The trickiest part of a replay system in a competitive title is almost always the determinism contract, not the serialization itself. You want to separate concerns: capture an immutable input stream (player inputs + timestamp deltas, not wall-clock), snapshot the authoritative game state at fixed tick boundaries, and guarantee that replaying those inputs through the exact same code reproduces byte-identical state. The classic failure modes I’ve hit are non-deterministic RNG leaking through unseeded generators, floating-point divergence across CPU architectures (denormals, FMA contraction, SIMD ordering), and any wall-clock or thread-timing dependency sneaking into authoritative logic. My recommendation is to run a headless deterministic build with a fixed tick rate and a seeded, explicit RNG (like a splitmix64 or PCG you control), then validate the replay pipeline in CI by comparing serialized state hashes frame-by-frame against a golden reference. If your simulation is multi-threaded, you’ll need to serialize and deserialize thread work queues at tick boundaries rather than relying on execution order.
On the serialization side, avoid serializing pointers or memory layouts directly—snapshot logical fields and reconstruct object identity explicitly, otherwise you’ll get corruption that only manifests months later on a different platform. Use a versioned schema with forward/backward field migration so replay files from older
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