fix(responses): keep the system prompt in input for json_object on the chat to responses bridge (#49957)

The bridge moves leading system messages into the Responses instructions
field. With response_format json_object, Azure requires the word json in
input and ignores instructions, so a request that follows the documented
rule 400s after translation. Send the system text as a system input item
when json_object is requested, the same shape already used for
system-only requests

Resolves LIT-10426

litellm/completion_extras/litellm_responses_transformation/transformation.py

@@ -1180,11 +1180,12 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
             include_namespace=True,
             replay_openai_history=provider == "openai",
         )
-        # OpenAI's Responses API rejects an empty input. For a system-only
-        # request, carry the system message as a system-role input item instead
-        # of instructions, mirroring how non-string system content is already
-        # handled in convert_chat_completion_messages_to_responses_api.
-        is_system_only_request: Final = not converted_input_items and converted_instructions is not None
+        # Keep the system text in input instead of instructions when the Responses API needs it there.
+        # It rejects an empty input, and json_object requires the word "json" in input, ignoring instructions.
+        typed_optional_params: Final = cast("Mapping[str, object]", optional_params)  # cast-ok: untyped dict
+        system_goes_in_input: Final = converted_instructions is not None and (
+            not converted_input_items or _payload_type(typed_optional_params.get("response_format")) == "json_object"
+        )
         target_input_items: Final = (
             _without_audio_input_parts(converted_input_items)
             if _drops_audio_input(model, litellm_params)
@@ -1196,12 +1197,13 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
                     "type": "message",
                     "role": "system",
                     "content": [{"type": "input_text", "text": converted_instructions}],
-                }
+                },
+                *target_input_items,
             ]
-            if is_system_only_request
+            if system_goes_in_input
             else target_input_items
         )
-        instructions: Final = None if is_system_only_request else converted_instructions
+        instructions: Final = None if system_goes_in_input else converted_instructions
 
         # Build responses API request using the reverse transformation logic
         responses_api_request: Final = ResponsesAPIOptionalRequestParams()

tests/unit/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py

@@ -1856,6 +1856,51 @@ def test_transform_request_system_only_message_maps_to_system_input_item():
     assert not result.get("instructions")
 
 
+_JSON_SYSTEM_PROMPT: Final = "You output JSON objects."
+_PLAIN_USER_ITEM: Final = {
+    "type": "message",
+    "role": "user",
+    "content": [{"type": "input_text", "text": "List three colors."}],
+}
+_COLORS_SCHEMA: Final = {
+    "type": "json_schema",
+    "json_schema": {"name": "colors", "schema": {"type": "object", "properties": {}}},
+}
+
+
+@pytest.mark.parametrize(
+    ("response_format", "expected_input", "expected_instructions"),
+    [
+        (
+            {"type": "json_object"},
+            [
+                {"type": "message", "role": "system", "content": [{"type": "input_text", "text": _JSON_SYSTEM_PROMPT}]},
+                _PLAIN_USER_ITEM,
+            ],
+            None,
+        ),
+        (_COLORS_SCHEMA, [_PLAIN_USER_ITEM], _JSON_SYSTEM_PROMPT),
+    ],
+    ids=["json_object_keeps_system_in_input", "json_schema_keeps_system_in_instructions"],
+)
+def test_transform_request_json_object_keeps_system_prompt_where_the_json_word_check_reads_it(
+    response_format, expected_input, expected_instructions
+):
+    result = LiteLLMResponsesTransformationHandler().transform_request(
+        model="gpt-5.4-mini",
+        messages=[
+            {"role": "system", "content": _JSON_SYSTEM_PROMPT},
+            {"role": "user", "content": "List three colors."},
+        ],
+        optional_params={"response_format": response_format},
+        litellm_params={},
+        headers={},
+        litellm_logging_obj=Mock(),
+    )
+
+    assert (result["input"], result.get("instructions")) == (expected_input, expected_instructions), result
+
+
 def test_transform_request_single_char_keys_not_matched():
     """Test that single-character keys are not incorrectly matched to 'metadata' or 'previous_response_id'