microsoft/semantic-kernel · error · ServiceInvalidRequestError

Unsupported content type in an assistant message: {type(item

Error message

Unsupported content type in an assistant message: {type(item)}

What it means

Raised while formatting an assistant-role message for Bedrock when an item is not TextContent, ImageContent (caught above as a specific error), or FunctionCallContent. It is the fallback guard for any unrecognized content type in an assistant turn, reporting the offending Python type in the message.

Source

Thrown at python/semantic_kernel/connectors/ai/bedrock/services/model_provider/utils.py:111

    contents: list[Any] = []
    for item in message.items:
        if isinstance(item, ImageContent):
            raise ServiceInvalidRequestError("Image content is not supported in an assistant message.")

        if isinstance(item, TextContent):
            contents.append({"text": item.text})
        elif isinstance(item, FunctionCallContent):
            contents.append({
                "toolUse": {
                    "toolUseId": item.id,
                    "name": item.name,
                    "input": item.arguments
                    if isinstance(item.arguments, Mapping)
                    else json.loads(item.arguments or "{}"),
                }
            })
        else:
            raise ServiceInvalidRequestError(f"Unsupported content type in an assistant message: {type(item)}")

    return {
        "role": "assistant",
        "content": contents,
    }


def _format_tool_message(message: ChatMessageContent) -> dict[str, Any]:
    """Format a tool message to the expected object for the client.

    Args:
        message: The tool message.

    Returns:
        The formatted tool message.
    """
    contents: list[Any] = []
    for item in message.items:

View on GitHub (pinned to c028a0c7dc)

Solutions

  1. Move FunctionResultContent items to a tool-role message; move annotations/file references out of the assistant turn or strip them before Bedrock serialization.
  2. Inspect the reported type in the error message to identify exactly which content class triggered it.
  3. Keep assistant messages limited to TextContent and FunctionCallContent when targeting Bedrock.

Example fix

// before
history.add_message(ChatMessageContent(role=AuthorRole.ASSISTANT, items=[FunctionResultContent(...)]))
// after
history.add_message(ChatMessageContent(role=AuthorRole.TOOL, items=[FunctionResultContent(...)]))
Defensive patterns

Strategy: validation

Validate before calling

from semantic_kernel.contents.text_content import TextContent
from semantic_kernel.contents.function_call_content import FunctionCallContent

def assistant_items_are_bedrock_supported(items) -> bool:
    return all(isinstance(i, (TextContent, FunctionCallContent)) for i in items)

Type guard

from semantic_kernel.contents.text_content import TextContent
from semantic_kernel.contents.function_call_content import FunctionCallContent

def is_bedrock_assistant_item(item: object) -> bool:
    return isinstance(item, (TextContent, FunctionCallContent))

Try / catch

from semantic_kernel.exceptions.service_exceptions import ServiceInvalidRequestError

try:
    await service.get_chat_message_contents(history, settings)
except ServiceInvalidRequestError as e:
    if "Unsupported content type in an assistant message" in str(e):
        logger.error("Offending item type in assistant message; redirect tool results to TOOL role")
    raise

Prevention

When it happens

Trigger: A ChatHistory assistant message contains a content item such as FunctionResultContent, AnnotationContent, FileReferenceContent, or any future content type not handled by _format_assistant_message.

Common situations: Mixing a tool result into an assistant message; adding annotation/file-reference items to assistant turns; a new content class was introduced in a newer SK version but the Bedrock converter has not been updated.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/99c81711a94b71a9. Report an issue: GitHub.