PrefectHQ/fastmcp · error · ValueError

No content in response from Anthropic

Error message

No content in response from Anthropic

What it means

_message_to_create_message_result raises ValueError when the Anthropic API returns a Message whose content list is empty. The handler must produce a CreateMessageResult with content, so an empty response cannot be converted and is surfaced as an explicit error.

Source

Thrown at fastmcp_slim/fastmcp/client/sampling/handlers/anthropic.py:340

                        content=[_image_content_to_anthropic_block(content)],
                    )
                )
                continue

            # Handle AudioContent - not supported by Anthropic
            if isinstance(content, AudioContent):
                raise ValueError("AudioContent is not supported by the Anthropic API")

            raise ValueError(f"Unsupported content type: {type(content)}")

        return anthropic_messages

    @staticmethod
    def _message_to_create_message_result(
        message: Message,
    ) -> CreateMessageResult:
        if len(message.content) == 0:
            raise ValueError("No content in response from Anthropic")

        # Join all text blocks to avoid dropping content
        text = "".join(
            block.text for block in message.content if isinstance(block, TextBlock)
        )
        if text:
            return CreateMessageResult(
                content=TextContent(type="text", text=text),
                role="assistant",
                model=message.model,
            )

        raise ValueError(
            f"No text content in response from Anthropic: {[type(b).__name__ for b in message.content]}"
        )

    def _select_model_from_preferences(
        self, model_preferences: ModelPreferences | str | list[str] | None

View on GitHub (pinned to 1f02114297)

Solutions

1. Increase max_tokens in the sampling handler config so the model can emit at least one content block.
2. Catch ValueError in the sampling callback and return a fallback CreateMessageResult or an error to the server.
3. Retry the request once — transient empty responses can occur during API incidents.
4. Log message.stop_reason to diagnose why the model returned nothing before retrying.

Example fix

// before
result = await handler(messages, params, context)
// after
try:
    result = await handler(messages, params, context)
except ValueError as e:
    if "No content" in str(e):
        return CreateMessageResult(content=TextContent(type="text", text="Model returned no content"), role="assistant", model=params.modelPreferences.model_hint if params.modelPreferences else "unknown")
    raise

null

null

try:
    result = await handler(messages, params, context)
except ValueError as e:
    if "No content in response from Anthropic" in str(e):
        result = CreateMessageResult(content=TextContent(type="text", text="(empty model response)"), role="assistant", model="unknown")
    else:
        raise

When it happens

Trigger: Anthropic returns a Message with content=[] — e.g. the model produced no blocks (often after a refusal, filtering, or a max_tokens stop with zero output).

Common situations: Hitting max_tokens=1 or very small limits; content filtering by Anthropic; unusual model parameters (temperature/top_p edge settings); API quirks during outages.

Related errors

• No response for completion
• Unsupported image MIME type for Anthropic: {content.mime_typ
• ImageContent is only supported in user messages for Anthropi
• AudioContent is not supported by the Anthropic API
• Unsupported content type for Anthropic: {type(item).__name__

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/092f2a6956edc1ec. Report an issue: GitHub.