When LangChain's @tool decorator is applied to a Python function, the function's documentation becomes part of the metadata shown to the language model. LangChain explicitly states that, by default, the function's docstring becomes the tool description , helping the model understand when the capability should be selected.

This metadata is operationally significant because tool selection is model-driven. A clear description communicates the tool's purpose, appropriate usage conditions, expected arguments, and semantic boundaries. LangChain's context-engineering guidance emphasizes that tool names, descriptions, argument names, and argument descriptions guide the model's reasoning about when and how a particular tool should be invoked.

Docstrings therefore affect agent reliability, not Python runtime performance. They do not determine which underlying model provider is used, and they have no cryptographic role in protecting function parameters. Poor or ambiguous descriptions can cause an LLM to choose an inappropriate tool or supply unsuitable arguments even when the Python implementation itself is technically correct.

Consequently, D precisely captures why docstrings are particularly important for @tool -defined LangChain functions. The uploaded source confirms the same answer.

Study Guide reference/topic: LangChain for AI Agents — @tool decorator, tool descriptions, docstrings, schemas, tool selection, and context engineering.

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