Build deterministic race-condition tests - identify shared mutable state, drive interleavings via barriers / latches / manual scheduling; use ThreadSanitizer (clang `-fsanitize=thread`) for C/C++ data race detection; run the Go race detector end-to-end (`go test -race`, GORACE tuning, `-count`/`-cpu` stress, goroutine-leak gating with go.uber.org/goleak - references/go.md); use jcstress (`@JCStressTest` + `@Actor` + `@Outcome`) for JVM stress; use Loom virtual-thread interleavings for parallel testing. Use when a defect only reproduces under load on shared in-process state (cache, counter, connection pool, lazy-init singleton), when writing the regression test for a race-condition incident before the fix lands, or when adding `-race` to a Go CI matrix.