How does routing work in LCEL?

Routing in LCEL means directing an input to one of several sub-chains based on a condition. The two main tools are RunnableBranch (declarative) and a plain Python function returning a Runnable (imperative).

RunnableBranch — takes a list of (condition, runnable) pairs and a default. The first condition that evaluates to True determines which runnable handles the input:

from langchain_core.runnables import RunnableBranch

router = RunnableBranch(
    (lambda x: "sql" in x["topic"].lower(), sql_chain),
    (lambda x: "python" in x["topic"].lower(), python_chain),
    general_chain,  # default
)
result = router.invoke({"topic": "How do I write a SQL JOIN?"})
# Routes to sql_chain

Lambda-based routing — a custom function that returns the appropriate runnable based on the classification output from an earlier chain step:

def route(info):
    if info["topic"] == "science":
        return science_chain
    return general_chain

full_chain = classify_chain | RunnableLambda(route)

A common production pattern is to first run a fast, cheap classification chain that returns a topic label, then route to specialised chains accordingly. This avoids sending every request through a heavyweight model.

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