fix(bedrock): gate in-place system role messages on model support for Claude Invoke (#32831)

Backport of #32831 to stable/1.91.x.
Cherry-picked from 5e23a5ab05 (litellm_internal_staging).

Adapted for this line: the fallback-generalizations feature does not exist on
stable/1.91.x, so the fallback rule for unmapped Claude 4.8+ models (and its
tests) is omitted; unmapped models fall back to hoist-all, which is the safe
default. Only supports_mid_conversation_system is added to the Opus 4.8 cost
map entries.

litellm/llms/bedrock/messages/invoke_transformations/anthropic_claude3_transformation.py

@@ -93,31 +93,48 @@ class AmazonAnthropicClaudeMessagesConfig(
             return [{"type": "text", "text": value}]
         return [value]
 
-    def _normalize_system_role_messages_for_bedrock(self, anthropic_messages_request: dict) -> None:
-        """Bedrock Invoke rejects a conversation that opens with ``role: "system"``
-        entries inside ``messages`` ("messages.0: use the top-level 'system'
-        parameter for the initial system prompt"); Anthropic Messages carries that
-        content in the top-level ``system`` field, so hoist the leading run of
-        system entries there. Mid-conversation system entries (e.g. Claude Code's
-        ``mid-conversation-system-2026-04-07`` reminders) are accepted by Invoke in
-        place and MUST stay in place: hoisting one mutates the ``system`` prefix
-        and invalidates the prompt cache for the entire message history.
+    @staticmethod
+    def _is_system_role_message(message: Any) -> bool:
+        return isinstance(message, dict) and message.get("role") == "system"
+
+    def _normalize_system_role_messages_for_bedrock(self, anthropic_messages_request: dict, model: str) -> None:
+        """Bedrock Invoke validates ``role: "system"`` entries inside ``messages``
+        per model. Models carrying ``supports_mid_conversation_system`` in the
+        cost map (the Opus 4.8 family) only reject a leading run ("messages.0:
+        use the top-level 'system' parameter for the initial system prompt") and
+        accept mid-conversation entries (e.g. Claude Code's
+        ``mid-conversation-system-2026-04-07`` reminders) in place, where they
+        MUST stay: hoisting one mutates the ``system`` prefix and invalidates the
+        prompt cache for the entire message history. Older Claude models (Opus
+        4.7, Sonnet 4.6, Haiku 4.5, ...) reject the role in every position
+        ("role 'system' is not supported on this model"), so without the flag
+        every system entry is hoisted into the top-level ``system`` field.
         Billing-header system blocks are stripped from the top-level ``system``
         field regardless of whether anything was hoisted."""
         messages = anthropic_messages_request.get("messages")
         if not isinstance(messages, list):
             return
-        leading_count = next(
-            (i for i, m in enumerate(messages) if not (isinstance(m, dict) and m.get("role") == "system")),
-            len(messages),
-        )
-        if leading_count:
-            anthropic_messages_request["messages"] = messages[leading_count:]
+        if _supports_factory(
+            model=model,
+            custom_llm_provider="bedrock",
+            key="supports_mid_conversation_system",
+        ):
+            leading_count = next(
+                (i for i, m in enumerate(messages) if not self._is_system_role_message(m)),
+                len(messages),
+            )
+            hoisted = messages[:leading_count]
+            remaining = messages[leading_count:]
+        else:
+            hoisted = [m for m in messages if self._is_system_role_message(m)]
+            remaining = [m for m in messages if not self._is_system_role_message(m)]
+        if hoisted:
+            anthropic_messages_request["messages"] = remaining
         system_content = [
             block
             for source in (
                 anthropic_messages_request.get("system"),
-                *(m.get("content") for m in messages[:leading_count]),
+                *(m.get("content") for m in hoisted),
             )
             for block in self._as_system_content_blocks(source)
         ]
@@ -653,7 +670,7 @@ class AmazonAnthropicClaudeMessagesConfig(
             litellm_params=litellm_params,
             headers=headers,
         )
-        self._normalize_system_role_messages_for_bedrock(anthropic_messages_request)
+        self._normalize_system_role_messages_for_bedrock(anthropic_messages_request, model=model)
         #########################################################
         ############## BEDROCK Invoke SPECIFIC TRANSFORMATION ###
         #########################################################

