address review: flat-string fallback prefill check + snowflake registry flag

Greptile findings:

1. build_mid_stream_continuation_messages now scans every entry of a flat string-format fallback list directly. get_fallback_model_group pops a single string mid-iteration, so a prefill-rejecting model at a non-first position was never capability-checked; since the continuation is built once and reused across all fallback hops, the chain still 400'd when it reached that model. Dict/standard format keeps using get_fallback_model_group (already returns the full group).

2. snowflake/claude-sonnet-4-6 gets supports_assistant_prefill:false in both the root map and the bundled backup (it existed only in root before; the backup is the offline/test source), completing the 4-6 sweep.

Tests: +snowflake registry param, +flat-string non-first-position -> user continuation, +flat-string all-supporting -> legacy prefill.

litellm/model_prices_and_context_window_backup.json

@@ -31306,6 +31306,22 @@
         "mode": "chat",
         "supports_computer_use": true
     },
+    "snowflake/claude-sonnet-4-6": {
+        "cache_read_input_token_cost": 0.0000003,
+        "input_cost_per_token": 0.000003,
+        "litellm_provider": "snowflake",
+        "max_input_tokens": 200000,
+        "max_output_tokens": 16384,
+        "max_tokens": 16384,
+        "mode": "chat",
+        "output_cost_per_token": 0.000015,
+        "supports_assistant_prefill": false,
+        "supports_function_calling": true,
+        "supports_prompt_caching": true,
+        "supports_response_schema": true,
+        "supports_system_messages": true,
+        "supports_vision": true
+    },
     "snowflake/deepseek-r1": {
         "litellm_provider": "snowflake",
         "max_input_tokens": 32768,

litellm/router_utils/fallback_event_handlers.py

@@ -65,18 +65,28 @@ def build_mid_stream_continuation_messages(
     """
     candidate_models: list[Optional[str]] = [model_group]
     if fallbacks is not None and model_group is not None:
-        try:
-            # Shallow copy: get_fallback_model_group POPS a matching entry from
-            # flat string-format fallback lists — mutating the live list here
-            # would silently drop one fallback target before the actual
-            # fallback execution runs.
-            fallback_model_group, _ = get_fallback_model_group(
-                fallbacks=list(fallbacks), model_group=model_group
-            )
-            if fallback_model_group:
-                candidate_models.extend(fallback_model_group)
-        except Exception:
-            pass
+        if fallbacks and all(isinstance(f, str) for f in fallbacks):
+            # Flat string-format lists (["model-a", "model-b", ...]) are tried in
+            # order for every model_group. get_fallback_model_group surfaces only
+            # ONE entry from them — it pops a single string mid-iteration and
+            # leaves the rest hidden — so a prefill-rejecting model at a non-first
+            # position would slip through, and the once-built continuation that is
+            # reused across every hop would 400 when the chain reaches it. Check
+            # every entry directly instead.
+            candidate_models.extend(fallbacks)
+        else:
+            try:
+                # list() copy: get_fallback_model_group POPS string entries from a
+                # mixed fallbacks list while iterating — mutating the live list
+                # here would silently drop a fallback target before the actual
+                # fallback execution runs.
+                fallback_model_group, _ = get_fallback_model_group(
+                    fallbacks=list(fallbacks), model_group=model_group
+                )
+                if fallback_model_group:
+                    candidate_models.extend(fallback_model_group)
+            except Exception:
+                pass
 
     if any(_prefill_explicitly_unsupported(m) for m in candidate_models):
         return messages + [

model_prices_and_context_window.json

@@ -43019,6 +43019,7 @@
     "cache_read_input_token_cost": 0.0000003,
     "litellm_provider": "snowflake",
     "mode": "chat",
+    "supports_assistant_prefill": false,
     "supports_function_calling": true,
     "supports_vision": true,
     "supports_prompt_caching": true,

tests/test_litellm/router_utils/test_fallback_event_handlers.py

@@ -32,6 +32,7 @@ def local_model_cost_map(monkeypatch):
         litellm.model_cost = original_model_cost
         litellm.get_model_info.cache_clear()
 
+
 MESSAGES = [{"role": "user", "content": "Plan my trip to Tokyo"}]
 PARTIAL = "Here are the best flight options I found so"
 
@@ -66,6 +67,7 @@ def _assert_legacy_prefill(result):
         "vertex_ai/claude-sonnet-4-6",
         "claude-opus-4-6",
         "openrouter/anthropic/claude-sonnet-4.6",  # dot-variant registry key
+        "snowflake/claude-sonnet-4-6",  # snowflake provider entry
     ],
 )
 def test_prefill_rejecting_models_get_user_continuation(model):
@@ -143,3 +145,32 @@ def test_fallbacks_list_is_not_mutated_by_capability_check():
         fallbacks=dict_format_fallbacks,
     )
     assert dict_format_fallbacks == [{"gpt-4": ["claude-sonnet-4-6"]}]
+
+
+def test_flat_string_fallback_prefill_rejecter_at_non_first_position():
+    """Flat string-format lists are tried in order for every model group, but
+    get_fallback_model_group surfaces only one entry. A prefill-rejecting model
+    anywhere in the list — not just position 0 — must flip the reused
+    continuation to the user-message form."""
+    result = build_mid_stream_continuation_messages(
+        messages=MESSAGES,
+        generated_content=PARTIAL,
+        model_group="gpt-4",
+        fallbacks=["gpt-4o", "claude-sonnet-4-6", "gpt-3.5-turbo"],
+    )
+    assert len(result) == 2
+    assert result[1]["role"] == "user"
+    assert PARTIAL in result[1]["content"]
+    assert all(m.get("prefix") is not True for m in result)
+
+
+def test_flat_string_fallback_all_prefill_supporting_keeps_legacy():
+    """A flat string-format list with no prefill-rejecting model keeps the
+    legacy prefill resume."""
+    result = build_mid_stream_continuation_messages(
+        messages=MESSAGES,
+        generated_content=PARTIAL,
+        model_group="gpt-4",
+        fallbacks=["gpt-4o", "gpt-3.5-turbo"],
+    )
+    _assert_legacy_prefill(result)