address review: check fallback targets for prefill support, drop public accessor, update capability pin test

- Greptile: the same continuation messages go to every fallback target, so the user-message continuation now engages when the primary OR any configured fallback target for the group is explicitly marked prefill-unsupported (via get_fallback_model_group)
- Greptile: drop the supports_assistant_prefill() public accessor - _supports_factory's absent-key=False default contradicts this feature's absent-key=legacy-prefill routing; the routing reads get_model_info directly
- CI: test_claude_sonnet_4_6_config pinned the stale capability value; prefill was removed in Sonnet 4.6 so the pin is now False

litellm/router.py

@@ -2242,6 +2242,7 @@ class Router:
                                 messages=messages,
                                 generated_content=e.generated_content,
                                 model_group=model_group,
+                                fallbacks=fallbacks,
                             )
                         )
                     self._update_kwargs_before_fallbacks(
@@ -2801,6 +2802,7 @@ class Router:
                                 messages=messages,
                                 generated_content=e.generated_content,
                                 model_group=model_group,
+                                fallbacks=fallbacks,
                             )
                         )
                     router_self._update_kwargs_before_fallbacks(

litellm/router_utils/fallback_event_handlers.py

@@ -43,6 +43,7 @@ def build_mid_stream_continuation_messages(
     messages: List[Any],
     generated_content: str,
     model_group: Optional[str],
+    fallbacks: Optional[List[Any]] = None,
 ) -> List[Any]:
     """Build the message list a mid-stream fallback uses to resume an interrupted stream.
 
@@ -56,8 +57,24 @@ def build_mid_stream_continuation_messages(
     a trailing USER message instead — the continuation pattern Anthropic's
     migration guide documents:
     https://platform.claude.com/docs/en/about-claude/models/migration-guide
+
+    The same continuation messages are sent to every deployment the fallback
+    chain tries, so the safe pattern engages when the primary model OR any
+    configured fallback target for this model group is explicitly marked as
+    not supporting prefill.
     """
-    if _prefill_explicitly_unsupported(model_group):
+    candidate_models: List[Optional[str]] = [model_group]
+    if fallbacks is not None and model_group is not None:
+        try:
+            fallback_model_group, _ = get_fallback_model_group(
+                fallbacks=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 + [
             {
                 "role": "user",

litellm/utils.py

@@ -2448,30 +2448,6 @@ def supports_url_context(model: str, custom_llm_provider: Optional[str] = None)
     )
 
 
-def supports_assistant_prefill(
-    model: str, custom_llm_provider: Optional[str] = None
-) -> bool:
-    """
-    Check if the given model supports assistant message prefill and return a boolean value.
-
-    Anthropic removed assistant prefill starting with Claude Sonnet 4.6 / Opus 4.6 —
-    a prefilled assistant message returns a 400 error on those models.
-    https://platform.claude.com/docs/en/about-claude/models/migration-guide
-
-    Parameters:
-    model (str): The model name to be checked.
-    custom_llm_provider (Optional[str]): The provider to be checked.
-
-    Returns:
-    bool: True if the model supports assistant prefill, False otherwise (including unknown models).
-    """
-    return _supports_factory(
-        model=model,
-        custom_llm_provider=custom_llm_provider,
-        key="supports_assistant_prefill",
-    )
-
-
 def supports_native_streaming(model: str, custom_llm_provider: Optional[str]) -> bool:
     """
     Check if the given model supports native streaming and return a boolean value.

tests/test_litellm/router_utils/test_fallback_event_handlers.py

@@ -93,3 +93,29 @@ def test_other_models_keep_legacy_prefill_resume(model):
     """Anything not explicitly marked supports_assistant_prefill=false keeps the
     pre-existing prefill-resume behavior (back-compat)."""
     _assert_legacy_prefill(_build(model))
+
+
+def test_prefill_rejecting_fallback_target_gets_user_continuation():
+    """The same continuation messages go to every fallback target — a primary
+    that supports prefill must still use the user continuation when any
+    configured fallback target rejects it (e.g. claude-3-5 → claude-sonnet-4-6)."""
+    result = build_mid_stream_continuation_messages(
+        messages=MESSAGES,
+        generated_content=PARTIAL,
+        model_group="claude-3-5-sonnet-20241022",
+        fallbacks=[{"claude-3-5-sonnet-20241022": ["claude-sonnet-4-6"]}],
+    )
+    assert len(result) == 2
+    assert result[1]["role"] == "user"
+    assert PARTIAL in result[1]["content"]
+
+
+def test_unrelated_fallback_groups_do_not_affect_prefill():
+    """Fallback config for OTHER model groups must not flip this group's behavior."""
+    result = build_mid_stream_continuation_messages(
+        messages=MESSAGES,
+        generated_content=PARTIAL,
+        model_group="gpt-4",
+        fallbacks=[{"some-other-model": ["claude-sonnet-4-6"]}],
+    )
+    _assert_legacy_prefill(result)

tests/test_litellm/test_claude_sonnet_4_6_config.py

@@ -48,7 +48,9 @@ def test_bedrock_sonnet_4_6_region_prefixes():
         assert model_info.get("supports_prompt_caching") is True
         assert model_info.get("supports_response_schema") is True
         assert model_info.get("supports_pdf_input") is True
-        assert model_info.get("supports_assistant_prefill") is True
+        # Assistant prefill was removed in Sonnet 4.6 — a prefilled assistant
+        # message returns a 400 on every platform (Anthropic migration guide).
+        assert model_info.get("supports_assistant_prefill") is False
         assert model_info.get("supports_reasoning") is True