What causes "Return type {type(return_value)} not in return types {return_types}" in microsoft/semantic-kernel?

Make every return path produce one of the declared return types.

microsoft/semantic-kernel · error · ValueError

Return type not in return types

Error message

Return type {type(return_value)} not in return types {return_types}

What it means

Raised at runtime by the @message_handler wrapper (strict mode) when the handler returns a value whose concrete type is not among its declared return types and the return is not typed Any. The runtime uses return-type metadata to route replies, so an undeclared return type breaks the contract.

Solutions

Make every return path produce one of the declared return types.
Widen the return annotation to a Union covering all returned types, or to Any.
Set strict=False to demote to a warning (only when acceptable).
Ensure error paths raise rather than returning an undeclared type.

Example fix

// before
@message_handler
async def handle(self, message: Req, ctx: MessageContext) -> Resp:
    if not ok:
        return None   # undeclared -> ValueError
    return Resp(...)
// after
@message_handler
async def handle(self, message: Req, ctx: MessageContext) -> Resp:
    if not ok:
        raise ValueError("not ok")
    return Resp(...)

Defensive patterns

Strategy: try-catch

Validate before calling

from semantic_kernel.agents.runtime.core.type_helpers import AnyType

def return_in_declared_types(value, return_types) -> bool:
    return AnyType in return_types or type(value) in return_types

Type guard

from typing import Any
from semantic_kernel.agents.runtime.core.type_helpers import AnyType

def return_matches_declared(value: Any, return_types) -> bool:
    return AnyType in return_types or type(value) in return_types

Try / catch

try:
    await handler.run(message, ctx)
except ValueError as e:
    if "Return type" in str(e) and "not in return types" in str(e):
        # fix the handler's return paths or widen the annotation
        raise
    raise

Prevention

Ensure every return path yields a declared return type.
Widen the return annotation to a Union or Any if multiple types are possible.
Raise on error paths instead of returning an undeclared type (e.g. None).
Set strict=False only if downgrading to a warning is acceptable.

When it happens

Trigger: A handler declared to return ResponseA actually returns ResponseB (or None when a response was declared), with strict=True. Common when the handler logic has multiple return paths returning different types.

Common situations: A bug in the handler returning None on an error path while a response type is declared; returning a subclass whose exact type() differs; refactoring the return type without updating the annotation.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/2d7e1dc69b0016cb. Report an issue: GitHub.

Appendix: source

Thrown at python/semantic_kernel/agents/runtime/core/routed_agent.py:160

        return_types = get_types(type_hints["return"])

        if return_types is None:
            raise AssertionError("Return type not found")

        # Convert target_types to list and stash

        @wraps(func)
        async def wrapper(self: AgentT, message: ReceivesT, ctx: MessageContext) -> ProducesT:
            if type(message) not in target_types:
                if strict:
                    raise CantHandleException(f"Message type {type(message)} not in target types {target_types}")
                logger.warning(f"Message type {type(message)} not in target types {target_types}")

            return_value = await func(self, message, ctx)

            if AnyType not in return_types and type(return_value) not in return_types:
                if strict:
                    raise ValueError(f"Return type {type(return_value)} not in return types {return_types}")
                logger.warning(f"Return type {type(return_value)} not in return types {return_types}")

            return return_value

        wrapper_handler = cast(MessageHandler[AgentT, ReceivesT, ProducesT], wrapper)
        wrapper_handler.target_types = list(target_types)
        wrapper_handler.produces_types = list(return_types)
        wrapper_handler.is_message_handler = True
        wrapper_handler.router = match or (lambda _message, _ctx: True)

        return wrapper_handler

    if func is None and not callable(func):
        return decorator
    if callable(func):
        return decorator(func)
    raise ValueError("Invalid arguments")

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