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The opt-in ssm_bf16_tau hybrid mode splits a gated-DeltaNet layer's recurrent SSM state into an f32 partition (s_l) and a bf16 partition (s_l_bf16). The recurrent state serialization paths (state_write_data / state_read_data) were never updated for the split: they read/wrote s_l using the FULL hparams.n_embd_s() (S_v*S_v*H) row width, but a split layer's s_l only holds S_v*S_v*n_f32, so the access overruns the smaller tensor (a ggml_backend tensor read out of bounds), and the bf16 fast-head partition was never persisted at all. This is what broke high-concurrency serving with --ssm-bf16-tau: the server's context-checkpoint feature serializes per-sequence state via state_seq_get_data. With a checkpoint enabled, even a single request triggered the out-of-bounds read; at higher concurrency the cell range starts at a higher base slot so the overrun reaches further (hard abort in a debug build, silent state corruption then 1-token-then-EOS on restore in a release build). The static batched-bench never exercises save/restore so it did not catch it; the GDN decode kernel and per-head partition offsets were already correct (decode with checkpoints disabled is fine at N=8/16/32). Fix: serialize the f32 partition and, when the layer is split, the bf16 partition right after it, each with its OWN row width (tensor ne[0]). head_slot is rebuilt deterministically at load (same model + tau), so it is not serialized. Non-split layers have ne[0] == n_embd_s() and no bf16 partition, so their on-disk format and behavior are byte-identical (the default f32 path and the bit-exact gate are unaffected). Verified on GB10/DGX with Qwen3.6-35B-A3B-NVFP4 + --ssm-bf16-tau 64 via a continuous-batching llama-server: with context checkpoints enabled, N=8, N=16 and N=32 (slot reuse + restore) all now produce full coherent 128-token output and the server stays up; pre-fix the same config aborted on the first checkpoint. Assisted-by: Claude:claude-opus-4-8[1m] [Claude Code] Signed-off-by: Ettore Di Giacinto <mudler@localai.io>