diff --git a/crates/ml/tests/fxcache_roundtrip_test.rs b/crates/ml/tests/fxcache_roundtrip_test.rs
new file mode 100644
index 000000000..0309ed4a7
--- /dev/null
+++ b/crates/ml/tests/fxcache_roundtrip_test.rs
@@ -0,0 +1,237 @@
+//! Roundtrip tests for the `.fxcache` binary format.
+//!
+//! Covers f64/bf16 serialization, cache-key lookup, header validation,
+//! and the empty-bars error path.
+
+use ml::fxcache::{find_fxcache, load_fxcache, write_fxcache, FxCacheData};
+use tempfile::TempDir;
+
+// ── Helpers ─────────────────────────────────────────────────────────────────
+
+/// Deterministic feature vector for bar `i`.
+fn make_features(n: usize) -> Vec<[f64; 42]> {
+    (0..n)
+        .map(|i| {
+            let mut row = [0.0_f64; 42];
+            for j in 0..42 {
+                row[j] = (i * 42 + j) as f64 * 0.01;
+            }
+            row
+        })
+        .collect()
+}
+
+/// Deterministic target vector for bar `i`.
+fn make_targets(n: usize) -> Vec<[f64; 4]> {
+    (0..n)
+        .map(|i| {
+            let mut row = [0.0_f64; 4];
+            for j in 0..4 {
+                row[j] = 5000.0 + (i * 4 + j) as f64 * 0.25;
+            }
+            row
+        })
+        .collect()
+}
+
+/// Deterministic OFI vector for bar `i`.
+fn make_ofi(n: usize) -> Vec<[f64; 8]> {
+    (0..n)
+        .map(|i| {
+            let mut row = [0.0_f64; 8];
+            for j in 0..8 {
+                row[j] = (i * 8 + j) as f64 * 0.001;
+            }
+            row
+        })
+        .collect()
+}
+
+/// A fixed 32-byte cache key for tests.
+fn test_cache_key() -> [u8; 32] {
+    let mut key = [0u8; 32];
+    for (i, slot) in key.iter_mut().enumerate() {
+        *slot = (i as u8).wrapping_mul(7).wrapping_add(13);
+    }
+    key
+}
+
+// ── Tests ───────────────────────────────────────────────────────────────────
+
+/// Write 100 bars as f64 (version 1), read back, verify bit-exact match.
+#[test]
+fn test_fxcache_f64_roundtrip() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("test_f64.fxcache");
+
+    let n = 100;
+    let features = make_features(n);
+    let targets = make_targets(n);
+    let ofi = make_ofi(n);
+    let key = test_cache_key();
+
+    let bytes_written =
+        write_fxcache(&path, &features, &targets, &ofi, key, false).unwrap();
+    assert!(bytes_written > 0, "expected nonzero bytes written");
+
+    let data: FxCacheData = load_fxcache(&path).unwrap();
+    assert_eq!(data.bar_count, n);
+    assert_eq!(data.cache_key, key);
+    assert_eq!(data.features.len(), n);
+    assert_eq!(data.targets.len(), n);
+    assert_eq!(data.ofi.len(), n);
+
+    // Bit-exact match for f64 roundtrip.
+    for i in 0..n {
+        assert_eq!(
+            data.features[i], features[i],
+            "feature mismatch at bar {i}"
+        );
+        assert_eq!(
+            data.targets[i], targets[i],
+            "target mismatch at bar {i}"
+        );
+        assert_eq!(data.ofi[i], ofi[i], "ofi mismatch at bar {i}");
+    }
+}
+
+/// Write 50 bars as bf16 (version 2), read back, verify within tolerance.
+/// bf16 has limited precision: ~3 significant digits.
+#[test]
+fn test_fxcache_bf16_roundtrip() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("test_bf16.fxcache");
+
+    let n = 50;
+    let features = make_features(n);
+    let targets = make_targets(n);
+    let ofi = make_ofi(n);
+    let key = test_cache_key();
+
+    let bytes_written =
+        write_fxcache(&path, &features, &targets, &ofi, key, true).unwrap();
+    assert!(bytes_written > 0, "expected nonzero bytes written");
+
+    let data: FxCacheData = load_fxcache(&path).unwrap();
+    assert_eq!(data.bar_count, n);
+    assert_eq!(data.cache_key, key);
+
+    // bf16 tolerance checks.
