diff --git a/tests/run.sh b/tests/run.sh
index ba59142..7b40609 100755
--- a/tests/run.sh
+++ b/tests/run.sh
@@ -1338,6 +1338,30 @@ fi
 # ------------------------------------------------------------ converter ----
 head_ "converter"
 
+# The fp8 converter path has two readers and a silent shape-preserving failure
+# mode, so keep its tile mapping under a small synthetic test instead of
+# relying on a multi-hour model conversion. Exit 77 is an explicit skip when
+# torch is unavailable, never a pass.
+#
+# Through uv, like every other torch checker here: torch is not a dependency
+# of this repo and is not a system package on the machines that run this. As
+# bare `python3` the whole check reported "torch not installed" and skipped
+# everywhere, CI included — where the Linux job is the one that installs uv,
+# so this is exactly where it does get to run.
+if ! command -v uv >/dev/null 2>&1; then
+    # The guard is on uv rather than python3 for the same reason: without uv
+    # run_uv exits 127 and the catch-all below would call that a failure.
+    sk "fp8 block-scale mapping" "uv not installed"
+else
+    out=$(run_uv run --quiet --with torch --no-project \
+              python tests/test_fp8_blocks.py 2>&1); rc=$?
+    case "$rc" in
+        0)  ok "fp8 block scales, partial tiles, missing companions, and reader agreement" ;;
+        77) sk "fp8 block-scale mapping" "torch not installed" ;;
+        *)  no "fp8 block-scale mapping"; printf '%s\n' "$out" | grep -E "FAIL|Error|Traceback" | head -5 ;;
+    esac
+fi
+
 # Resume is the one converter behaviour that cannot be checked by looking at
 # a finished container: it is about the partial states a crash leaves. The
 # quantizer is stubbed out, so this needs neither torch nor source weights.
diff --git a/tests/test_fp8_blocks.py b/tests/test_fp8_blocks.py
new file mode 100644
index 0000000..dbba939
--- /dev/null
+++ b/tests/test_fp8_blocks.py
@@ -0,0 +1,152 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
+# Copyright 2026 SQLite Cloud, Inc.
+"""Regression checks for fp8 block-scale dequantization.
+
+These tests use small synthetic tensors so they do not need model weights. A
+missing torch installation is an explicit skip, matching tests/run.sh's rule
+that unavailable prerequisites must never look like a pass.
+
+The one case nothing here can catch, stated because it is the reason the tile
+size is read from the checkpoint's config rather than inferred from the two
+shapes: a *compatible but wrong* block size. 300 rows against 3 scale rows
+admits both 128 (the truth, with a partial last tile) and 100 (a clean split).
+Both satisfy the shape check below, both produce a tensor of the right size,
+and the wrong one applies every scale to the wrong rows. No assertion over
+shapes can separate them, which is why `unblock_scale` takes `block` as an
+argument instead of deriving it — the check that matters happened before this
+file was reached.
+"""
+
+import json
+import os
+import struct
+import sys
+import tempfile
+
+try:
+    import torch
+except ImportError:
+    print("SKIP: torch is not installed")
+    raise SystemExit(77)
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "tools"))
+from convert import ShardReader
+from mxfp4 import ST, unblock_scale
+
+
+def expected_dequant(q, scale, block):
+    """Reference the tile lookup with explicit row and column indices."""
+    bm, bn = block
+    rows = torch.arange(q.shape[0]) // bm
+    cols = torch.arange(q.shape[1]) // bn
+    return q.float() * scale[rows[:, None], cols[None, :]]
+
+
+def write_safetensors_model(root, block, include_scale=True):
+    """Write the smallest safetensors model both readers can consume."""
