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What the interviewer is testing
Whether you understand the basic agent loop, why interleaving reasoning with tool results beats reasoning alone, and what controls a production loop needs.
Answer framework
- Define it: ReAct (reason plus act) interleaves a short reasoning step, an action such as a tool call, and an observation of the result, repeated until done.
- Why it helps: each step is grounded in real tool output instead of the model's memory, which reduces made-up facts.
- Relation to tool calling: modern APIs implement the action step as structured function calls.
- Controls: a step limit, a token or cost budget, a clear stop condition and error handling for failed tools.
- Alternatives: plan-then-execute when the steps are known up front.
Sample answer
ReAct is the loop most tool-using agents are built on. The model thinks briefly about what it needs, takes an action such as searching documentation or querying a database, reads the observation that comes back, and decides the next step. It repeats until it can answer or hits a limit.
The value is grounding. A model reasoning alone can drift into confident guesses. In ReAct, each step can be checked against real results: if the search shows the API was renamed, the next thought builds on that rather than on stale training data. The loop also makes behavior easier to debug, because the trace shows what the model tried and what it saw.
In production I implement the action step with native tool calling rather than parsing free text, and I wrap the loop with controls: a maximum number of steps, a cost budget, timeouts and retries for tools, detection of repeated identical calls, and a defined way to stop and hand off. For tasks where the steps are known in advance, I would use a plan-then-execute flow or a fixed workflow instead, since it is cheaper and more predictable.
Likely follow-ups
Common mistakes
- Describing ReAct as a model rather than a prompting and control pattern.
- Running the loop without step or cost limits.
- Parsing actions from free text when structured tool calls are available.
- Using an open-ended loop for a task with fixed steps.
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Related questions
Based on question #42 in the full question bank.