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DeerFlow’s reflection system (backend/src/reflection/resolvers.py) enables dynamic loading of Python modules, classes, and variables from string paths. This powers the configuration-driven architecture, allowing users to add new models, tools, and providers without modifying code.

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Core Functions

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resolve_variable()

def resolve_variable[T](
    variable_path: str,
    expected_type: type[T] | tuple[type, ...] | None = None,
) -> T:
    """Resolve a variable from a path.
    
    Args:
        variable_path: Path to variable (e.g., "module.submodule:variable_name")
        expected_type: Optional type(s) to validate against (uses isinstance())
    
    Returns:
        The resolved variable.
    
    Raises:
        ImportError: If module path invalid or attribute doesn't exist.
        ValueError: If resolved variable doesn't match expected_type.
    """
Location: backend/src/reflection/resolvers.py:28-73

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resolve_class()

def resolve_class[T](
    class_path: str,
    base_class: type[T] | None = None
) -> type[T]:
    """Resolve a class from a module path.
    
    Args:
        class_path: Path to class (e.g., "langchain_openai:ChatOpenAI")
        base_class: Base class to validate subclass relationship
    
    Returns:
        The resolved class.
    
    Raises:
        ImportError: If module path invalid or attribute doesn't exist.
        ValueError: If resolved object is not a class or not a subclass of base_class.
    """
Location: backend/src/reflection/resolvers.py:76-98

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Path Format

Both functions use the format: module.path:attribute_name Examples: 
# Standard library
"os.path:join"  # → os.path.join function
"json:dumps"    # → json.dumps function

# Third-party packages
"langchain_openai:ChatOpenAI"      # → ChatOpenAI class
"langchain.chat_models:BaseChatModel"  # → BaseChatModel class

# DeerFlow modules
"src.tools.builtins:bash_tool"    # → bash_tool variable
"src.sandbox:LocalSandbox"         # → LocalSandbox class
"src.models.factory:create_chat_model"  # → create_chat_model function
Path Components:
  • Before :: Module path (using . separators)
  • After :: Attribute name (variable, class, or function)

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Usage Patterns

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1. Model Instantiation

Configuration (config.yaml): 
models:
  - name: gpt-4o
    use: langchain_openai:ChatOpenAI
    model: gpt-4o
    temperature: 0.7
Resolution (backend/src/models/factory.py:26): 
from src.reflection import resolve_class
from langchain.chat_models import BaseChatModel

# Load model config
model_config = get_app_config().get_model_config("gpt-4o")

# Resolve class dynamically
model_class = resolve_class(model_config.use, BaseChatModel)
# → imports langchain_openai
# → returns ChatOpenAI class
# → validates ChatOpenAI is subclass of BaseChatModel

# Instantiate with config parameters
model_instance = model_class(
    model="gpt-4o",
    temperature=0.7
)
Benefits:
  • Add new models without code changes
  • Type safety via base_class validation
  • Clear error messages for invalid configurations

​
2. Tool Loading

Configuration (config.yaml): 
tools:
  - name: tavily_search
    use: src.community.tavily.tools:tavily_search_tool
    group: web
  
  - name: bash
    use: src.sandbox.tools:bash_tool
    group: sandbox
Resolution (backend/src/tools/__init__.py): 
from src.reflection import resolve_variable
from langchain_core.tools import BaseTool

def load_tools_from_config(config):
    tools = []
    for tool_config in config.tools:
        # Resolve tool variable
        tool = resolve_variable(
            tool_config.use,
            expected_type=BaseTool  # Validate it's a LangChain tool
        )
        tools.append(tool)
    return tools

​
3. Sandbox Provider Selection

Configuration (config.yaml): 
sandbox:
  use: src.sandbox.local.provider:LocalSandboxProvider
  # or
  # use: src.community.aio_sandbox.provider:AioSandboxProvider
Resolution (backend/src/sandbox/__init__.py): 
from src.reflection import resolve_class
from src.sandbox.sandbox import SandboxProvider

def get_sandbox_provider() -> SandboxProvider:
    config = get_app_config()
    
    # Resolve provider class
    provider_class = resolve_class(
        config.sandbox.use,
        base_class=SandboxProvider
    )
    
    # Instantiate provider
    return provider_class(config.sandbox)

​
Error Handling

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Missing Module

try:
    tool = resolve_variable("nonexistent.module:tool")
except ImportError as e:
    print(e)
    # ImportError: Could not import module nonexistent.module.
    #              Missing dependency 'nonexistent'.
    #              Install with `uv add nonexistent`, then restart.

