheap and your eval set already exists — so the cost of re-checking is a few hours, and the cost of not re-checking is being one generation behind indefinitely.
Narrow the field
Order a model selection process from cheapest filter to most expensive.
Tap a piece, then tap where it goes — or drag it. Tap the tray to take one back.
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Test your knowledge
The first stage of a model-selection funnel should be:
The short version
Hiring, not shopping. You do not interview everyone who applied: you screen out whoever fails a hard requirement, shortlist on the CV, and then give the short list the same work sample. The CV is public evidence; the work sample is your eval; only one of them predicts the job.
In more depth
Model Selection
There are more usable models than any team can evaluate, and the set changes every few months. Run the decision as a funnel, cheapest filters first.
The four stages
API vs open weights
API wins on time-to-first-call, peak capability and zero ops. Open weights win on data residency, cost at very high volume, latency floor, finetuning freedom, version stability and vendor risk. The common end state is both, with a router in front.
Reading a benchmark
Benchmarks saturate, aggregate scores hide the shape, setups are rarely comparable, and contamination is hard to rule out. A 20-point gap between a public score and a fresh variant of the same test is the signature of memorisation. Treat every public number as an upper bound.
Breakeven
API = pure variable; self-host = mostly fixed. Two lines, one crossing. Utilisation is the variable nobody models: a GPU at 30% occupancy costs the same as one at 95%.
Interview Questions
References & Further Reading
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