+    for i in 0..n {
+        for j in 0..42 {
+            let diff = (data.features[i][j] - features[i][j]).abs();
+            assert!(
+                diff < 0.02,
+                "feature[{i}][{j}]: expected {}, got {}, diff={diff}",
+                features[i][j],
+                data.features[i][j]
+            );
+        }
+        for j in 0..4 {
+            let diff = (data.targets[i][j] - targets[i][j]).abs();
+            // bf16 at ~5000 has resolution of ~4, so allow up to 4.0.
+            assert!(
+                diff <= 4.0,
+                "target[{i}][{j}]: expected {}, got {}, diff={diff}",
+                targets[i][j],
+                data.targets[i][j]
+            );
+        }
+        for j in 0..8 {
+            let diff = (data.ofi[i][j] - ofi[i][j]).abs();
+            assert!(
+                diff < 0.02,
+                "ofi[{i}][{j}]: expected {}, got {}, diff={diff}",
+                ofi[i][j],
+                data.ofi[i][j]
+            );
+        }
+    }
+}
+
+/// Write a file with a known cache key, verify `find_fxcache` locates it.
+/// Also verify that a different key returns `None`.
+#[test]
+fn test_fxcache_find() {
+    let dir = TempDir::new().unwrap();
+    let key = test_cache_key();
+    let hex_key = hex::encode(key);
+    let path = dir.path().join(format!("{hex_key}.fxcache"));
+
+    let features = make_features(5);
+    let targets = make_targets(5);
+    let ofi = make_ofi(5);
+
+    write_fxcache(&path, &features, &targets, &ofi, key, false).unwrap();
+
+    // Should find the file.
+    let found = find_fxcache(dir.path(), &key);
+    assert!(found.is_some(), "expected to find fxcache file");
+    assert_eq!(found.unwrap(), path);
+
+    // Wrong key should miss.
+    let mut wrong_key = [0xFFu8; 32];
+    wrong_key[0] = 0x00;
+    let miss = find_fxcache(dir.path(), &wrong_key);
+    assert!(miss.is_none(), "expected miss for wrong key");
+}
+
+/// Write garbage bytes to a file, verify `load_fxcache` returns an error
+/// whose message contains "magic".
+#[test]
+fn test_fxcache_header_validation() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("garbage.fxcache");
+
+    // Write 128 bytes of garbage (more than header size).
+    std::fs::write(&path, vec![0xABu8; 128]).unwrap();
+
+    let err = load_fxcache(&path).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.to_lowercase().contains("magic"),
+        "expected error about magic bytes, got: {msg}"
+    );
+}
+
+/// Attempting to write 0 bars should fail (writer rejects empty data).
+/// Attempting to read a hand-crafted header with bar_count=0 should also fail.
+#[test]
+fn test_fxcache_empty() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("empty.fxcache");
+
+    // Writer rejects 0 bars.
+    let features: Vec<[f64; 42]> = vec![];
+    let targets: Vec<[f64; 4]> = vec![];
+    let ofi: Vec<[f64; 8]> = vec![];
+    let key = test_cache_key();
+
+    let err = write_fxcache(&path, &features, &targets, &ofi, key, false).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("0 bars") || msg.contains("empty"),
+        "expected empty/0-bars error from writer, got: {msg}"
+    );
+
+    // Reader also rejects a header with bar_count=0.
+    // Hand-craft a valid-magic header with bar_count=0.
+    let mut header = [0u8; 64];
+    header[0..8].copy_from_slice(b"FXCACHE\0");
+    header[8..10].copy_from_slice(&1u16.to_le_bytes()); // version=1
+    header[10..12].copy_from_slice(&42u16.to_le_bytes()); // feat_dim
+    header[12..14].copy_from_slice(&4u16.to_le_bytes()); // target_dim
+    header[14..16].copy_from_slice(&8u16.to_le_bytes()); // ofi_dim
+    header[16..24].copy_from_slice(&0u64.to_le_bytes()); // bar_count=0
+    // cache_key + reserved stay zeroed.
+
+    let path_reader = dir.path().join("empty_header.fxcache");
+    std::fs::write(&path_reader, &header).unwrap();
+
+    let err = load_fxcache(&path_reader).unwrap_err();
+    let msg = format!("{err:#}");
+    assert!(
+        msg.contains("zero") || msg.contains("bar_count"),
+        "expected bar_count/zero error from reader, got: {msg}"
+    );
+}