diff --git a/crates/ml-backtesting/build.rs b/crates/ml-backtesting/build.rs
index 343f076d5..f827135f7 100644
--- a/crates/ml-backtesting/build.rs
+++ b/crates/ml-backtesting/build.rs
@@ -11,9 +11,21 @@ fn main() -> Result<(), String> {
 
     // Per pearl_build_rs_rerun_if_env_changed.md: pair every env::var
     // with rerun-if-env-changed.
-    // Default sm_86 covers RTX 3050 (local dev). Production Argo workflows
-    // override via FOXHUNT_CUDA_ARCH=sm_89 (L40S Ada) or sm_90 (H100).
-    let arch = std::env::var("FOXHUNT_CUDA_ARCH").unwrap_or_else(|_| "sm_86".into());
+    //
+    // Arch detection (mirrors crates/ml-alpha/build.rs:detect_arch — the
+    // original "mirror" docstring above was aspirational, B-9 cluster
+    // alpha-rl-mbg2n on H100 caught the gap: ml-alpha picked sm_90 via
+    // CUDA_COMPUTE_CAP but ml-backtesting fell through to default sm_86
+    // → no kernel image error at LobSimCuda::new). Priority:
+    //   1. CUDA_COMPUTE_CAP (numeric, e.g. "90") — set by alpha-rl-template
+    //      from nvidia-smi inside the compile pod
+    //   2. FOXHUNT_CUDA_ARCH (sm_-prefixed, e.g. "sm_90") — set by
+    //      lob-backtest-sweep-template
+    //   3. nvidia-smi --query-gpu=compute_cap at build time
+    //   4. Default sm_86 (RTX 3050 Ti local dev)
+    let arch = detect_arch();
+    eprintln!("  ml-backtesting: compiling kernels for {arch}");
+    println!("cargo:rerun-if-env-changed=CUDA_COMPUTE_CAP");
     println!("cargo:rerun-if-env-changed=FOXHUNT_CUDA_ARCH");
     println!("cargo:rerun-if-env-changed=CUDA_PATH");
     println!("cargo:rerun-if-env-changed=NVCC");
@@ -60,3 +72,38 @@ fn main() -> Result<(), String> {
     }
     Ok(())
 }
+
+/// Returns the `sm_<NN>` arch string, honoring (in order):
+/// `CUDA_COMPUTE_CAP` numeric env → `FOXHUNT_CUDA_ARCH` sm_-prefixed env
+/// → `nvidia-smi --query-gpu=compute_cap` → default `sm_86`.
+fn detect_arch() -> String {
+    // 1. Numeric CUDA_COMPUTE_CAP (e.g. "90") from alpha-rl-template.
+    if let Ok(cap) = std::env::var("CUDA_COMPUTE_CAP") {
+        let trimmed = cap.trim();
+        if !trimmed.is_empty() {
+            return format!("sm_{trimmed}");
+        }
+    }
+    // 2. sm_-prefixed FOXHUNT_CUDA_ARCH (e.g. "sm_90") from lob-backtest-sweep.
+    if let Ok(arch) = std::env::var("FOXHUNT_CUDA_ARCH") {
+        let trimmed = arch.trim();
+        if !trimmed.is_empty() {
+            return trimmed.to_string();
+        }
+    }
+    // 3. Query the GPU on this machine (e.g. "9.0" → "90" → "sm_90").
+    if let Ok(output) = Command::new("nvidia-smi")
+        .args(["--query-gpu=compute_cap", "--format=csv,noheader"])
+        .output()
+    {
+        if output.status.success() {
+            let s = String::from_utf8_lossy(&output.stdout);
+            let cap = s.trim().replace('.', "");
+            if !cap.is_empty() {
+                return format!("sm_{cap}");
+            }
+        }
+    }
+    // 4. Default — RTX 3050 Ti local dev.
+    "sm_86".to_string()
+}