PrefectHQ/fastmcp · error · ValueError

Elicitation response missing required 'value' field.

Error message

Elicitation response missing required 'value' field.

What it means

For raw schema forms (dict or nested-list shorthand like {"low": {"title": ...}} or [["a","b"]]), FastMCP expects the user's accepted response content to be an object with a single 'value' field. If the client's response content is not a dict or lacks 'value', handle_elicit_accept cannot extract the answer and raises.

Source

Thrown at fastmcp_slim/fastmcp/server/elicitation.py:325

    )


def handle_elicit_accept(
    config: ElicitConfig, content: Any
) -> AcceptedElicitation[Any]:
    """Handle an accepted elicitation response.

    Args:
        config: The elicitation configuration from parse_elicit_response_type
        content: The response content from the client

    Returns:
        AcceptedElicitation with the extracted/validated data
    """
    # For raw schemas (dict/nested-list syntax), extract value directly
    if config.is_raw:
        if not isinstance(content, dict) or "value" not in content:
            raise ValueError("Elicitation response missing required 'value' field.")
        return AcceptedElicitation[Any](data=content["value"])

    # For typed schemas, validate with Pydantic
    if config.response_type is not None:
        type_adapter = get_cached_typeadapter(config.response_type)
        validated_data = type_adapter.validate_python(content)
        if isinstance(validated_data, ScalarElicitationType):
            return AcceptedElicitation[Any](data=validated_data.value)
        return AcceptedElicitation[Any](data=validated_data)

    # For None response_type, expect empty response
    if content:
        raise ValueError(
            f"Elicitation expected an empty response, but received: {content}"
        )
    return AcceptedElicitation[dict[str, Any]](data={})

View on GitHub (pinned to 1f02114297)

Solutions

1. Check the responding client's elicitation implementation — it must wrap the answer in {"value": ...} per the generated schema
2. Inspect the raw response content (log it in middleware) to confirm what was actually returned
3. In tests, always return content of the form {"value": <answer>} for raw-syntax elicitations
4. If the response path is custom (proxy/middleware), stop altering the content payload

Example fix

// before
handle_elicit_accept(config, content=None)
// after
handle_elicit_accept(config, content={"value": "low"})

def has_value_content(content: object) -> bool:
    return isinstance(content, dict) and "value" in content

if not has_value_content(response.content):
    raise ValueError("client returned malformed elicitation content")

def is_valid_raw_content(content: object) -> bool:
    return isinstance(content, dict) and "value" in content

try:
    accepted = await ctx.elicit(response_type={"low": {"title": "Low"}})
except ValueError as e:
    if "missing required 'value'" in str(e):
        logger.warning("client returned malformed elicitation content; retrying with typed model")
        return None
    raise

When it happens

Trigger: ctx.elicit() with a dict or [[...]] response_type, then the accepted response's content is None, a scalar, or a dict without the 'value' key — typically a client that returned a malformed or non-standard accept payload.

Common situations: Custom or non-conforming MCP clients echoing back wrong content shapes; middleware or proxies transforming the elicitation response; writing tests that construct AcceptedElicitation content manually and forgetting the 'value' wrapper.

Understand the failure class

Background: "Missing required field" and "field is required" errors: why libraries reject payloads that omit mandatory fields — this error's family across 20 libraries.

Related errors

• Elicitation expected an empty response, but received: {conte
• ctx.elicit() requires a response_type. The empty-schema form
• response_title and response_description are only supported w
• Dict response_type cannot be empty.
• Invalid list response_type format. Received: {lst}

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/3287ffe16dadf9bf. Report an issue: GitHub.