FakeChatModel with Tools

[qvalentin]

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Example Code

# Passing the following model to `create_react_agent` with tools does
# not work because `GenericFakeChatModel` has no bind_tools
# and also does not handle the `tool_calls`in  `AIMessage`.


from langchain_core.language_models import GenericFakeChatModel, LanguageModelInput

GenericFakeChatModel(
            messages=iter([
                AIMessage(content="42"),
                AIMessage(
                    content="I want to call a tool",
                    tool_calls=[
                        ToolCall(
                            id="tool_call_1",
                            name="HelloTool",
                            args={"name": "World"},
                        )
                    ],
                ),
            ])
        )

Description

• I want to test my agent using the predefined FakeChatModels (GenericFakeChatModel)
• The GenericFakeChatModel does not seem to support tools, but it seems to support function_calls with additional_kwargs (https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/language_models/fake_chat_models.py#L304)

My questions: Is there another FakeChatModel I can use for such tests? Should GenericFakeChatModel be extended to also handle Tools? How could you extend GenericFakeChatModel? (I'm open to contributing)

System Info

System Information

OS: Darwin
OS Version: Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:49 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6000
Python Version: 3.13.5 (main, Jun 11 2025, 15:36:57) [Clang 17.0.0 (clang-1700.0.13.3)]

Package Information

langchain_core: 0.3.60
langsmith: 0.3.42
langchain_azure_ai: 0.1.2
langchain_google_vertexai: 2.0.24
langchain_mcp_adapters: 0.1.4
langchain_openai: 0.3.17
langgraph_sdk: 0.1.69

Optional packages not installed

langserve

Other Dependencies

aiohttp: 3.11.18
anthropic[vertexai]: Installed. No version info available.
azure-ai-inference[opentelemetry]: Installed. No version info available.
azure-core: 1.34.0
azure-cosmos: 4.9.0
azure-identity: 1.22.0
azure-monitor-opentelemetry: Installed. No version info available.
bottleneck: 1.5.0
google-cloud-aiplatform: 1.93.1
google-cloud-storage: 2.19.0
httpx: 0.28.1
httpx-sse: 0.4.0
jsonpatch<2.0,>=1.33: Installed. No version info available.
langchain-core<0.4,>=0.3.36: Installed. No version info available.
langchain-core<1.0.0,>=0.3.59: Installed. No version info available.
langchain-mistralai: Installed. No version info available.
langsmith-pyo3: Installed. No version info available.
langsmith<0.4,>=0.1.126: Installed. No version info available.
mcp>=1.9.1: Installed. No version info available.
numexpr: 2.10.2
numpy: 2.2.5
openai-agents: Installed. No version info available.
openai<2.0.0,>=1.68.2: Installed. No version info available.
opentelemetry-api: 1.31.1
opentelemetry-exporter-otlp-proto-http: 1.31.1
opentelemetry-instrumentation-threading: 0.52b1
opentelemetry-sdk: 1.31.1
opentelemetry-semantic-conventions-ai: 0.4.8
orjson: 3.10.18
packaging: 24.2
packaging<25,>=23.2: Installed. No version info available.
pyarrow: 19.0.1
pydantic: 2.11.4
pydantic>=2.7.4: Installed. No version info available.
pytest: 8.3.5
PyYAML>=5.3: Installed. No version info available.
requests: 2.32.3
requests-toolbelt: 1.0.0
rich: 14.0.0
tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
tiktoken<1,>=0.7: Installed. No version info available.
typing-extensions>=4.7: Installed. No version info available.
validators: 0.35.0
zstandard: 0.23.0