diff --git a/studio/backend/routes/settings.py b/studio/backend/routes/settings.py
index 3e4409ae3b..139538a3bd 100644
--- a/studio/backend/routes/settings.py
+++ b/studio/backend/routes/settings.py
@@ -355,16 +355,17 @@ def get_openai_auto_switch_overrides(
 
 def _bare_model_id(model_id: str) -> Optional[str]:
     """``repo`` for a ``repo:QUANT`` key, or None when there is no quant suffix."""
-    from core.inference.llama_cpp import _GGUF_KNOWN_QUANT_RE
     from hub.utils.gguf import extract_quant_label
+    from utils.openai_auto_switch_settings import split_quant_suffix
 
     head, sep, tail = model_id.rpartition(":")
     if not sep or not head or not tail:
         return None
-    if "/" not in tail and "\\" not in tail and len(tail) <= _MAX_VARIANT_SUFFIX_LEN:
-        # Must actually look like a quant, not just like a short path segment.
-        if _GGUF_KNOWN_QUANT_RE.fullmatch(tail) is not None:
-            return head
+    # Must actually look like a quant, not just like a short path segment. The
+    # label may carry a bits-per-weight modifier ("IQ4_XS-3.53bpw"), which keeps
+    # two files at the same base quant distinct, so that form counts too.
+    if split_quant_suffix(model_id) is not None:
+        return head
     # A .gguf with no recognizable quant token is still labelled by the scanner,
     # which falls back to the filename stem, so the UI stores keys like
     # "/models/custom.gguf:custom". Refusing those dropped the bare entry's
diff --git a/studio/backend/tests/test_openai_auto_switch.py b/studio/backend/tests/test_openai_auto_switch.py
index 721c9e2b22..cd77be0c77 100644
--- a/studio/backend/tests/test_openai_auto_switch.py
+++ b/studio/backend/tests/test_openai_auto_switch.py
@@ -4697,6 +4697,44 @@ def test_unknown_quant_label_on_a_gguf_still_carries_flags_over(monkeypatch):
     assert resp.overrides["/models/custom.gguf:custom"]["llama_extra_args"] == ["--flash-attn"]
 
 
+def test_bpw_qualified_variants_still_carry_flags_over(monkeypatch):
+    # utils/models/model_config.py keeps a bits-per-weight modifier on the label
+    # so two files at the same base quant stay distinct, and that form reaches
+    # the override keys. The known-quant pattern does not accept it, so the bare
+    # entry was missed and the first qualified save dropped its launch flags.
+    import routes.settings as settings_route
+
+    _mock_override_store(monkeypatch)
+    settings.set_model_override("unsloth/Repo-GGUF", llama_extra_args = ["--flash-attn"])
+    resp = settings_route.update_openai_auto_switch_override(
+        settings_route.ModelOverridePayload(
+            model_id = "unsloth/Repo-GGUF:IQ4_XS-3.53bpw", max_seq_length = 4096
+        ),
+        "tester",
+    )
+    assert resp.overrides["unsloth/Repo-GGUF:IQ4_XS-3.53bpw"]["llama_extra_args"] == [
+        "--flash-attn"
+    ]
+
+
+def test_a_posix_path_variant_folds_while_the_path_does_not(monkeypatch):
+    # The browser lowercases the quant but keeps POSIX path casing, so the
+    # migrated key is "/models/Foo:q4_k_m" while the scanner asks for
+    # "/models/Foo:Q4_K_M". The path itself must still be case-sensitive.
+    _mock_override_store(monkeypatch)
+    settings.set_model_override("/models/Foo:q4_k_m", max_seq_length = 8192)
+    assert settings.get_model_override("/models/Foo:Q4_K_M")["max_seq_length"] == 8192
+    assert settings.get_model_override("/models/foo:Q4_K_M") == {}
+
+
+def test_a_posix_colon_filename_is_not_folded_as_a_variant(monkeypatch):
+    # "/models/foo:Bar.gguf" is one filename, not path + quant, so folding its
+    # tail would let it reach a different file's settings.
+    _mock_override_store(monkeypatch)
+    settings.set_model_override("/models/foo:bar.gguf", max_seq_length = 8192)
+    assert settings.get_model_override("/models/foo:Bar.gguf") == {}
+
+
 def test_a_suffix_the_scanner_would_not_derive_carries_nothing_over(monkeypatch):
     # Only the exact label the scanner derives for this filename is accepted, so
     # an unrelated colon suffix cannot reach into another model's flags.
diff --git a/studio/backend/utils/openai_auto_switch_settings.py b/studio/backend/utils/openai_auto_switch_settings.py
index 24fa7704f0..59827121df 100644
--- a/studio/backend/utils/openai_auto_switch_settings.py
+++ b/studio/backend/utils/openai_auto_switch_settings.py
@@ -502,6 +502,47 @@ def _fold_case_insensitive_path(model_id: str) -> Optional[str]:
     return trimmed.casefold()
 
