diff --git a/crates/ml-alpha/tests/gpu_log_ring_invariants.rs b/crates/ml-alpha/tests/gpu_log_ring_invariants.rs
new file mode 100644
index 000000000..3ee5afe2d
--- /dev/null
+++ b/crates/ml-alpha/tests/gpu_log_ring_invariants.rs
@@ -0,0 +1,291 @@
+//! Invariant tests for the GPU log ring: allocator, magic validation,
+//! ring wrap, drainer fall-behind detection.
+//!
+//! Per `feedback_no_cpu_test_fallbacks`: unit tests #1-#6 exercise the
+//! HOST-SIDE invariants only (struct layout, slot arithmetic, magic
+//! validation, drain dispatch). They write LogRecord structs directly
+//! into the mapped-pinned buffer — `#[repr(C)]` makes the host write
+//! binary-identical to what a kernel would write.
+//!
+//! Test #7 is the GPU oracle path: launches the tick kernel via cudarc,
+//! confirms the device-side step counter increments.
+
+#![cfg(feature = "cuda-diag-log")]
+
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::time::Duration;
+
+use ml_alpha::gpu_log::{
+    spawn_drain_task, LogHeader, LogRecord, LogRing, KID_SMOOTHNESS_CONTROLLER, MAGIC,
+    N_RECORDS_PER_STEP, N_RT_PER_KERNEL, N_SLOTS, PAYLOAD_F32, RT_INPUT, RT_OUTPUT, RT_STATE,
+};
+use ml_alpha::pinned_mem::MappedRecordBuffer;
+use ml_core::device::MlDevice;
+
+// ── helpers ──────────────────────────────────────────────────────────
+
+/// Acquire a CUDA device — required even for "unit" tests because
+/// `LogRing::alloc()` calls `cuMemHostAlloc` which needs an active
+/// context on the calling thread. Per `feedback_no_cpu_test_fallbacks`,
+/// the only valid CPU↔GPU memory channel is mapped-pinned (see
+/// `feedback_no_htod_htoh_only_mapped_pinned`), so a CUDA-less unit
+/// test is not possible for the ring allocator.
+fn test_device() -> MlDevice {
+    MlDevice::cuda(0).expect("CUDA 0 required for gpu_log_ring tests")
+}
+
+/// Compute the slot index for a (step, kernel_id, record_type) tuple.
+/// Mirrors the kernel-side arithmetic in `gpu_log_ring.cu`.
+fn slot_of(step: u32, kernel_id: u8, record_type: u8) -> usize {
+    ((step as usize) * N_RECORDS_PER_STEP
+        + (kernel_id as usize) * N_RT_PER_KERNEL
+        + record_type as usize)
+        & (N_SLOTS - 1)
+}
+
+/// Build a `LogRecord` with the given fields.
+fn make_record(step: u32, kid: u8, rt: u8, payload: &[f32]) -> LogRecord {
+    let mut rec = LogRecord {
+        header: LogHeader {
+            magic: MAGIC,
+            step,
+            kernel_id: kid,
+            record_type: rt,
+            payload_words: payload.len().min(PAYLOAD_F32) as u8,
+            reserved: 0,
+            _padding: 0,
+        },
+        payload: [0.0; PAYLOAD_F32],
+    };
+    for (i, &v) in payload.iter().take(PAYLOAD_F32).enumerate() {
+        rec.payload[i] = v;
+    }
+    rec
+}
+
+/// Allocate a host-visible step-counter shadow (1 i32).
+fn alloc_host_counter() -> Arc<MappedRecordBuffer<i32>> {
+    let buf = unsafe { MappedRecordBuffer::<i32>::new(1) }
+        .expect("alloc step counter shadow");
+    buf.write_record(0, 0);
+    Arc::new(buf)
+}
+
+// ── unit tests (require CUDA context for mapped-pinned alloc only) ───
+
+#[test]
+fn ring_alloc_zeros_magic() {
+    let _dev = test_device();
+    let ring = LogRing::alloc().expect("alloc ring");
+    // Every slot's magic must be 0 (zero-initialised — must not match MAGIC).
+    for i in 0..N_SLOTS {
+        let rec = ring.buffer.read_record(i);
+        assert_eq!(
+            rec.header.magic, 0,
+            "slot {i} magic should be 0 on fresh alloc, got 0x{:x}",
+            rec.header.magic
+        );
+    }
+}
+
+#[test]
+fn host_write_round_trip() {
+    let _dev = test_device();
+    let ring = LogRing::alloc().expect("alloc ring");
+    let payload = [1.0_f32, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0];
+    let rec = make_record(42, KID_SMOOTHNESS_CONTROLLER, RT_INPUT, &payload);
+
+    // Write at the canonical slot for (step=42, KID_SMOOTHNESS_CONTROLLER, RT_INPUT).
+    let slot = slot_of(42, KID_SMOOTHNESS_CONTROLLER, RT_INPUT);
+    ring.buffer.write_record(slot, rec);
+
+    // Read back — all fields must match.
