Prompt Credit Assignment: Finding the Dead Weight in Your System Prompt
Most teams discover their system prompt has a weight problem the same way — a cost review, a latency spike, or an engineer who finally reads the thing end to end. What they find is typically a 2,000-token document that grew organically over six months, with three versions of "be concise" scattered across different sections, instructions that reference a product workflow that was deprecated in February, and a dozen rules that the model visibly ignores on every run. The prompt is large. Most of it isn't doing anything.
This is the prompt credit assignment problem: figuring out which instructions in a multi-thousand-token system prompt actually drive model behavior, and which are just dead weight that burns tokens and dilutes attention. The bad news is that most teams skip this entirely — they add instructions when behavior breaks and never subtract. The good news is there is a repeatable engineering discipline for it.
