diff --git a/strix/interface/utils.py b/strix/interface/utils.py
index b1e01c9da..99a0b0039 100644
--- a/strix/interface/utils.py
+++ b/strix/interface/utils.py
@@ -1720,13 +1720,20 @@ def _split_workspace_spec(spec: str) -> tuple[str, str | None]:
 
     A Windows drive letter's colon (``C:\\path\\file``) is the only colon in a
     bare path. ``rpartition`` still finds it, leaving a single-letter ``raw``
-    with the rest of the path in what would be ``dest`` -- that split is
-    detected here and rejected, so the whole spec is returned as the path.
+    with the rest of the path in what would be ``dest``. That collides with a
+    real single-letter source that declares an absolute ``/workspace/...``
+    destination (``a:/workspace/input.txt``), so the drive-letter case is only
+    taken when ``dest`` is not one of those: the explicit form keeps its
+    existing meaning either way.
     """
     raw, sep, dest = spec.rpartition(":")
     if not sep or not dest.strip():
         return spec, None
-    if re.fullmatch(r"[A-Za-z]", raw) and dest[:1] in ("\\", "/"):
+    if (
+        re.fullmatch(r"[A-Za-z]", raw)
+        and dest[:1] in ("\\", "/")
+        and not dest.startswith("/workspace/")
+    ):
         return spec, None
     return raw, dest
 
diff --git a/tests/test_workspace_files.py b/tests/test_workspace_files.py
index 45335ed28..ce8136bc1 100644
--- a/tests/test_workspace_files.py
+++ b/tests/test_workspace_files.py
@@ -59,6 +59,23 @@ def test_a_windows_drive_letter_is_not_mistaken_for_a_dest_separator(
     assert _split_workspace_spec(spec) == expected
 
 
+def test_a_single_letter_source_with_a_workspace_destination_is_not_a_drive_letter() -> None:
+    assert _split_workspace_spec("a:/workspace/input.txt") == ("a", "/workspace/input.txt")
+
+
+def test_a_single_letter_file_can_declare_an_absolute_workspace_destination(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    (tmp_path / "a").write_text("x", encoding="utf-8")
+    monkeypatch.chdir(tmp_path)
+
+    # A relative, single-letter spec: "a:/workspace/input.txt" is exactly the
+    # shape rpartition(":") could confuse for a Windows drive letter.
+    resolved = resolve_workspace_files(["a:/workspace/input.txt"])
+
+    assert resolved[0]["workspace_path"] == "/workspace/input.txt"
+
+
 @pytest.mark.skipif(sys.platform != "win32", reason="tmp_path is only drive-letter-shaped here")
 def test_a_bare_windows_path_resolves_end_to_end(tmp_path: Path) -> None:
     source = tmp_path / "wordlist.txt"