+    q = torch.tensor([[1.0, -2.0, 3.0, -4.0, 0.5],
+                      [-1.0, 2.0, -3.0, 4.0, -0.5]], dtype=torch.float8_e4m3fn)
+    scale = torch.tensor([[1.0, 2.0]], dtype=torch.float32)
+    tensors = {"weight": ("F8_E4M3", list(q.shape),
+                           bytes(q.contiguous().view(torch.uint8).flatten().tolist()))}
+    if include_scale:
+        tensors["weight_scale_inv"] = ("F32", list(scale.shape),
+                                         bytes(scale.contiguous().view(torch.uint8).flatten().tolist()))
+
+    header = {}
+    payload = bytearray()
+    for name, (dtype, shape, raw) in tensors.items():
+        start = len(payload)
+        payload.extend(raw)
+        header[name] = {"dtype": dtype, "shape": shape,
+                        "data_offsets": [start, len(payload)]}
+    header_bytes = json.dumps(header, separators=(",", ":")).encode()
+    while (8 + len(header_bytes)) % 8:
+        header_bytes += b" "
+    with open(os.path.join(root, "shard.safetensors"), "wb") as f:
+        f.write(struct.pack("<Q", len(header_bytes)))
+        f.write(header_bytes)
+        f.write(payload)
+
+    with open(os.path.join(root, "model.safetensors.index.json"), "w") as f:
+        json.dump({"weight_map": {name: "shard.safetensors" for name in tensors}}, f)
+    with open(os.path.join(root, "config.json"), "w") as f:
+        json.dump({"quantization_config": {"weight_block_size": list(block)}}, f)
+    return q.float(), scale
+
+
+def test_aligned_tiles_apply_each_scale():
+    q = torch.arange(16, dtype=torch.float32).reshape(4, 4)
+    scale = torch.tensor([[2.0, 3.0], [5.0, 7.0]])
+    result = unblock_scale(q, scale, (2, 2))
+    assert torch.equal(result, expected_dequant(q, scale, (2, 2)))
+
+
+def test_partial_last_row_and_column_are_cropped_after_mapping():
+    q = torch.arange(15, dtype=torch.float32).reshape(3, 5)
+    scale = torch.tensor([[2.0, 3.0], [5.0, 7.0]])
+    result = unblock_scale(q, scale, (2, 3))
+    assert torch.equal(result, expected_dequant(q, scale, (2, 3)))
+
+
+def test_missing_scale_companion_is_rejected():
+    """Both readers refuse, not just the one that is easy to remember.
+
+    ST and ShardReader each carry their own copy of this guard, and covering
+    only ST left the convert.py one free to return the tensor unscaled: a
+    mutation that deleted its `raise` kept the whole suite green. That is the
+    silent-wrong-answer this file exists to prevent, in the reader #26's own
+    description called out as "a second reader and was easy to miss".
+    """
+    for label, read in (("mxfp4.ST", lambda r: ST(r).tensor("weight")),
+                        ("convert.ShardReader", lambda r: ShardReader(r).get("weight"))):
+        with tempfile.TemporaryDirectory() as root:
+            write_safetensors_model(root, (2, 3), include_scale=False)
+            try:
+                read(root)
+            except KeyError as exc:
+                assert "weight_scale_inv" in str(exc), f"{label}: {exc}"
+            else:
+                raise AssertionError(
+                    f"{label} accepted an fp8 tensor with no scale companion")
+
+
+def test_gross_scale_shape_mismatch_is_rejected():
+    q = torch.zeros(3, 5)
+    scale = torch.ones(1, 1)
+    try:
+        unblock_scale(q, scale, (2, 3))
+    except ValueError as exc:
+        assert "does not match" in str(exc)
+    else:
+        raise AssertionError("a gross fp8 scale shape mismatch was accepted")
+
+
+def test_safetensor_readers_agree_on_configured_block_size():
+    with tempfile.TemporaryDirectory() as root:
+        expected_q, expected_scale = write_safetensors_model(root, (2, 3))
+        from_st = ST(root).tensor("weight")
+        from_converter = ShardReader(root).get("weight")
+        expected = expected_dequant(expected_q, expected_scale, (2, 3))
+        assert torch.equal(from_st, expected)
+        assert torch.equal(from_converter, expected)
+        assert torch.equal(from_st, from_converter)
+
+
+def main():
+    tests = [
+        test_aligned_tiles_apply_each_scale,
+        test_partial_last_row_and_column_are_cropped_after_mapping,
+        test_missing_scale_companion_is_rejected,
+        test_gross_scale_shape_mismatch_is_rejected,
+        test_safetensor_readers_agree_on_configured_block_size,
+    ]
+    for test in tests:
+        test()
+        print(f"PASS: {test.__name__}")
+    print(f"PASS: {len(tests)} fp8 block-scale checks")
+
+
+if __name__ == "__main__":
+    main()