fix(anthropic): handle mid-conversation system messages (#26773)

Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

python/sglang/srt/entrypoints/anthropic/protocol.py

@@ -124,7 +124,7 @@ AnthropicContentBlock = Annotated[
 
 
 class AnthropicMessage(BaseModel):
-    role: Literal["user", "assistant"]
+    role: Literal["user", "assistant", "system"]
     content: Union[str, list[AnthropicContentBlock]]
 
 
@@ -379,6 +379,64 @@ class AnthropicMessagesRequest(BaseModel):
     output_config: Optional[AnthropicOutputConfig] = None
     betas: Optional[list[str]] = None
 
+    @model_validator(mode="before")
+    @classmethod
+    def move_mid_conversation_system_messages(cls, values: dict) -> dict:
+        """Fold mid-conversation ``role: "system"`` turns into the top-level
+        ``system`` field — some clients (e.g. Claude Code) emit them there."""
+        messages = values.get("messages", [])
+        if not messages:
+            return values
+
+        clean_messages = []
+        extracted_system_texts = []
+
+        for msg in messages:
+            # ``mode="before"`` sees raw dicts (HTTP path) but also already-
+            # constructed ``AnthropicMessage`` objects (programmatic path, e.g.
+            # ``handle_count_tokens``), so normalize to a dict first.
+            if isinstance(msg, BaseModel):
+                msg = msg.model_dump()
+            if msg.get("role") == "system":
+                content = msg.get("content", "")
+                if isinstance(content, str) and content.strip():
+                    extracted_system_texts.append(content.strip())
+                elif isinstance(content, list):
+                    for block in content:
+                        if isinstance(block, dict) and block.get("type") == "text":
+                            text = block.get("text", "").strip()
+                            if text:
+                                extracted_system_texts.append(text)
+            else:
+                clean_messages.append(msg)
+
+        if extracted_system_texts:
+            existing_system = values.get("system")
+            combined_system = []
+
+            if existing_system:
+                if isinstance(existing_system, str):
+                    if existing_system.strip():
+                        combined_system.append(existing_system.strip())
+                elif isinstance(existing_system, list):
+                    for block in existing_system:
+                        if isinstance(block, BaseModel):
+                            block = block.model_dump()
+                        if isinstance(block, dict) and block.get("type") == "text":
+                            text = block.get("text", "").strip()
+                            if text:
+                                combined_system.append(text)
+
+            combined_system.extend(extracted_system_texts)
+
+            # Join into a string — ``system`` is ``str | list[AnthropicContentBlock]``,
+            # so a ``list[str]`` would fail validation.
+            if combined_system:
+                values["system"] = "\n".join(combined_system)
+
+        values["messages"] = clean_messages
+        return values
+
     @field_validator("model")
     @classmethod
     def _validate_model(cls, v):