model: per-block activation recompute (--recompute)

Wrap each transformer block's forward in the checkpoint primitive when
recompute is enabled (Phase T13 / KI-3). To make the block forward a pure
segment fn (no `&self` borrow, so it can re-run in the backward closure),
extract the block body + its helpers (linear / norm_gamma / attention /
swiglu_mlp) into free functions parameterised by (cfg, compute_dtype) and add
`Block::block_params()` (the 11 leaves in the params() per-block order). The
non-recompute path calls `block_forward` directly — identical graph to before.

- `TinyTransformer::with_recompute(bool)` builder (opt-in; default off keeps the
  unchanged tape / bit-identical numerics).
- `--recompute` flag wired into bin/train and bin/train_ddp (DDP: each rank
  checkpoints independently).

Correctness gate: tests/recompute.rs builds two identical models (recompute
on/off), runs the same batched loss+backward, and asserts the forward logits,
the loss, and EVERY parameter grad match within tight fp tol — parameterised
over fp32 and bf16 (T12 composition).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-distributed/src/bin/train_ddp.rs

@@ -85,6 +85,10 @@ fn main() {
     // bf16 mixed precision (Phase T12): fp32 master weights, bf16 linears +
     // activations. Opt-in; default fp32 reproduces v0–v4 numerics.
     let bf16 = args.iter().any(|a| a == "--bf16");
+    // Activation recomputation (Phase T13): per-block gradient checkpointing — each
+    // rank checkpoints its own forward/backward; exact grads, lower peak activation
+    // memory (lets dim1024 batch32 fit). Opt-in; default off.
+    let recompute = args.iter().any(|a| a == "--recompute");
     let ckpt: Option<PathBuf> = args
         .iter()
         .position(|a| a == "--ckpt")
@@ -167,18 +171,23 @@ fn main() {
     if bf16 {
         println!("bf16 mixed precision: ON (fp32 master weights)");
     }
+    if recompute {
+        println!("activation recompute: ON (per-block gradient checkpointing)");
+    }
     let results = launch(
         &devices,
         &train_corpus,
         valid.as_ref(),
         &dcfg,
         move |device| {
-            let m = build_model(cfg, device);
+            let mut m = build_model(cfg, device);
             if bf16 {
-                m.with_compute_dtype(xtrain_tensor::DType::BF16)
+                m = m.with_compute_dtype(xtrain_tensor::DType::BF16);
-            } else {
-                m
             }
+            if recompute {
+                m = m.with_recompute(true);
+            }
+            m
         },
     );
     let r0 = &results[0];

crates/xtrain-model/src/model.rs

@@ -37,6 +37,16 @@ pub struct TinyTransformer {
     /// `docs/11-bf16-mixed-precision.md`). The cast op's backward upcasts the bf16
     /// weight grad back to fp32, so AdamW/clip/DDP stay fp32 and unchanged.
     compute_dtype: DType,
+    /// Activation recomputation / gradient checkpointing (Phase T13, KI-3). When
+    /// `true`, each transformer block's forward runs through
+    /// [`xtrain_autodiff::checkpoint`]: the block's internal activations are NOT
+    /// kept on the tape during forward (only the block input is), and the block
+    /// forward is re-run during backward to recover them. Trades ~one extra forward
+    /// per block for a large drop in peak activation memory → lets dim1024 batch32
+    /// fit. Default `false` = the unchanged path (every activation stored), so
+    /// existing numerics are bit-identical; recompute is mathematically exact, so
+    /// grads match the non-checkpointed path within fp tolerance.
+    recompute: bool,
 }
 
 impl TinyTransformer {
@@ -79,6 +89,7 @@ impl TinyTransformer {
             final_norm,
             lm_head,
             compute_dtype: DType::F32,
+            recompute: false,
         }
     }
 
