Your Agent Failed in Prod. Good Luck Reproducing It.

Chasing bitwise determinism in LLM agents is futile. Even at temperature zero, GPU floating-point non-determinism, batch variance, and MoE routing yield different outputs, making bugs unreproducible. The fix is replayability: record every input/output at node boundaries to freeze session state.

Chronicle, a proof-of-concept, captures full traces with model version and build ID. Replaying stubs LLM calls, enabling deterministic testing of tools and guardrails in CI at zero model cost. This turns production incidents into repeatable test cases. Only deterministic testing (tool logic) benefits; behavioral testing (LLM tone) still requires separate review.

Engineering leaders must shift observability to boundary-level state recording, logging model version, build ID, and RAG chunks. This requires tooling investment but prevents hidden errors like a $190K trade. Replayability is not a silver bullet: behavioral regressions still need LLM-as-judge, and storage adds complexity. Leaders should pilot replayability now while watching for mature tools.