 
+# A quant label may carry a bits-per-weight modifier, because two files at the
+# same base quant are kept distinct by it ("IQ4_XS-3.53bpw"). The two label
+# helpers disagree on whether to keep it, so anything reading a stored key has
+# to accept both forms.
+_BPW_SUFFIX = re.compile(r"-[0-9]+(?:\.[0-9]+)?bpw$", re.IGNORECASE)
+_MAX_QUANT_SUFFIX_LEN = 64
+
+
+def split_quant_suffix(value: str) -> Optional[tuple[str, str]]:
+    """``(head, quant)`` for a ``head:QUANT`` key, or None when there is none.
+
+    The suffix has to be a real quant label, so an ordinary colon inside a POSIX
+    filename is left alone: "/models/foo:bar.gguf" is one valid filename, and
+    splitting it would graft /models/foo's launch flags onto a different model.
+    """
+    from core.inference.llama_cpp import _GGUF_KNOWN_QUANT_RE
+
+    head, sep, tail = value.rpartition(":")
+    if not sep or not head or not tail:
+        return None
+    if len(tail) > _MAX_QUANT_SUFFIX_LEN or "/" in tail or "\\" in tail:
+        return None
+    if _GGUF_KNOWN_QUANT_RE.fullmatch(_BPW_SUFFIX.sub("", tail)) is None:
+        return None
+    return head, tail
+
+
+def _fold_posix_path_variant(value: str) -> str:
+    """A POSIX path id with only its quant suffix folded.
+
+    The browser lowercases the variant but keeps the path casing, so a stored
+    "/models/Foo:q4_k_m" has to be reachable from "/models/Foo:Q4_K_M" without
+    also making "/models/Foo.gguf" reachable from "/models/foo.gguf".
+    """
+    split = split_quant_suffix(value)
+    if split is None:
+        return value
+    head, quant = split
+    return f"{head}:{quant.casefold()}"
+
+
 def get_model_overrides() -> dict[str, dict]:
     """Per-model launch configs keyed by model id (see normalize_model_override)."""
     raw = _cached_setting(MODEL_OVERRIDES_SETTING_KEY, None)
@@ -544,11 +585,22 @@ def resolve_model_override_key(model_id: str) -> Optional[str]:
     # every migrated Windows entry unreachable until the user saved it again.
     if _looks_like_filesystem_path(model_id):
         folded = _fold_case_insensitive_path(model_id)
-        if folded is None:
-            return None
+        if folded is not None:
 
-        def fold(key: str) -> Optional[str]:
-            return _fold_case_insensitive_path(key)
+            def fold(key: str) -> Optional[str]:
+                return _fold_case_insensitive_path(key)
+        else:
+            # POSIX: the path itself stays case-sensitive, but the browser
+            # lowercases the quant suffix while keeping the path casing, so a
+            # migrated "/models/Foo:q4_k_m" has to stay reachable from the
+            # scanner's "/models/Foo:Q4_K_M".
+            folded = _fold_posix_path_variant(model_id)
+
+            def fold(key: str) -> Optional[str]:
+                # A path only ever folds onto another path.
+                if not _looks_like_filesystem_path(key):
+                    return None
+                return None if _fold_case_insensitive_path(key) else _fold_posix_path_variant(key)
     else:
         folded = model_id.casefold()
 
diff --git a/studio/frontend/src/features/model-picker/api/migrate-model-overrides.ts b/studio/frontend/src/features/model-picker/api/migrate-model-overrides.ts
index b5f908a109..3cfa571db3 100644
--- a/studio/frontend/src/features/model-picker/api/migrate-model-overrides.ts
+++ b/studio/frontend/src/features/model-picker/api/migrate-model-overrides.ts
@@ -75,8 +75,13 @@ export async function backfillModelOverrides(): Promise<void> {
   const local = listPerModelConfigs().filter(
     // A quant means it is a GGUF, which is the only thing API auto-switch
     // resolves. Backfilling a safetensors config would claim an API behaviour
-    // that does not exist.
-    (entry) => entry.ggufVariant != null && !isDefaultConfig(entry.config),
+    // that does not exist. A standalone .gguf picked directly carries no quant
+    // to select between, so it is stored with a null variant and would fail
+    // that test despite being exactly what auto-switch does resolve; the flag
+    // is then set and its settings stay browser-only for good.
+    (entry) =>
+      (entry.ggufVariant != null || entry.modelId.toLowerCase().endsWith(".gguf")) &&
+      !isDefaultConfig(entry.config),
   );
   if (local.length === 0) {
     markRan();
diff --git a/tests/studio/test_model_picker_contracts.py b/tests/studio/test_model_picker_contracts.py
index 0a66f4efe3..79efdbf61e 100644
--- a/tests/studio/test_model_picker_contracts.py
+++ b/tests/studio/test_model_picker_contracts.py
@@ -733,3 +733,16 @@ def test_api_reach_copy_is_limited_to_gguf_models():
     src = " ".join(_read("features/hub/catalog/hub-model-settings-view.tsx").split())
     assert "{target.isGguf ?" in src
     assert "Saved settings apply everywhere Studio loads this model." in src
+
+
+def test_backfill_includes_a_standalone_gguf_with_no_variant():
+    """A standalone .gguf picked directly has no quant to choose between, so it
+    is stored with a null variant. The quant filter classified it like
+    safetensors and skipped it, and since the done flag is set on the same pass
+    those settings stayed browser-only permanently while API auto-switch, which
+    does resolve that model, kept loading it with defaults.
+    """
+    src = " ".join(_read("features/model-picker/api/migrate-model-overrides.ts").split())
+    assert 'entry.modelId.toLowerCase().endsWith(".gguf")' in src
+    # Still excluded for safetensors, which auto-switch does not resolve.
+    assert "entry.ggufVariant != null ||" in src