Fable
Beside the fast river cart, more than just railings were added
The mountain road had sharp turns and drops, and when the driver knew the route well, he often felt experience was enough. One rainy night, the wheels slipped on wet stone; writing "caution" afterward could not stop the next cart from repeating it.
The road chief set up wooden barriers outside dangerous bends, laid gravel on steep slopes, and restricted heavy carts from traveling at night. Drivers could still pass, but dangerous actions would be stopped or require confirmation before entering the cliff edge.
They also check whether cargo is overweight, whether routes cross boundaries, whether wheels are abnormal; notices are only reminders, and real guardrails must be able to read state and influence execution.
When a heavy rainstorm hits, the system guides vehicles into a refuge, records the reason, and waits for manual release. Guardrails can both reject high-risk actions and provide degradation and audit paths.
Guardrails are control layers arranged around input, output, tools, permissions, and state transitions; reliable guardrails should take effect at the execution boundary, not merely assign responsibility to prompts.