Syndrome-Consistency Gating for Quantum Error Correction: Decoder-Independent Hardware Validation Across IBM and Rigetti

Quantum error correction (QEC) decoders produce corrections but provide noper-shot signal of correction reliability. We introduce syndrome-consistency gating: adecoder-independent shot acceptance filter that uses redundant syndrome copies to identifyunreliable decoder outputs. The gate is veto-only (it may block shots but never overrides thedecoder) and fails closed (blocks all shots on gate failure). We validate this approach on twoindependent hardware vendors. The primary dataset comprises 768,000 shots on IBM Torino(Heron r2, 156 qubits); a cross-vendor dataset of 64,000 shots on Rigetti Ankaa-3 (84qubits) provides supporting evidence; and an exploratory dataset of 64,000 shots on IBMFez preceded both. Total: ~896,000 hardware shots. All experiments use repetition codes.On IBM Torino with a distance-5 code, strict gating with two redundant copies (f=2) achieves100% logical error rate (LER) reduction (0.188% to 0.00%) at 36.3% allow rate, with all 30observed logical errors concentrated exclusively in blocked shots. Under practicalparameters, LER reduction reaches 85.6% (0.081% to 0.012%) at 53.3% allow rate with13.7× error concentration. On Rigetti Ankaa-3, gated active broadcast achieves zero logicalerrors in 16,076 accepted shots versus 9 errors in 24,000 passive shots (p = 0.014). Wecharacterise the allow-rate scaling as exponential decay (R² ≥ 0.98) and identify f=2 as thepractical operating point.