​
Missing Attribute

try:
    model = resolve_class("langchain_openai:NonexistentModel")
except ImportError as e:
    print(e)
    # ImportError: Module langchain_openai does not define a
    #              NonexistentModel attribute/class

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Type Validation Failure

try:
    # Expect a function, but got a class
    func = resolve_variable(
        "langchain_openai:ChatOpenAI",
        expected_type=Callable  # ChatOpenAI is a class, not callable
    )
except ValueError as e:
    print(e)
    # ValueError: langchain_openai:ChatOpenAI is not an instance of Callable,
    #             got type

​
Subclass Validation Failure

try:
    # Expect subclass of BaseChatModel, but got wrong type
    model_class = resolve_class(
        "langchain_core.tools:BaseTool",  # This is a tool, not a model
        base_class=BaseChatModel
    )
except ValueError as e:
    print(e)
    # ValueError: langchain_core.tools:BaseTool is not a subclass of
    #             BaseChatModel

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Dependency Hints

The reflection system provides actionable install hints for missing dependencies: 
MODULE_TO_PACKAGE_HINTS = {
    "langchain_google_genai": "langchain-google-genai",
    "langchain_anthropic": "langchain-anthropic",
    "langchain_openai": "langchain-openai",
    "langchain_deepseek": "langchain-deepseek",
}

def _build_missing_dependency_hint(module_path: str, err: ImportError) -> str:
    """Build actionable hint when module import fails."""
    module_root = module_path.split(".", 1)[0]
    missing_module = getattr(err, "name", None) or module_root
    
    # Prefer provider package hints for known integrations
    package_name = MODULE_TO_PACKAGE_HINTS.get(module_root)
    if package_name is None:
        package_name = MODULE_TO_PACKAGE_HINTS.get(
            missing_module,
            missing_module.replace("_", "-")
        )
    
    return (
        f"Missing dependency '{missing_module}'. "
        f"Install it with `uv add {package_name}` "
        f"(or `pip install {package_name}`), then restart DeerFlow."
    )
Example Error: 
ImportError: Could not import module langchain_google_genai.
Missing dependency 'google-generativeai'.
Install it with `uv add langchain-google-genai` (or `pip install langchain-google-genai`), then restart DeerFlow.

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Implementation Details

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Path Parsing

try:
    module_path, variable_name = variable_path.rsplit(":", 1)
except ValueError as err:
    raise ImportError(
        f"{variable_path} doesn't look like a variable path. "
        "Example: parent_package.sub_package.module:variable_name"
    ) from err
Validation:
  • Must contain exactly one : separator
  • Module path uses . separators (standard Python import format)
  • Attribute name cannot contain . or :

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Module Import

from importlib import import_module

try:
    module = import_module(module_path)
except ImportError as err:
    module_root = module_path.split(".", 1)[0]
    err_name = getattr(err, "name", None)
    
    if isinstance(err, ModuleNotFoundError) or err_name == module_root:
        # Missing module - provide install hint
        hint = _build_missing_dependency_hint(module_path, err)
        raise ImportError(f"Could not import module {module_path}. {hint}") from err
    else:
        # Other import error - preserve original message
        raise ImportError(f"Error importing module {module_path}: {err}") from err
Error Distinction:
  • ModuleNotFoundError: Package not installed → Install hint
  • Other ImportError: Syntax error, circular import, etc. → Original error

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Attribute Resolution

try:
    variable = getattr(module, variable_name)
except AttributeError as err:
    raise ImportError(
        f"Module {module_path} does not define a {variable_name} "
        "attribute/class"
    ) from err

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Type Validation (resolve_variable)

if expected_type is not None:
    if not isinstance(variable, expected_type):
        type_name = (
            expected_type.__name__
            if isinstance(expected_type, type)
            else " or ".join(t.__name__ for t in expected_type)
        )
        raise ValueError(
            f"{variable_path} is not an instance of {type_name}, "
            f"got {type(variable).__name__}"
        )
Supports:
  • Single type: expected_type=BaseTool
  • Multiple types: expected_type=(BaseTool, StructuredTool)
  • Uses isinstance() for validation

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Class Validation (resolve_class)

model_class = resolve_variable(class_path, expected_type=type)

if not isinstance(model_class, type):
    raise ValueError(f"{class_path} is not a valid class")

if base_class is not None and not issubclass(model_class, base_class):
    raise ValueError(
        f"{class_path} is not a subclass of {base_class.__name__}"
    )
Two-Step Validation:
  1. Verify resolved object is a class (using isinstance(obj, type))
  2. Verify class is subclass of base_class (using issubclass())

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Advanced Usage

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1. Dynamic Tool Registration

from src.reflection import resolve_variable
from langchain_core.tools import BaseTool

class ToolRegistry:
    def __init__(self):
        self._tools = {}
    
    def register_from_config(self, tool_configs):
        for config in tool_configs:
            tool = resolve_variable(
                config["use"],
                expected_type=BaseTool
            )
            self._tools[config["name"]] = tool
    
    def get_tool(self, name: str) -> BaseTool:
        return self._tools[name]

# Usage
registry = ToolRegistry()
registry.register_from_config([
    {"name": "search", "use": "src.community.tavily:tavily_search_tool"},
    {"name": "bash", "use": "src.sandbox.tools:bash_tool"}
])

search_tool = registry.get_tool("search")

​
2. Plugin System

from src.reflection import resolve_class

class Plugin:
    def execute(self):
        raise NotImplementedError

class PluginLoader:
    def load_plugin(self, plugin_path: str) -> Plugin:
        plugin_class = resolve_class(plugin_path, base_class=Plugin)
        return plugin_class()

