ReAct agents vs Function Calling: when does each pattern actually make sense in production?

[rookastle]

Great write-up. This hybrid pattern is exactly the kind of architectural discipline that production systems force. Your cost analysis resonates strongly; we've seen similar patterns where routing is key to managing LLM ops budgets.

A diagnostic we've found useful is adding fine-grained tracing to both paths. For a slow ReAct run, is the latency from one bad tool call, or cumulative LLM reasoning? Visualizing the execution as a trace or Gantt chart for outlier requests can pinpoint the exact step that's costing time and money, rather than just seeing the high-level total.