+    let read = ring.buffer.read_record(slot);
+    assert_eq!(read.header.magic, MAGIC);
+    assert_eq!(read.header.step, 42);
+    assert_eq!(read.header.kernel_id, KID_SMOOTHNESS_CONTROLLER);
+    assert_eq!(read.header.record_type, RT_INPUT);
+    assert_eq!(read.header.payload_words, 10);
+    for (i, &expected) in payload.iter().enumerate() {
+        assert_eq!(read.payload[i], expected, "payload[{i}] mismatch");
+    }
+    // Untouched payload slots remain zero.
+    for i in payload.len()..PAYLOAD_F32 {
+        assert_eq!(read.payload[i], 0.0, "untouched payload[{i}] should be 0");
+    }
+}
+
+#[tokio::test(flavor = "current_thread")]
+async fn magic_validation_skips_torn() {
+    let _dev = test_device();
+    let ring = Arc::new(LogRing::alloc().expect("alloc ring"));
+    let counter = alloc_host_counter();
+    let stop = Arc::new(AtomicBool::new(false));
+
+    // Slot for step=1, KID_SMOOTHNESS_CONTROLLER, RT_INPUT — valid record.
+    let valid_slot = slot_of(1, KID_SMOOTHNESS_CONTROLLER, RT_INPUT);
+    // Slot for step=1, KID_SMOOTHNESS_CONTROLLER, RT_STATE — torn (magic=0).
+    let torn_slot = slot_of(1, KID_SMOOTHNESS_CONTROLLER, RT_STATE);
+    // Slot for step=1, KID_SMOOTHNESS_CONTROLLER, RT_OUTPUT — header.step mismatched.
+    let mismatch_slot = slot_of(1, KID_SMOOTHNESS_CONTROLLER, RT_OUTPUT);
+
+    let valid_payload = [100.0_f32; 10];
+    ring.buffer.write_record(
+        valid_slot,
+        make_record(1, KID_SMOOTHNESS_CONTROLLER, RT_INPUT, &valid_payload),
+    );
+
+    // Torn record: write with magic=0 so drainer must skip.
+    let mut torn = make_record(1, KID_SMOOTHNESS_CONTROLLER, RT_STATE, &[1.0; 10]);
+    torn.header.magic = 0;
+    ring.buffer.write_record(torn_slot, torn);
+
+    // Mismatch record: claims step=99 but lives in slot for step=1 — drainer must skip.
+    let mismatch = make_record(99, KID_SMOOTHNESS_CONTROLLER, RT_OUTPUT, &[2.0; 10]);
+    ring.buffer.write_record(mismatch_slot, mismatch);
+
+    counter.write_record(0, 1);
+
+    let h = spawn_drain_task(ring.clone(), counter.clone(), stop.clone());
+    // Give drainer one tick (>500 ms cadence + a bit of slack).
+    tokio::time::sleep(Duration::from_millis(700)).await;
+    stop.store(true, Ordering::Relaxed);
+    // Wait for the next sleep boundary so the drain task observes the stop flag.
+    let _ = tokio::time::timeout(Duration::from_secs(2), h).await;
+    // Pass if no panic — torn slot was skipped, mismatch slot was skipped.
+    // (We don't intercept tracing output here; the absence of panic is the
+    // pass condition — drainer must defensively handle both cases.)
+}
+
+#[test]
+fn ring_wrap_modulo_arithmetic() {
+    // For step values past N_SLOTS/N_RECORDS_PER_STEP, slot indices wrap.
+    let wrap_step = (N_SLOTS / N_RECORDS_PER_STEP) as u32; // = 1024
+    // step 0 and step `wrap_step` for (KID=0, RT=0) hash to the same slot.
+    assert_eq!(
+        slot_of(0, 0, 0),
+        slot_of(wrap_step, 0, 0),
+        "wrap_step={wrap_step} should collide with step=0 at same (kid, rt)"
+    );
+    // step 0 and step (wrap_step * 2) collide on the next cycle, etc.
+    assert_eq!(slot_of(0, 0, 0), slot_of(wrap_step * 2, 0, 0));
+    // Different (kid, rt) at same step never collide (within one cycle).
+    assert_ne!(slot_of(0, 0, 0), slot_of(0, 0, 1));
+    assert_ne!(slot_of(0, 0, 0), slot_of(0, 1, 0));
+    // All slots stay in [0, N_SLOTS).
+    for step in 0..(wrap_step * 3) {
+        for kid in 0..8u8 {
+            for rt in 0..4u8 {
+                let s = slot_of(step, kid, rt);
+                assert!(
+                    s < N_SLOTS,
+                    "slot {s} out of bounds for (step={step}, kid={kid}, rt={rt})"
+                );
+            }
+        }
+    }
+}
+
+#[tokio::test(flavor = "current_thread")]
+async fn step_gap_detection_emits_warn() {
+    let _dev = test_device();
+    let ring = Arc::new(LogRing::alloc().expect("alloc ring"));
+    let counter = alloc_host_counter();
+    let stop = Arc::new(AtomicBool::new(false));
+
+    // Drainer ring window = N_SLOTS / N_RECORDS_PER_STEP = 1024 steps.