# User's plugin
class MyPlugin(Plugin):
    def execute(self):
        print("My plugin executed!")

# Load dynamically
loader = PluginLoader()
plugin = loader.load_plugin("user.plugins:MyPlugin")
plugin.execute()

​
3. Configuration-Driven Middleware

from src.reflection import resolve_class
from langchain.agents.middleware import AgentMiddleware

def load_middlewares_from_config(config):
    middlewares = []
    for mw_config in config.middlewares:
        mw_class = resolve_class(
            mw_config["use"],
            base_class=AgentMiddleware
        )
        mw_instance = mw_class(**mw_config.get("params", {}))
        middlewares.append(mw_instance)
    return middlewares

# config.yaml
# middlewares:
#   - use: src.agents.middlewares:TitleMiddleware
#     params: {}
#   - use: src.agents.middlewares:MemoryMiddleware
#     params:
#       agent_name: null

​
Testing

import pytest
from src.reflection import resolve_variable, resolve_class
from langchain.chat_models import BaseChatModel

def test_resolve_variable_success():
    func = resolve_variable("os.path:join")
    assert callable(func)
    assert func("a", "b") == "a/b"  # or "a\\b" on Windows

def test_resolve_variable_with_type():
    from langchain_core.tools import BaseTool
    from src.sandbox.tools import bash_tool
    
    tool = resolve_variable(
        "src.sandbox.tools:bash_tool",
        expected_type=BaseTool
    )
    assert tool == bash_tool

def test_resolve_class_success():
    from langchain_openai import ChatOpenAI
    
    model_class = resolve_class(
        "langchain_openai:ChatOpenAI",
        base_class=BaseChatModel
    )
    assert model_class == ChatOpenAI
    assert issubclass(model_class, BaseChatModel)

def test_resolve_missing_module():
    with pytest.raises(ImportError, match="Install it with `uv add"):
        resolve_variable("nonexistent.module:variable")

def test_resolve_missing_attribute():
    with pytest.raises(ImportError, match="does not define"):
        resolve_variable("os:nonexistent_function")

def test_resolve_wrong_type():
    with pytest.raises(ValueError, match="not an instance of"):
        resolve_variable(
            "langchain_openai:ChatOpenAI",
            expected_type=int  # ChatOpenAI is a class, not an int
        )

def test_resolve_class_not_subclass():
    from langchain_core.tools import BaseTool
    
    with pytest.raises(ValueError, match="not a subclass of"):
        resolve_class(
            "langchain_core.tools:BaseTool",
            base_class=BaseChatModel  # BaseTool is not a chat model
        )

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Performance Considerations

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Caching

Reflection calls are typically executed once at startup, not per-request: 
# Good: Resolve once, reuse instance
class ModelFactory:
    def __init__(self, config):
        self._model_class = resolve_class(
            config.use,
            BaseChatModel
        )
    
    def create_model(self, **kwargs):
        return self._model_class(**kwargs)  # Reuse resolved class

# Bad: Resolve on every call
def create_model(config, **kwargs):
    model_class = resolve_class(config.use, BaseChatModel)  # Slow!
    return model_class(**kwargs)

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Import Cost

First import of a module may be slow (e.g., TensorFlow, PyTorch): 
# Strategy: Lazy loading
class LazyModel:
    def __init__(self, model_path: str):
        self._model_path = model_path
        self._model_class = None
    
    def _ensure_loaded(self):
        if self._model_class is None:
            self._model_class = resolve_class(
                self._model_path,
                BaseChatModel
            )
    
    def invoke(self, *args, **kwargs):
        self._ensure_loaded()  # Import on first use
        return self._model_class(*args, **kwargs)

​
Security Considerations

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Path Injection

Risk: User-controlled paths could import arbitrary modules. Mitigation: Validate paths against allowlist: 
ALLOWED_MODULE_PREFIXES = [
    "langchain",
    "langchain_",
    "src."
]

def resolve_safe(variable_path: str):
    module_path = variable_path.split(":")[0]
    
    if not any(module_path.startswith(prefix) for prefix in ALLOWED_MODULE_PREFIXES):
        raise ValueError(f"Module path not allowed: {module_path}")
    
    return resolve_variable(variable_path)

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Code Execution

Reflection enables arbitrary code execution if config is user-controlled. Best Practice: Load config from trusted sources only (local files, not user input).

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Best Practices

  1. Always validate types: 
    tool = resolve_variable(path, expected_type=BaseTool)
  2. Use base_class for classes: 
    model_class = resolve_class(path, base_class=BaseChatModel)
  3. Cache resolved values: 
    self._cached_class = resolve_class(path, BaseClass)
  4. Handle errors gracefully: 
    try:
    tool = resolve_variable(path)
except ImportError as e:
    logger.error(f"Failed to load tool: {e}")
    return default_tool
  5. Document expected types in config schema: 
    # config.yaml
tools:
  - name: my_tool
    use: src.tools:my_tool  # Must be a BaseTool instance

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See Also