Agents for Humans: Designing a Safe Mechanic Patch Loop with Strands Agents
How CAST uses Strands Agents, constrained tools, versioned mechanics, and explicit failure handling to turn natural-language design intent into executable game-rule patches without letting the model freely rewrite the application.
Why we did not let the model write arbitrary game code
CAST is a live playtest agent for game and interaction designers.
A designer can play a prototype, describe a mechanic change, and compare the exact same recorded play under the old and new rules.
The natural-language part sounds simple:
“Only attach the cargo when I approach slowly.”
But implementing that safely raised a more interesting question:
How much control should the agent actually have?
One option would be to give a model access to the source code and ask it to rewrite whatever is necessary.
We deliberately did not build CAST that way.
Instead, CAST uses the Strands Agents SDK with a constrained mechanic runtime and explicit tools.
The agent can reason about what should change, but the actual changes have to fit a bounded set of executable primitives.
That makes the system easier to inspect, replay, patch, and revert.
Intent is flexible; execution is constrained
The designer speaks in mechanic-level language.
For example:
“Only attach when I approach slowly.”
The agent does not need the designer to say:
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distance < 0.30
AND
speed < 0.25
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