+    // Jump the counter past the window so drainer must clamp.
+    let big_jump = ((N_SLOTS / N_RECORDS_PER_STEP) + 10) as i32;
+    counter.write_record(0, big_jump);
+
+    let h = spawn_drain_task(ring.clone(), counter.clone(), stop.clone());
+    tokio::time::sleep(Duration::from_millis(700)).await;
+    stop.store(true, Ordering::Relaxed);
+    let _ = tokio::time::timeout(Duration::from_secs(2), h).await;
+
+    // Pass if no panic — drainer clamped `last_drained_step` to `oldest_safe_step`
+    // and emitted a warn. We don't tap tracing here; the absence of panic +
+    // the drainer's known structure (warns then clamps) is the pass condition.
+}
+
+#[tokio::test(flavor = "current_thread")]
+async fn decoder_dispatch_unknown_falls_through() {
+    let _dev = test_device();
+    let ring = Arc::new(LogRing::alloc().expect("alloc ring"));
+    let counter = alloc_host_counter();
+    let stop = Arc::new(AtomicBool::new(false));
+
+    // Write a valid record with kernel_id=99 (out of declared range — no decoder registered).
+    let unknown_kid: u8 = 99;
+    let rec = make_record(1, unknown_kid, RT_INPUT, &[7.0; 10]);
+    let slot = ((1usize) * N_RECORDS_PER_STEP
+        + (unknown_kid as usize) * N_RT_PER_KERNEL
+        + RT_INPUT as usize)
+        & (N_SLOTS - 1);
+    ring.buffer.write_record(slot, rec);
+    counter.write_record(0, 1);
+
+    let h = spawn_drain_task(ring.clone(), counter.clone(), stop.clone());
+    tokio::time::sleep(Duration::from_millis(700)).await;
+    stop.store(true, Ordering::Relaxed);
+    let _ = tokio::time::timeout(Duration::from_secs(2), h).await;
+
+    // The drainer's iteration is `for kid in 0..N_KERNELS` — so kid=99 will
+    // never be visited by the drainer's slot walk. But the `decode()` function
+    // itself MUST handle unknown kernel_id without panic. Exercise it directly:
+    ml_alpha::gpu_log::decode(unknown_kid, RT_INPUT, 1, &[7.0; 10]);
+    ml_alpha::gpu_log::decode(0, 99, 1, &[7.0; 10]); // unknown record_type
+                                                      // Pass if no panic.
+}
+
+// ── CUDA integration test #7 ─────────────────────────────────────────
+
+/// Fire the `gpu_log_tick` kernel via cudarc twice; confirm that the
+/// device-side step counter increments from 0 → 1 → 2. This validates
+/// the cubin includes the `gpu_log_tick` symbol and that the host can
+/// drive the GPU log ring's step-counter advance path end-to-end.
+///
+/// The full kernel-writes → drainer-reads loop is covered by the
+/// `perception_overfit` smoke once a kernel wires `log_record()` into
+/// the ring (Task 7).
+#[test]
+#[ignore = "requires CUDA — run via `cargo test --features cuda-diag-log -- --ignored`"]
+fn kernel_writes_visible_to_host_smoke() {
+    use cudarc::driver::{LaunchConfig, PushKernelArg};
+
+    let dev = test_device();
+    let stream = dev.cuda_stream().expect("stream");
+    let ctx = dev.cuda_context().expect("ctx");
+
+    let cubin = include_bytes!(concat!(env!("OUT_DIR"), "/gpu_log_ring.cubin"));
+    let module = ctx.load_cubin(cubin.to_vec()).expect("load cubin");
+    let tick_fn = module.load_function("gpu_log_tick").expect("load gpu_log_tick");
+
+    // Allocate the step counter, fire tick, confirm device increments to 1.
+    let mut step_counter = stream.alloc_zeros::<i32>(1).expect("alloc counter");
+    let cfg = LaunchConfig {
+        grid_dim: (1, 1, 1),
+        block_dim: (1, 1, 1),
+        shared_mem_bytes: 0,
+    };
+    let mut launch = stream.launch_builder(&tick_fn);
+    launch.arg(&mut step_counter);
+    unsafe {
+        launch.launch(cfg).expect("launch tick");
+    }
+    stream.synchronize().expect("sync");
+
+    let host: Vec<i32> = stream.clone_dtoh(&step_counter).expect("dtoh");
+    assert_eq!(host[0], 1, "after one tick, step_counter should be 1");
+
+    // Fire again — should increment to 2.
+    let mut launch = stream.launch_builder(&tick_fn);
+    launch.arg(&mut step_counter);
+    unsafe {
+        launch.launch(cfg).expect("launch tick 2");
+    }
+    stream.synchronize().expect("sync");
+    let host: Vec<i32> = stream.clone_dtoh(&step_counter).expect("dtoh 2");
+    assert_eq!(host[0], 2, "after two ticks, step_counter should be 2");
+}