A bug you can't reproduce, you can't fix
GDC-L1-QA-000301
Statement
Reproduction is the heart of fixing bugs: a defect you can reliably trigger you can diagnose and verify fixed; one you can't reproduce you can only guess at. Invest in the things that make bugs reproducible — clear repro steps, determinism where feasible, good logging, and captured state — so bugs become tractable instead of haunting.
02
Rationale
The debugging loop is reproduce → diagnose → fix → verify, and it stalls entirely at step one: if you can't make the bug happen on demand, you can't watch it, can't be sure of the cause, and can't confirm your fix actually worked [S-games-user-research]. So reproducibility is the single most valuable property for bug-fixing, and it doesn't happen by accident — it's enabled by disciplined repro steps in reports, determinism (same inputs → same behavior, which also aids automated testing, QA-0002, and reproducible builds, CONTENT-0005), comprehensive logging, and the ability to capture and replay state. Non-reproducible bugs ("it happened once") are the ones that ship and recur. Making bugs reproducible is upstream of fixing them.
03
Applies when
All bug-fixing and QA. Especially critical for intermittent, timing-dependent, or emergent bugs, and for anything that must be verified fixed.
04
Does not apply / Exceptions
Some bugs are genuinely rare/environmental and full determinism is impractical (network races, hardware-specific issues) — there you invest in observability (logging, crash dumps, telemetry) to reconstruct what happened, since you can't reproduce it directly. And chasing perfect reproducibility for a trivial cosmetic bug isn't worth it (risk-weight it, QA-0001).
05
Implementation
Require clear repro steps in bug reports. Build for determinism where feasible (fixed seeds, deterministic simulation) — it aids repro, automated testing (QA-0002), and reproducible builds (CONTENT-0005). Log richly and capture state/crash dumps so hard-to-repro bugs can be reconstructed (observability). Verify fixes by reproducing the original and confirming it's gone. Treat non-reproducible recurring bugs as a signal to improve observability.
06
Disagreement
Little on the principle; the nuance is how much to invest in determinism/observability infrastructure versus living with some irreproducible bugs. High-reliability and competitive/networked games invest heavily; smaller games lean on logging and accept some mystery bugs.
07
Notes
The debugging-foundation principle of QA; enabled by determinism (shared with QA-0002 and CONTENT-0005) and observability. Pairs with blameless problem-solving (TEAM-0003 — reproduce and fix the system, don't blame). Confidence 4.
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Connected principles
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