X_STEP_OUTPUT_UNSERIALIZABLE
Means
a step returned a value JSON cannot hold
Typical cause
step.run(name, fn) answered a bigint, a cycle, NaN, an infinity, an invalid Date, or text jsonb refuses (a NUL, a lone surrogate), so no attempt could replay it
Fix
return plain JSON from the step: String(id) for a bigint, the ids rather than the object graph, null rather than NaN or an invalid Date, text with no NUL and no lone surrogate