tool_emulator.py — langchain Source File

Source Code

"""Tool emulator middleware for testing."""

from __future__ import annotations

from typing import TYPE_CHECKING, Any, Generic

from langchain_core.language_models.chat_models import BaseChatModel
from langchain_core.messages import HumanMessage, ToolMessage

from langchain.agents.middleware.types import AgentMiddleware, AgentState, ContextT
from langchain.chat_models.base import init_chat_model

if TYPE_CHECKING:
    from collections.abc import Awaitable, Callable

    from langgraph.types import Command

    from langchain.agents.middleware.types import ToolCallRequest
    from langchain.tools import BaseTool


class LLMToolEmulator(AgentMiddleware[AgentState[Any], ContextT], Generic[ContextT]):
    """Emulates specified tools using an LLM instead of executing them.

    This middleware allows selective emulation of tools for testing purposes.

    By default (when `tools=None`), all tools are emulated. You can specify which
    tools to emulate by passing a list of tool names or `BaseTool` instances.

    Examples:
        !!! example "Emulate all tools (default behavior)"

            ```python
            from langchain.agents.middleware import LLMToolEmulator

            middleware = LLMToolEmulator()

            agent = create_agent(
                model="openai:gpt-4o",
                tools=[get_weather, get_user_location, calculator],
                middleware=[middleware],
            )
            ```

        !!! example "Emulate specific tools by name"

            ```python
            middleware = LLMToolEmulator(tools=["get_weather", "get_user_location"])
            ```

        !!! example "Use a custom model for emulation"

            ```python
            middleware = LLMToolEmulator(
                tools=["get_weather"], model="anthropic:claude-sonnet-4-5-20250929"
            )
            ```

        !!! example "Emulate specific tools by passing tool instances"

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