diff --git a/src/tether/exporters/fp16_convert.py b/src/tether/exporters/fp16_convert.py
index 7c6bfe5..d389fcd 100644
--- a/src/tether/exporters/fp16_convert.py
+++ b/src/tether/exporters/fp16_convert.py
@@ -115,6 +115,35 @@ def parity_gate(
     }
 
 
+def _remove_stale_external_data(dst: Path) -> list[Path]:
+    """Delete external-data leftovers from a PRIOR conversion of *this* model.
+
+    ``onnx.save`` writes the FP16 weights to ``{dst.stem}.bin`` next to ``dst``.
+    A leftover ``.bin``/``.data`` from an earlier run at the same path would sit
+    alongside the new one and inflate our size accounting, so we clear it first.
+
+    Scope is strictly this model's stem. The previous implementation globbed
+    every ``*.bin`` / ``*.data`` in ``dst.parent`` and unlinked them — which
+    destroyed *other* models' weight files whenever a user converted into a
+    shared export directory (real data-loss bug). ``dst`` itself (the ``.onnx``)
+    is never touched.
+
+    Returns the list of paths actually removed (for logging / tests).
+    """
+    removed: list[Path] = []
+    for old in dst.parent.glob(f"{dst.stem}*"):
+        if old == dst:
+            continue
+        if old.suffix not in (".bin", ".data"):
+            continue
+        try:
+            old.unlink()
+            removed.append(old)
+        except OSError:
+            pass
+    return removed
+
+
 def convert_fp32_to_fp16(
     fp32_onnx_path: str | Path,
     fp16_onnx_path: str | Path,
@@ -205,15 +234,10 @@ def convert_fp32_to_fp16(
         )
 
     logger.info("[fp16] Saving %s...", dst)
-    # Remove any leftover external data from a prior run at this path —
-    # onnx.save would otherwise leave both the old and new .bin on disk
-    # and our size accounting would double-count.
-    for pat in ("*.bin", "*.data"):
-        for old in dst.parent.glob(pat):
-            try:
-                old.unlink()
-            except Exception:
-                pass
+    # Clear leftover external data from a prior conversion of THIS model only.
+    # (Scoped to dst.stem — see _remove_stale_external_data; a blanket
+    # *.bin/*.data sweep would delete other models' weights in a shared dir.)
+    _remove_stale_external_data(dst)
 
     onnx.save(
         model_fp16,
diff --git a/tests/test_fp16_convert_external_data.py b/tests/test_fp16_convert_external_data.py
new file mode 100644
index 0000000..0c3776f
--- /dev/null
+++ b/tests/test_fp16_convert_external_data.py
@@ -0,0 +1,61 @@
+"""Regression tests for fp16_convert external-data cleanup scoping.
+
+Guards the data-loss bug where the pre-save cleanup globbed every *.bin/*.data
+in the destination's parent directory and unlinked them — destroying OTHER
+models' weight files when a user converted into a shared export dir. The fix
+scopes deletion strictly to the model being converted (dst.stem).
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+from tether.exporters.fp16_convert import _remove_stale_external_data
+
+
+def _touch(p: Path) -> Path:
+    p.write_bytes(b"x")
+    return p
+
+
+def test_removes_this_models_stale_bin(tmp_path: Path) -> None:
+    dst = tmp_path / "model_fp16.onnx"
+    stale = _touch(tmp_path / "model_fp16.bin")
+    removed = _remove_stale_external_data(dst)
+    assert stale in removed
+    assert not stale.exists()
+
+
+def test_removes_legacy_data_variants_for_this_model(tmp_path: Path) -> None:
+    dst = tmp_path / "model_fp16.onnx"
+    legacy1 = _touch(tmp_path / "model_fp16.data")
+    legacy2 = _touch(tmp_path / "model_fp16.onnx.data")
+    removed = _remove_stale_external_data(dst)
+    assert legacy1 in removed and legacy2 in removed
+    assert not legacy1.exists() and not legacy2.exists()
+
+
+def test_preserves_other_models_weights(tmp_path: Path) -> None:
+    """The core data-loss guard: a sibling model's external data must survive."""
+    dst = tmp_path / "model_fp16.onnx"
+    _touch(tmp_path / "model_fp16.bin")  # ours — will be removed
+    other_bin = _touch(tmp_path / "other_model.bin")
+    other_data = _touch(tmp_path / "unrelated.onnx.data")
+    other_onnx = _touch(tmp_path / "other_model.onnx")
+
+    removed = _remove_stale_external_data(dst)
+
+    assert other_bin.exists(), "sibling model .bin was destroyed"
+    assert other_data.exists(), "unrelated .onnx.data was destroyed"
+    assert other_onnx.exists()
+    assert other_bin not in removed and other_data not in removed
+
+
+def test_never_removes_dst_itself(tmp_path: Path) -> None:
+    dst = _touch(tmp_path / "model_fp16.onnx")
+    _remove_stale_external_data(dst)
+    assert dst.exists(), "dst .onnx must never be unlinked"
+
+
+def test_no_leftovers_is_noop(tmp_path: Path) -> None:
+    dst = tmp_path / "model_fp16.onnx"
+    assert _remove_stale_external_data(dst) == []