litellm/model_prices_and_context_window_backup.json

@@ -1481,6 +1481,7 @@
     "anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1516,6 +1517,7 @@
     "global.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1551,6 +1553,7 @@
     "us.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1586,6 +1589,7 @@
     "eu.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1621,6 +1625,7 @@
     "au.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,

litellm/types/utils.py

@@ -143,6 +143,7 @@ class ProviderSpecificModelInfo(TypedDict, total=False):
     supports_web_search: Optional[bool]
     supports_reasoning: Optional[bool]
     supports_adaptive_thinking: Optional[bool]
+    supports_mid_conversation_system: Optional[bool]
     supports_url_context: Optional[bool]
     supports_none_reasoning_effort: Optional[bool]
     supports_minimal_reasoning_effort: Optional[bool]

litellm/utils.py

@@ -5412,6 +5412,7 @@ def _get_model_info_helper(
                 supports_url_context=_model_info.get("supports_url_context", None),
                 supports_reasoning=_model_info.get("supports_reasoning", None),
                 supports_adaptive_thinking=_model_info.get("supports_adaptive_thinking", None),
+                supports_mid_conversation_system=_model_info.get("supports_mid_conversation_system", None),
                 supports_none_reasoning_effort=_model_info.get("supports_none_reasoning_effort", None),
                 supports_minimal_reasoning_effort=_model_info.get("supports_minimal_reasoning_effort", None),
                 supports_low_reasoning_effort=_model_info.get("supports_low_reasoning_effort", None),

model_prices_and_context_window.json

@@ -1481,6 +1481,7 @@
     "anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1516,6 +1517,7 @@
     "global.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1551,6 +1553,7 @@
     "us.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1586,6 +1589,7 @@
     "eu.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1621,6 +1625,7 @@
     "au.anthropic.claude-opus-4-8": {
         "supports_parallel_tool_use_config": true,
         "bedrock_converse_supports_strict_tools": false,
+        "supports_mid_conversation_system": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,

tests/test_litellm/llms/bedrock/messages/invoke_transformations/test_anthropic_claude3_transformation.py

@@ -1893,14 +1893,15 @@ def test_bedrock_invoke_transform_merges_list_content_system_role_into_system():
     ]
 
 
-def test_bedrock_invoke_transform_keeps_mid_conversation_system_role_in_place():
+def test_bedrock_invoke_transform_keeps_mid_conversation_system_role_in_place(local_model_cost_map):
     """Regression test for the Bedrock prompt-cache collapse: hoisting a
     mid-conversation ``role: "system"`` message (e.g. Claude Code's
     ``mid-conversation-system-2026-04-07`` reminders) into the top-level
     ``system`` field mutates the cache prefix and invalidates the cached message
-    history, so such entries must be forwarded in place. Invoke only rejects a
-    system entry at ``messages.0``. Billing-header blocks must still be stripped
-    from the top-level ``system`` field even when nothing is hoisted."""
+    history, so on models flagged ``supports_mid_conversation_system`` (the Opus
+    4.8 family, which Invoke accepts the role on) such entries must be forwarded
+    in place. Billing-header blocks must still be stripped from the top-level
+    ``system`` field even when nothing is hoisted."""
     from litellm.types.router import GenericLiteLLMParams
 
     cfg = AmazonAnthropicClaudeMessagesConfig()
@@ -1932,10 +1933,11 @@ def test_bedrock_invoke_transform_keeps_mid_conversation_system_role_in_place():
     ]
 
 
-def test_bedrock_invoke_transform_hoists_only_leading_system_run():
-    """Only the leading run of ``role: "system"`` messages is hoisted into the
-    top-level ``system`` field; a later system entry keeps its position in
-    ``messages`` so the serialized prefix stays stable across turns."""
+def test_bedrock_invoke_transform_hoists_only_leading_system_run(local_model_cost_map):
+    """On models flagged ``supports_mid_conversation_system``, only the leading
+    run of ``role: "system"`` messages is hoisted into the top-level ``system``
+    field; a later system entry keeps its position in ``messages`` so the
+    serialized prefix stays stable across turns."""
     from litellm.types.router import GenericLiteLLMParams
 
     cfg = AmazonAnthropicClaudeMessagesConfig()
@@ -1966,6 +1968,76 @@ def test_bedrock_invoke_transform_hoists_only_leading_system_run():
     ]
 