@@ -103,16 +114,17 @@ impl TinyTransformer {
         self.compute_dtype
     }
 
-    /// Project `x` (activation, in the compute dtype) by weight `w` (an fp32
+    /// Enable per-block activation recomputation / gradient checkpointing (Phase
-    /// master leaf). In bf16 mode the weight is cast to bf16 via the autograd
+    /// T13). Builder-style and opt-in; default off keeps the unchanged tape (every
-    /// `cast` op (whose backward upcasts the grad to fp32); in fp32 mode this is
+    /// activation stored). On, each block's forward is wrapped in
-    /// just `matmul(x, w)`. The activation `x` already carries `compute_dtype`.
+    /// [`xtrain_autodiff::checkpoint`] — exact grads, lower peak activation memory.
-    fn linear(&self, x: &Var, w: &Var) -> Var {
+    pub fn with_recompute(mut self, recompute: bool) -> Self {
-        match self.compute_dtype {
+        self.recompute = recompute;
-            DType::F32 => ops::matmul(x, w),
+        self
-            DType::BF16 => ops::matmul(x, &ops::cast(w, DType::BF16)),
+    }
-            _ => unreachable!(),
+
-        }
+    pub fn recompute(&self) -> bool {
+        self.recompute
     }
 
     /// All learnable parameters, in a stable order. The optimizer (a hand-written
@@ -171,32 +183,36 @@ impl TinyTransformer {
             h = ops::cast(&h, DType::BF16);
         }
         for b in &self.blocks {
-            // --- Attention sub-block (pre-norm + residual) ---
+            h = if self.recompute {
-            let normed = ops::rms_norm(&h, &self.norm_gamma(&b.attn_norm), self.cfg.eps);
+                // Activation recomputation (T13): run the whole block forward inside
-            let attn = self.attention(b, &normed, batch, seq);
+                // `checkpoint` so its internal activations aren't kept on the tape;
-            h = ops::add(&h, &attn);
+                // the block forward is re-run in backward to recover the grads. The
-
+                // segment fn captures only `Copy` config (no borrow of `self`) and
-            // --- MLP sub-block (pre-norm + residual) ---
+                // receives the block's params via the slice, in `block_params` order.
-            let normed = ops::rms_norm(&h, &self.norm_gamma(&b.ffn_norm), self.cfg.eps);
+                let (cfg, cdt) = (self.cfg, self.compute_dtype);
-            let mlp = self.swiglu_mlp(b, &normed);
+                let seg = move |x: &Var, p: &[Var]| block_forward(cfg, cdt, batch, seq, x, p);
-            h = ops::add(&h, &mlp);
+                xtrain_autodiff::checkpoint::checkpoint(seg, &h, &b.block_params())
+            } else {
+                block_forward(
+                    self.cfg,
+                    self.compute_dtype,
+                    batch,
+                    seq,
+                    &h,
+                    &b.block_params(),
+                )
+            };
         }
 
-        let h = ops::rms_norm(&h, &self.norm_gamma(&self.final_norm), self.cfg.eps);
+        let h = ops::rms_norm(
+            &h,
+            &norm_gamma(self.compute_dtype, &self.final_norm),
+            self.cfg.eps,
+        );
         // lm_head matmul in compute dtype. Logits stay bf16 in bf16 mode — the
         // cross_entropy op upcasts to fp32 internally (no persistent fp32 logits
         // buffer, a real saving at vocab 50257), and its backward casts dx back.
-        self.linear(&h, &self.lm_head) // [batch*seq, vocab]
+        linear(self.compute_dtype, &h, &self.lm_head) // [batch*seq, vocab]
-    }
-
-    /// A norm/QK-norm gamma in the compute dtype. fp32 master leaf → bf16 (cast
-    /// op, grad upcast) in bf16 mode; identity in fp32 mode.
-    fn norm_gamma(&self, gamma: &Var) -> Var {
-        match self.compute_dtype {
-            DType::F32 => gamma.clone(),
-            DType::BF16 => ops::cast(gamma, DType::BF16),
-            _ => unreachable!(),
-        }
     }
 
     /// Cross-entropy mean loss of `forward(ids)` against `targets` (`[seq]` I32).
@@ -213,68 +229,146 @@ impl TinyTransformer {
         let logits = self.forward_batched(ids, batch);
         ops::cross_entropy(&logits, targets)
     }
+}
 
