Studio: accept system-role messages in Claude Code requests (#6006)

Normalize misplaced system-role messages in /v1/messages by hoisting their content into the top-level Anthropic system field, fixing the 422 that newer Claude Code clients trigger. Null and non-text system content is ignored rather than stringified.

Fixes #6001

studio/backend/models/inference.py

@@ -1431,6 +1431,47 @@ AnthropicContentBlock = Union[
 ]
 
 
+def _anthropic_content_to_system_text(content: Any) -> str:
+    """Convert misplaced system message content into Anthropic system text."""
+    if content is None:  # null content must not become the literal "None"
+        return ""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        parts: list[str] = []
+        for block in content:
+            if isinstance(block, dict) and block.get("type") == "text":
+                text = block.get("text")
+                if isinstance(text, str):
+                    parts.append(text)
+                    continue
+            if block is not None:
+                parts.append(str(block))
+        return "\n\n".join(part for part in parts if part)
+    return str(content)
+
+
+def _merge_anthropic_system(system: Any, additions: list[str]) -> Any:
+    if not additions:
+        return system
+
+    addition_blocks = [
+        {"type": "text", "text": text} for text in additions if text.strip()
+    ]
+    if not addition_blocks:
+        return system
+
+    if system is None:
+        return (
+            addition_blocks[0]["text"] if len(addition_blocks) == 1 else addition_blocks
+        )
+    if isinstance(system, str):
+        return "\n\n".join([system, *[block["text"] for block in addition_blocks]])
+    if isinstance(system, list):
+        return [*system, *addition_blocks]
+    return system
+
+
 class AnthropicMessage(BaseModel):
     role: Literal["user", "assistant"]
     content: Union[str, list[AnthropicContentBlock]]
@@ -1474,6 +1515,39 @@ class AnthropicMessagesRequest(BaseModel):
     cancel_id: Optional[str] = None
     model_config = {"extra": "allow"}
 
+    @model_validator(mode = "before")
+    @classmethod
+    def normalize_system_messages(cls, data: Any) -> Any:
+        if not isinstance(data, dict):
+            return data
+
+        messages = data.get("messages")
+        if not isinstance(messages, list):
+            return data
+
+        normalized_messages: list[Any] = []
+        system_additions: list[str] = []
+        changed = False
+
+        for message in messages:
+            if isinstance(message, dict) and message.get("role") == "system":
+                system_additions.append(
+                    _anthropic_content_to_system_text(message.get("content", ""))
+                )
+                changed = True
+                continue
+            normalized_messages.append(message)
+
+        if not changed:
+            return data
+
+        normalized = dict(data)
+        normalized["messages"] = normalized_messages
+        normalized["system"] = _merge_anthropic_system(
+            normalized.get("system"), system_additions
+        )
+        return normalized
+
 
 # ── Response models ────────────────────────────────────────────
 

studio/backend/tests/test_anthropic_messages.py

@@ -78,6 +78,51 @@ class TestAnthropicModels:
         )
         assert req.system == "You are helpful."
 
+    def test_system_role_message_normalized_to_system_field(self):
+        req = AnthropicMessagesRequest(
+            max_tokens = 50,
+            messages = [
+                {"role": "system", "content": "You are helpful."},
+                {"role": "user", "content": "Hi"},
+            ],
+        )
+        assert req.system == "You are helpful."
+        assert len(req.messages) == 1
+        assert req.messages[0].role == "user"
+
+    def test_system_role_message_merges_with_existing_system_field(self):
+        req = AnthropicMessagesRequest(
+            max_tokens = 50,
+            system = "Base instructions.",
+            messages = [
+                {"role": "user", "content": "Hi"},
+                {"role": "system", "content": "Additional instructions."},
+                {"role": "assistant", "content": "Hello."},
+            ],
+        )
+        assert req.system == "Base instructions.\n\nAdditional instructions."
+        assert [msg.role for msg in req.messages] == ["user", "assistant"]
+
+    def test_system_role_message_with_null_content_ignored(self):
+        req = AnthropicMessagesRequest(
+            max_tokens = 50,
+            system = "Base.",
+            messages = [
+                {"role": "system", "content": None},
+                {
+                    "role": "system",
+                    "content": [
+                        None,
+                        {"type": "text", "text": "Use short answers."},
+                    ],
+                },
+                {"role": "user", "content": "Hi"},
+            ],
+        )
+        assert req.system == "Base.\n\nUse short answers."
+        assert "None" not in str(req.system)
+        assert [msg.role for msg in req.messages] == ["user"]
+
     def test_tools_field_parses(self):
         req = AnthropicMessagesRequest(
             max_tokens = 100,
@@ -158,6 +203,20 @@ class TestAnthropicMessagesToOpenAI:
         assert result[0] == {"role": "system", "content": "Be brief."}
         assert result[1] == {"role": "user", "content": "Hello"}
 
+    def test_top_level_system_request_translates_unchanged(self):
+        req = AnthropicMessagesRequest(
+            messages = [{"role": "user", "content": "Hello"}],
+            system = "Be brief.",
+        )
+        result = anthropic_messages_to_openai(
+            [m.model_dump() for m in req.messages],
+            req.system,
+        )
+        assert result == [
+            {"role": "system", "content": "Be brief."},
+            {"role": "user", "content": "Hello"},
+        ]
+
     def test_system_as_block_list(self):
         system = [
             {"type": "text", "text": "Be brief."},