Just-in-Time (JIT) agent harness construction is an execution paradigm where a task-specific operational scaffold is synthesized dynamically on the spot at inference time rather than precompiled ahead of time. Under the JIT paradigm, a trained generator receives the target task specification, operational protocol, available capability registry, and prior harness context to assemble an adapted harness instance. By tailoring memory management, planning representations, and action loops to the exact problem at hand, JIT construction amortizes harness optimization and shifts agent design from repeated offline search to runtime synthesis and evolutionary revision.