Seed Control

seed-control — catch unseeded randomness before it breaks reproducibility

The engine is seed-control.js (portable Node, zero deps). It statically scans .py / .js / .ts source for random draws that have no seed governing them. An unseeded draw makes a run non-repeatable: the motivating case is a backtest or simulation whose frozen regression tests are pinned byte-exact — one stray random.random(), np.random.rand(), or Math.random() with no seed silently turns a byte-for-byte-stable report into a moving target, and the frozen test starts flapping for reasons unrelated to any real change.

Purely read-only: it opens files, never writes to the scanned tree (canary experiments are confined to a temp dir).

Rules (same-file heuristic)

Math.random() is always flagged because there is no way to seed it; the fix is a seeded PRNG (mulberry32, seedrandom, etc.). A per-line # seed-ok (Python) or // seed-ok (JS/TS) comment suppresses that one finding when the randomness is intentionally unseeded (jitter, nonces, sampling you don't need to reproduce).

Commands

node seed-control.js scan <path> [<path>...]
node seed-control.js --canary

• scan takes files or directories. Directories recurse over .py/.js/.ts and skip node_modules, .git, __pycache__, .venv/venv, dist, build. Each finding prints as file:line:snippet. Exit 1 if any finding, else 0.

Examples

• Guard a backtest before freezing its report (must stay byte-exact): node seed-control.js scan /path/to/backtest — any hit is a draw that can drift the frozen baseline; seed it or mark # seed-ok, then re-run.
• Scan a web app's frontend/backend for stray Math.random() (token/ID generation that should use a seeded or crypto source): node seed-control.js scan /path/to/app.
• Check a single file: node seed-control.js scan sim.py.

Windows notes

• PowerShell 5.1 and Git Bash both run it the same way; paths with spaces go in double quotes: node seed-control.js scan "D:\path with space".
• Pure Node stdlib — no &&/|| chaining, ternary, or encoding gotchas involved; nothing is written, so no Set-Content / UTF-8 concerns apply.

Known limitations (read before trusting a clean result)

• Same-file heuristic, no cross-module tracking. A seed set in conftest.py, a shared set_seed() helper, or a fixture in another module does NOT count — the scanner only sees random.seed(/np.random.seed(/default_rng( in the same file as the draw. This yields false positives for centrally-seeded projects; triage each hit and add # seed-ok where a real upstream seed exists. Conversely, a seed in one file does not vouch for an unseeded draw in another — each file is judged alone.
• Textual, not semantic. It matches random./np.random./Math.random( token patterns; it does not parse scope or data flow. A random.seed() guarded behind a branch that never runs still reads as "seeded". Aliased imports (from random import random as r; import numpy.random as npr) are NOT matched — only the random. / np.random. / Math.random spellings are.
• Python random module only for the bare form — secrets.token_* (which is intentionally unseedable and correct for tokens) is not flagged.
• Seeding is necessary, not sufficient, for reproducibility: thread/process ordering, dict iteration, hardware floats, and library-internal RNGs can still cost you determinism. This scan removes the most common and silent cause, not all of them.

Exit codes

0 clean · 1 findings or canary failure · 2 usage error (no files, unknown command).

Verification (the done-check)

node seed-control.js --canary

Self-tests both directions in a throwaway temp dir: an unseeded random.random() IS flagged, a random.seed()ed file stays clean, unseeded np.random is flagged while np.random.seed()/default_rng() clear it, Math.random() is flagged in .js and .ts, and both # seed-ok / // seed-ok suppressions silence a line — plus end-to-end exit 1 (findings) vs. exit 0 (clean). MUST print CANARY PASS 16/16 before you trust a scan.