test_middleware_backwards_compat.py — langchain Source File

Source Code

"""Test backwards compatibility for middleware type parameters.

This file verifies that middlewares written BEFORE the ResponseT change still work.
All patterns that were valid before should remain valid.

Run type check: uv run --group typing mypy <this file>
Run tests: uv run --group test pytest <this file> -v
"""

from __future__ import annotations

from typing import TYPE_CHECKING, Any

import pytest
from langchain_core.language_models.fake_chat_models import GenericFakeChatModel
from langchain_core.messages import AIMessage, HumanMessage
from typing_extensions import TypedDict

from langchain.agents import create_agent
from langchain.agents.middleware.types import (
    AgentMiddleware,
    AgentState,
    ContextT,
    ModelRequest,
    ModelResponse,
    before_model,
)

if TYPE_CHECKING:
    from collections.abc import Awaitable, Callable

    from langgraph.runtime import Runtime


# =============================================================================
# OLD PATTERN 1: Completely unparameterized AgentMiddleware
# This was the most common pattern for simple middlewares
# =============================================================================
class OldStyleMiddleware1(AgentMiddleware):
    """Middleware with no type parameters at all - most common old pattern."""

    def before_model(self, state: AgentState[Any], runtime: Runtime[None]) -> dict[str, Any] | None:
        # Simple middleware that just logs or does something
        return None

    def wrap_model_call(
        self,
        request: ModelRequest,  # No type param
        handler: Callable[[ModelRequest], ModelResponse],  # No type params
    ) -> ModelResponse:  # No type param
        return handler(request)


# =============================================================================
# OLD PATTERN 2: AgentMiddleware with only 2 type parameters (StateT, ContextT)
# This was the pattern before ResponseT was added
# =============================================================================
class OldStyleMiddleware2(AgentMiddleware[AgentState[Any], ContextT]):
    """Middleware with 2 type params - the old signature before ResponseT."""

// ... (216 more lines)