-    /// Multi-head causal self-attention over a flattened batch. `x`:[batch*seq,dim]
+impl Block {
-    /// (already normed), laid out sequence-major. The Q/K/V/O projections are big
+    /// The block's learnable leaves, in the fixed order the segment forward
-    /// `[batch*seq, dim]` GEMMs; the scaled-dot-product attention itself runs as a
+    /// (`block_forward`) indexes them — matches the per-block slice in
-    /// fused BATCHED op over the `batch·n_heads` (sequence,head) blocks — each
+    /// [`TinyTransformer::params`]. This is the param order `checkpoint` passes to
-    /// attends within its own `[seq,seq]` causal window (NO cross-sequence
+    /// the recompute closure.
-    /// attention), with RoPE positions reset per sequence (`period = seq`). Causal
+    fn block_params(&self) -> Vec<Var> {
-    /// masking is applied inside the fused op's softmax kernel (no additive
+        vec![
-    /// `[seq,seq]` mask tensor).
+            self.attn_norm.clone(),
-    fn attention(&self, b: &Block, x: &Var, batch: usize, seq: usize) -> Var {
+            self.wq.clone(),
-        let (nh, hd) = (self.cfg.n_heads, self.cfg.head_dim);
+            self.wk.clone(),
-        let total = batch * seq;
+            self.wv.clone(),
-        let bh = batch * nh;
+            self.q_norm.clone(),
-        let scale = 1.0 / (hd as f32).sqrt();
+            self.k_norm.clone(),
+            self.wo.clone(),
+            self.ffn_norm.clone(),
+            self.w_gate.clone(),
+            self.w_up.clone(),
+            self.w_down.clone(),
+        ]
+    }
+}
 