 
+def test_bedrock_invoke_transform_hoists_mid_conversation_system_for_older_claude(local_model_cost_map):
+    """Regression test for Claude Code 400s on pre-Opus-4.8 Bedrock models:
+    Invoke rejects ``role: "system"`` in every position on Opus 4.7, Sonnet 4.6,
+    Haiku 4.5, etc. ("role 'system' is not supported on this model"), so on
+    models without ``supports_mid_conversation_system`` every system entry must
+    be hoisted into the top-level ``system`` field, mid-conversation ones
+    included."""
+    from litellm.types.router import GenericLiteLLMParams
+
+    cfg = AmazonAnthropicClaudeMessagesConfig()
+    messages = [
+        {"role": "user", "content": "read the file"},
+        {"role": "system", "content": "[Truncated: PARTIAL view of big1.txt]"},
+        {"role": "assistant", "content": "reading"},
+        {"role": "user", "content": "continue"},
+    ]
+
+    result = cfg.transform_anthropic_messages_request(
+        model="us.anthropic.claude-opus-4-7",
+        messages=copy.deepcopy(messages),
+        anthropic_messages_optional_request_params={
+            "max_tokens": 256,
+            "stream": False,
+            "system": [{"type": "text", "text": "Base."}],
+        },
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+
+    assert result["messages"] == [
+        {"role": "user", "content": "read the file"},
+        {"role": "assistant", "content": "reading"},
+        {"role": "user", "content": "continue"},
+    ]
+    assert result["system"] == [
+        {"type": "text", "text": "Base."},
+        {"type": "text", "text": "[Truncated: PARTIAL view of big1.txt]"},
+    ]
+
+
+def test_bedrock_invoke_transform_hoists_all_system_for_unmapped_model(local_model_cost_map):
+    """A model with no cost-map entry and no fallback-generalization rule gets
+    the hoist-everything behavior: the safe default is a mutated cache prefix,
+    never a provider 400 from forwarding a role the model may not accept."""
+    from litellm.types.router import GenericLiteLLMParams
+
+    cfg = AmazonAnthropicClaudeMessagesConfig()
+    messages = [
+        {"role": "user", "content": "hi"},
+        {"role": "system", "content": "mid-conversation reminder"},
+        {"role": "assistant", "content": "hello"},
+        {"role": "user", "content": "continue"},
+    ]
+
+    result = cfg.transform_anthropic_messages_request(
+        model="us.anthropic.claude-opus-3-9",
+        messages=copy.deepcopy(messages),
+        anthropic_messages_optional_request_params={"max_tokens": 256, "stream": False},
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+
+    assert result["messages"] == [
+        {"role": "user", "content": "hi"},
+        {"role": "assistant", "content": "hello"},
+        {"role": "user", "content": "continue"},
+    ]
+    assert result["system"] == [{"type": "text", "text": "mid-conversation reminder"}]
+
+
 def test_as_system_content_blocks_handles_each_shape():
     """``_as_system_content_blocks`` normalizes every system shape: ``None`` -> empty,
     a string -> a single text block, a list -> a shallow copy, and any other value

tests/test_litellm/test_utils.py

@@ -836,6 +836,7 @@ def test_aaamodel_prices_and_context_window_json_is_valid():
                 "supports_xhigh_reasoning_effort": {"type": "boolean"},
                 "supports_max_reasoning_effort": {"type": "boolean"},
                 "supports_adaptive_thinking": {"type": "boolean"},
+                "supports_mid_conversation_system": {"type": "boolean"},
                 "supports_sampling_params": {"type": "boolean"},
                 "supports_output_config": {"type": "boolean"},
                 "supports_speed": {"type": "boolean"},