PrefectHQ/fastmcp · error · ValueError

Unsupported content type for Anthropic: {type(item).name

Error message

Unsupported content type for Anthropic: {type(item).__name__}

What it means

ValueError from `_convert_to_anthropic_messages` (list-content branch): a content block of a type the Anthropic converter doesn't recognize was encountered, and it names the concrete class so you can see what leaked in.

Source

Thrown at fastmcp_slim/fastmcp/client/sampling/handlers/anthropic.py:242

                                TextBlockParam(type="text", text=sub_item.text)
                                for sub_item in item.content
                                if isinstance(sub_item, TextContent)
                            ]
                            if len(text_blocks) == 1:
                                result_content = text_blocks[0]["text"]
                            elif text_blocks:
                                result_content = text_blocks

                        content_blocks.append(
                            ToolResultBlockParam(
                                type="tool_result",
                                tool_use_id=item.tool_use_id,
                                content=result_content,
                                is_error=item.is_error if item.is_error else False,
                            )
                        )
                    else:
                        raise ValueError(
                            f"Unsupported content type for Anthropic: {type(item).__name__}"
                        )

                if content_blocks:
                    anthropic_messages.append(
                        MessageParam(
                            role=message.role,
                            content=content_blocks,
                        )
                    )
                continue

            # Handle ToolUseContent (assistant's tool calls)
            if isinstance(content, ToolUseContent):
                anthropic_messages.append(
                    MessageParam(
                        role="assistant",
                        content=[

View on GitHub (pinned to 1f02114297)

Solutions

Inspect `type(item).__name__` from the message to identify the offending block
Pre-process messages to convert/strip unsupported blocks (e.g. extract text from EmbeddedResource)
Update fastmcp-slim — newer versions may support the type
Report or subclass the handler to add conversion for the custom type

Example fix

// before
messages = [SamplingMessage(role="user", content=[EmbeddedResource(...)])]  # raises

// after
text = resource_to_text(EmbeddedResource(...))
messages = [SamplingMessage(role="user", content=[TextContent(type="text", text=text)])]

Defensive patterns

Strategy: type-guard

Validate before calling

ALLOWED = (TextContent, ImageContent, AudioContent, ToolResultContent)
bad = [b for m in messages for b in as_list(m.content) if not isinstance(b, ALLOWED)]
assert not bad, f"unsupported blocks: {[type(b).__name__ for b in bad]}"

Type guard

def is_anthropic_convertible(block) -> bool:
    from mcp.types import TextContent, ImageContent, AudioContent, ToolResultContent
    return isinstance(block, (TextContent, ImageContent, AudioContent, ToolResultContent))

Try / catch

try:
    return await handler(messages, params, ctx)
except ValueError as e:
    if e.args and e.args[0].startswith("Unsupported content type"):
        return await handler(convert_blocks(messages), params, ctx)
    raise

Prevention

Convert EmbeddedResource and custom blocks to TextContent before sampling
Pin fastmcp-slim versions consistent with the MCP types you emit
Sanitize message content in middleware before provider dispatch
Log the offending class name (it's in the message) to find the producing tool

When it happens

Trigger: A SamplingMessage's list content contains a block that is not TextContent, ImageContent, AudioContent, or ToolResultContent (e.g. EmbeddedResource, AudioContent subclasses, or a custom content type) passed to AnthropicSamplingHandler.

Common situations: Servers returning EmbeddedResource or other MCP content types in sampling contexts; custom middleware injecting non-standard blocks; version drift where newer MCP content types predate the converter.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/5d5e720f68d12ca2. Report an issue: GitHub.