-        // Project, qk-norm + RoPE, then lay out as a batched [B*nh, seq, hd] tensor.
+/// Project `x` (activation, in the compute dtype) by weight `w` (an fp32 master
-        //   [B*S,dim] @ [dim,dim] = [B*S,dim]
+/// leaf). In bf16 mode the weight is cast to bf16 via the autograd `cast` op (whose
-        //   reshape   [B*S, nh, hd]
+/// backward upcasts the grad to fp32); in fp32 mode this is just `matmul(x, w)`.
-        //   qk-norm   per-head RMSNorm over hd (Qwen3-style; Q/K only, before RoPE)
+fn linear(cdt: DType, x: &Var, w: &Var) -> Var {
-        //   rope      [B*S, nh, hd] with per-sequence position (period = seq)
+    match cdt {
-        //   reshape   [B, S, nh, hd] → transpose(1,2) → [B, nh, S, hd] → [B*nh, S, hd]
+        DType::F32 => ops::matmul(x, w),
-        let to_bh = |proj: Var, norm: Option<&Var>| -> Var {
+        DType::BF16 => ops::matmul(x, &ops::cast(w, DType::BF16)),
-            let r = ops::reshape(&proj, &[total, nh, hd]);
+        _ => unreachable!(),
-            let r = match norm {
+    }
-                // Per-head RMSNorm: flatten the (B*S,nh) head rows, norm over hd,
+}
-                // restore. RoPE follows on the normed Q/K (mirrors xserv qwen3.rs).
+
-                Some(gamma) => {
+/// A norm/QK-norm gamma in the compute dtype. fp32 master leaf → bf16 (cast op,
-                    let flat = ops::reshape(&r, &[total * nh, hd]);
+/// grad upcast) in bf16 mode; identity in fp32 mode.
-                    let normed = ops::rms_norm(&flat, &self.norm_gamma(gamma), self.cfg.eps);
+fn norm_gamma(cdt: DType, gamma: &Var) -> Var {
-                    let r = ops::reshape(&normed, &[total, nh, hd]);
+    match cdt {
-                    ops::rope(&r, self.cfg.rope_theta, seq)
+        DType::F32 => gamma.clone(),
-                }
+        DType::BF16 => ops::cast(gamma, DType::BF16),
-                None => r,
+        _ => unreachable!(),
-            };
+    }
-            let r = ops::reshape(&r, &[batch, seq, nh, hd]);
+}
-            let t = ops::transpose_4d12(&r); // [B, nh, S, hd]
+
-            ops::reshape(&t, &[bh, seq, hd]) // [B*nh, S, hd]
+/// One transformer block's forward: pre-norm + multi-head causal attention +
+/// residual, then pre-norm + SwiGLU MLP + residual. Pure in `(cfg, cdt, batch,
+/// seq, input, params)` (no `&self`) so it can be the segment fn of
+/// [`xtrain_autodiff::checkpoint`] for activation recomputation (T13). `params` is
+/// the block's leaves in [`Block::block_params`] order.
+fn block_forward(cfg: Config, cdt: DType, batch: usize, seq: usize, h: &Var, p: &[Var]) -> Var {
+    let (attn_norm, wq, wk, wv) = (&p[0], &p[1], &p[2], &p[3]);
+    let (q_norm, k_norm, wo) = (&p[4], &p[5], &p[6]);
+    let (ffn_norm, w_gate, w_up, w_down) = (&p[7], &p[8], &p[9], &p[10]);
+
+    // --- Attention sub-block (pre-norm + residual) ---
+    let normed = ops::rms_norm(h, &norm_gamma(cdt, attn_norm), cfg.eps);
+    let attn = attention(
+        cfg, cdt, batch, seq, &normed, wq, wk, wv, q_norm, k_norm, wo,
+    );
+    let h = ops::add(h, &attn);
+
+    // --- MLP sub-block (pre-norm + residual) ---
+    let normed = ops::rms_norm(&h, &norm_gamma(cdt, ffn_norm), cfg.eps);
+    let mlp = swiglu_mlp(cdt, &normed, w_gate, w_up, w_down);
+    ops::add(&h, &mlp)
+}
+
+/// Multi-head causal self-attention over a flattened batch. `x`:[batch*seq,dim]
+/// (already normed), laid out sequence-major. The Q/K/V/O projections are big
+/// `[batch*seq, dim]` GEMMs; the scaled-dot-product attention itself runs as a
+/// fused BATCHED op over the `batch·n_heads` (sequence,head) blocks — each attends
+/// within its own `[seq,seq]` causal window (NO cross-sequence attention), with
+/// RoPE positions reset per sequence (`period = seq`). Causal masking is applied
+/// inside the fused op's softmax kernel (no additive `[seq,seq]` mask tensor).
+#[allow(clippy::too_many_arguments)]
+fn attention(
+    cfg: Config,
+    cdt: DType,
+    batch: usize,
+    seq: usize,
+    x: &Var,
+    wq: &Var,
+    wk: &Var,
+    wv: &Var,
+    q_norm: &Var,
+    k_norm: &Var,
+    wo: &Var,
+) -> Var {
+    let (nh, hd) = (cfg.n_heads, cfg.head_dim);
+    let total = batch * seq;
+    let bh = batch * nh;
+    let scale = 1.0 / (hd as f32).sqrt();
+
+    // Project, qk-norm + RoPE, then lay out as a batched [B*nh, seq, hd] tensor.
+    //   [B*S,dim] @ [dim,dim] = [B*S,dim]
+    //   reshape   [B*S, nh, hd]
+    //   qk-norm   per-head RMSNorm over hd (Qwen3-style; Q/K only, before RoPE)
+    //   rope      [B*S, nh, hd] with per-sequence position (period = seq)
+    //   reshape   [B, S, nh, hd] → transpose(1,2) → [B, nh, S, hd] → [B*nh, S, hd]
+    let to_bh = |proj: Var, norm: Option<&Var>| -> Var {
+        let r = ops::reshape(&proj, &[total, nh, hd]);
+        let r = match norm {
+            // Per-head RMSNorm: flatten the (B*S,nh) head rows, norm over hd,
+            // restore. RoPE follows on the normed Q/K (mirrors xserv qwen3.rs).
+            Some(gamma) => {
+                let flat = ops::reshape(&r, &[total * nh, hd]);
+                let normed = ops::rms_norm(&flat, &norm_gamma(cdt, gamma), cfg.eps);
+                let r = ops::reshape(&normed, &[total, nh, hd]);
+                ops::rope(&r, cfg.rope_theta, seq)
+            }
+            None => r,
         };
+        let r = ops::reshape(&r, &[batch, seq, nh, hd]);
+        let t = ops::transpose_4d12(&r); // [B, nh, S, hd]
+        ops::reshape(&t, &[bh, seq, hd]) // [B*nh, S, hd]
+    };
 
