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Notes on building a deterministic execution engine — the decisions, and the ones that turned out wrong.

• A determinism tool whose own output was not deterministic
  31 July 2026 · 2 min read

  The regression diff printed its differences in a different order on every run, because it iterated a Go map. For the one package whose job is proving determinism, that is not cosmetic.

• Adding parallelism to an engine whose whole point is determinism
  31 July 2026 · 3 min read

  Fan-out is easy. Fan-out that gives the same answer regardless of which branch finishes first, and refuses rather than arbitrates when two branches disagree, took longer.