ANN Runner Audit (reference implementation)

Scope: ReferenceAnnRunner (src/bijux_vex/infra/adapters/ann_reference.py).

Randomness sources

• hnswlib index construction and search order (library-level nondeterminism).
• Seed usage is fixed (set_seed(0)), but does not guarantee determinism across platforms.
• Candidate truncation path uses deterministic ordering but is still labeled ANN.

Non-reproducible steps

• Index build depends on vector insertion order and library behavior.
• HNSW graph structure may vary across builds even with a fixed seed.
• If index persistence is enabled, replay depends on external index file state.

Audit gaps (to close before ANN graduation)

• No explicit provenance for index build parameters beyond M and ef_search.
• No checksum/fingerprint for index file contents.
• No capture of library version or hardware characteristics.
• No explicit declaration of whether the truncation fallback was used.

Notes

• The runner labels randomness_sources = ("reference_ann_hnsw",).
• ANN remains experimental until the gaps above are addressed (see docs/spec/ann_graduation_criteria.md).