-        let q = to_bh(self.linear(x, &b.wq), Some(&b.q_norm));
+    let q = to_bh(linear(cdt, x, wq), Some(q_norm));
-        let k = to_bh(self.linear(x, &b.wk), Some(&b.k_norm));
+    let k = to_bh(linear(cdt, x, wk), Some(k_norm));
-        let v = to_bh(self.linear(x, &b.wv), None);
+    let v = to_bh(linear(cdt, x, wv), None);
 
-        // Fused batched causal SDPA over all B*nh (sequence,head) blocks at once
+    // Fused batched causal SDPA over all B*nh (sequence,head) blocks at once
-        // (2 batched GEMMs + 1 causal-softmax kernel; no per-head/per-seq loop).
+    // (2 batched GEMMs + 1 causal-softmax kernel; no per-head/per-seq loop).
-        let out = ops::attention(&q, &k, &v, scale); // [B*nh, S, hd]
+    let out = ops::attention(&q, &k, &v, scale); // [B*nh, S, hd]
 
-        // Back to [B*S, dim]: [B*nh,S,hd] → [B,nh,S,hd] → transpose(1,2) →
+    // Back to [B*S, dim]: [B*nh,S,hd] → [B,nh,S,hd] → transpose(1,2) →
-        // [B,S,nh,hd] → [B*S, dim].
+    // [B,S,nh,hd] → [B*S, dim].
-        let out = ops::reshape(&out, &[batch, nh, seq, hd]);
+    let out = ops::reshape(&out, &[batch, nh, seq, hd]);
-        let out = ops::transpose_4d12(&out); // [B, S, nh, hd]
+    let out = ops::transpose_4d12(&out); // [B, S, nh, hd]
-        let concat = ops::reshape(&out, &[total, nh * hd]); // [B*S, dim]
+    let concat = ops::reshape(&out, &[total, nh * hd]); // [B*S, dim]
-        self.linear(&concat, &b.wo) // out projection
+    linear(cdt, &concat, wo) // out projection
-    }
+}
 
-    /// SwiGLU MLP: `down( silu(gate(x)) ∘ up(x) )`. `x`:[batch*seq,dim].
+/// SwiGLU MLP: `down( silu(gate(x)) ∘ up(x) )`. `x`:[batch*seq,dim].
-    fn swiglu_mlp(&self, b: &Block, x: &Var) -> Var {
+fn swiglu_mlp(cdt: DType, x: &Var, w_gate: &Var, w_up: &Var, w_down: &Var) -> Var {
-        let gate = self.linear(x, &b.w_gate); // [seq, ffn_hidden]
+    let gate = linear(cdt, x, w_gate); // [seq, ffn_hidden]
-        let up = self.linear(x, &b.w_up); // [seq, ffn_hidden]
+    let up = linear(cdt, x, w_up); // [seq, ffn_hidden]
-        let act = ops::swiglu(&gate, &up); // silu(gate) ∘ up
+    let act = ops::swiglu(&gate, &up); // silu(gate) ∘ up
-        self.linear(&act, &b.w_down) // [seq, dim]
+    linear(cdt, &act, w_down) // [seq, dim]
-    }
 }
 
