1Z0-1127-25 LangChain and AI Application Development Practice Question
A developer is using LangChain's LCEL to build a RAG pipeline. They want to add streaming of the final answer to the user. Which LCEL feature enables streaming output from the model?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Calling .stream() on the composed chain
The | operator in LCEL composes components but does not inherently stream. .stream() is a method on runnable objects that yields output chunks. .invoke() returns the full output. .batch() processes multiple inputs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Setting streaming=True in the model constructor
Why it's wrong here
Model constructors may support streaming, but in LCEL the chain's .stream() method is the standard way to stream the output.
- ✓
Calling .stream() on the composed chain
Why this is correct
.stream() allows the chain to yield output tokens as they are generated, enabling streaming to the user.
- ✗
Using .invoke() with a callable
Why it's wrong here
.invoke() returns the final output synchronously after completion, not streaming.
- ✗
The | operator between components
Why it's wrong here
The | operator composes a chain but does not enable streaming; it just pipes outputs to inputs.
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