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Deterministic fixture worlds

Fixture worlds exist so agents can:

  • run tests deterministically,
  • reproduce bugs exactly,
  • measure changes reliably.

Key rules​

  • no ambient time
  • no ambient network
  • no ambient filesystem scanning
  • all I/O goes through explicit, world-scoped builtins

Deterministic filesystem (solve-fs)​

  • the runner mounts a fixture directory as . (read-only)
  • path resolution is safe and deterministic
  • directory listing order is canonicalized

Request/response cassettes (solve-rr)​

  • request bytes map to response fixtures
  • optional deterministic latency modeling via fixture metadata

Seeded KV (solve-kv)​

  • the KV store is reset per case from a seeded dataset
  • keys and values are bytes
  • iteration is deterministic (if exposed)

Why this matters for agentic coding​

If an agent can’t reproduce failures exactly, it can’t repair reliably.

Fixture worlds create the “closed environment” needed for a robust autonomous loop.