 /// Materialise a parameter's value back to a host `Vec<f32>` (for the GD step

crates/xtrain-model/tests/recompute.rs

@@ -0,0 +1,156 @@
+// T13 activation-recomputation correctness gate (the HARD gate).
+//
+// Gradient checkpointing is mathematically EXACT: the backward re-runs the same
+// `segment_fn` from the same saved input and the same (unchanged) parameter
+// values, so the recomputed activations equal the originals and the recovered
+// grads equal the non-checkpointed grads — checkpointing trades compute for
+// memory, never correctness. This test makes that a closed loop on-GPU:
+//
+//   build two identical models (same init), one with `--recompute` on, one off,
+//   run the SAME batched loss + backward on both, and assert
+//     1. the forward logits match (recompute doesn't touch forward output)
+//     2. the loss matches
+//     3. EVERY parameter's grad matches within a tight fp tolerance.
+//
+// Composition is covered by parameterising over fp32 AND bf16 (T12): the
+// recompute path is the unchanged block forward, so it runs the same dtype path.
+#![cfg(not(no_cuda))]
+
+use xtrain_cuda::device;
+use xtrain_model::{Config, TinyTransformer, batched_ids_tensor};
+use xtrain_tensor::{DType, Device};
+
+fn fill(n: usize, seed: u64, scale: f32) -> Vec<f32> {
+    let mut state = seed
+        .wrapping_mul(2862933555777941757)
+        .wrapping_add(3037000493);
+    (0..n)
+        .map(|_| {
+            state = state
+                .wrapping_mul(6364136223846793005)
+                .wrapping_add(1442695040888963407);
+            (((state >> 33) as f32 / (1u64 << 31) as f32) - 0.5) * 2.0 * scale
+        })
+        .collect()
+}
+
+fn build(cfg: Config, device: Device, dtype: DType, recompute: bool) -> TinyTransformer {
+    let mut seed = 1u64;
+    let m = TinyTransformer::new(cfg, device, |shape| {
+        seed = seed.wrapping_add(1);
+        let n: usize = shape.iter().product();
+        if shape.len() == 1 {
+            fill(n, seed, 0.02).iter().map(|v| v + 1.0).collect()
+        } else {
+            fill(n, seed, 0.08)
+        }
+    });
+    m.with_compute_dtype(dtype).with_recompute(recompute)
+}
+
+fn host(t: &xtrain_tensor::Tensor) -> Vec<f32> {
+    t.to_device(Device::Cpu).as_slice::<f32>().to_vec()
+}
+
+fn run(dtype: DType, logit_tol: f32, grad_tol: f32) {
+    assert!(device::device_count().unwrap() > 0, "no CUDA device");
+    device::set_device(0).unwrap();
+    let device = Device::Cuda(0);
+
+    // A few layers so checkpointing actually wraps multiple blocks.
+    let mut cfg = Config::tiny();
+    cfg.vocab = 16;
+    cfg.n_layers = 4;
+    let batch = 3usize;
+    let seq = 6usize;
+    let seqs: Vec<Vec<i32>> = (0..batch)
+        .map(|b| {
+            (0..seq)
+                .map(|i| ((b * 7 + i * 3 + 1) % cfg.vocab) as i32)
+                .collect()
+        })
+        .collect();
+    let tgts: Vec<Vec<i32>> = (0..batch)
+        .map(|b| {
+            (0..seq)
+                .map(|i| ((b * 5 + i * 2 + 2) % cfg.vocab) as i32)
+                .collect()
+        })
+        .collect();
+    let ids = batched_ids_tensor(&seqs, device);
+    let tgt = batched_ids_tensor(&tgts, device);
+
+    // --- recompute OFF (reference) ---
+    let off = build(cfg, device, dtype, false);
+    let off_logits = host(&off.forward_batched(&ids, batch).value());
+    let off_loss = off.loss_batched(&ids, &tgt, batch);
+    let off_loss_val = host(&off_loss.value())[0];
+    off_loss.backward();
+    let off_grads: Vec<Vec<f32>> = off
+        .params()
+        .iter()
+        .map(|p| host(&p.grad().expect("off grad")))
+        .collect();
+
+    // --- recompute ON ---
+    let on = build(cfg, device, dtype, true);
+    let on_logits = host(&on.forward_batched(&ids, batch).value());
+    let on_loss = on.loss_batched(&ids, &tgt, batch);
+    let on_loss_val = host(&on_loss.value())[0];
+    on_loss.backward();
+    let on_grads: Vec<Vec<f32>> = on
+        .params()
+        .iter()
+        .map(|p| host(&p.grad().expect("on grad")))
+        .collect();
+
+    // 1. Forward logits — recompute must not change the forward output.
+    let logit_rel = off_logits
+        .iter()
+        .zip(&on_logits)
+        .map(|(a, b)| (a - b).abs() / a.abs().max(1e-4))
+        .fold(0.0f32, f32::max);
+    // 2. Loss.
+    let loss_rel = (off_loss_val - on_loss_val).abs() / off_loss_val.abs().max(1e-4);
+    println!(
+        "[{dtype:?}] recompute on/off: loss {off_loss_val:.6}/{on_loss_val:.6} (rel {loss_rel:.2e}), \
+         logits max rel {logit_rel:.2e}"
+    );
+    assert!(
+        logit_rel < logit_tol,
+        "[{dtype:?}] logits diverged: {logit_rel:.2e}"
+    );
+    assert!(
+        loss_rel < logit_tol,
+        "[{dtype:?}] loss diverged: {loss_rel:.2e}"
+    );
+
+    // 3. Every parameter grad — the load-bearing gate.
+    let mut max_grad_rel = 0.0f32;
+    for (off_g, on_g) in off_grads.iter().zip(&on_grads) {
+        for (a, b) in off_g.iter().zip(on_g) {
+            let rel = (a - b).abs() / a.abs().max(1e-3);
+            max_grad_rel = max_grad_rel.max(rel);
+        }
+    }
+    println!("[{dtype:?}] recompute on/off: grad max rel err = {max_grad_rel:.3e}");
+    assert!(
+        max_grad_rel < grad_tol,
+        "[{dtype:?}] recompute grads diverged from non-recompute: {max_grad_rel:.3e}"
+    );
+}
+
+#[test]
+fn recompute_matches_non_recompute_fp32() {
+    // fp32: recompute runs the identical deterministic kernels → grads match to
+    // (near) bit-exact; allow a hair for any nondeterministic GPU reduction.
+    run(DType::F32, 1e-5, 1e-4);
+}
+
+#[test]
+fn recompute_matches_non_recompute_bf16() {
+    // bf16 (T12 composition): same bf16 path on recompute. The recompute is still
+    // exact w.r.t. the bf16 forward, so on/off match tightly (looser tol only for
+    // bf16 rounding, not for any recompute discrepancy).
+    run(DType::BF16, 5e-3, 5e-3);
+}

crates/xtrain-train/src/bin/train.rs

@@ -112,6 +112,10 @@ fn main() {
     // bf16 mixed precision (Phase T12): fp32 master weights, bf16 linears +
     // activations. Opt-in; default fp32 reproduces v0–v4 numerics.
     let bf16 = args.iter().any(|a| a == "--bf16");
+    // Activation recomputation (Phase T13): per-block gradient checkpointing —
+    // exact grads, lower peak activation memory (lets dim1024 batch32 fit). Opt-in;
+    // default off stores every activation (unchanged numerics).
+    let recompute = args.iter().any(|a| a == "--recompute");
     let ckpt: PathBuf = PathBuf::from(
         args.iter()
             .position(|a| a == "--ckpt")
@@ -175,6 +179,10 @@ fn main() {
         model = model.with_compute_dtype(DType::BF16);
         println!("bf16 mixed precision: ON (fp32 master weights)");
     }
+    if recompute {
+        model = model.with_recompute(true);
+        println!("activation recompute: ON (per-block gradient checkpointing)");
+    }
 
     // Eval-only mode: load a checkpoint and score it on the held-out val set, then
     // exit. Used to put an EXISTING model (e.g. v0) and a new one on the same