# Revision: Cheat Sheet and Self-Check — LangGraph Agents & Multi-Agent Systems

Source: https://www.geekswithgeeks.com/en/langgraph-agents/z-revision

> Review the key ideas of the whole course.

## Cheat sheet

**Model**: `StateGraph(State)`; nodes return partial updates; reducers (`operator.add`, `add_messages`) accumulate; `compile()` gives an app with `invoke`/`stream`. **Flow**: conditional edges for routing, loops with a termination rule and a cap, `recursion_limit` as the safety net, `Command(update, goto)` for dynamic hand-offs, `Send` for parallel map-reduce. **Agents**: model node ↔ tools node loop; validate tool names and arguments; narrow, well-documented tools with recoverable errors; step, token, time and cost limits. **Memory**: checkpointer + `thread_id` (short-term), store with namespaces (long-term), state history for replay and time travel, trim and summarise long histories. **Control**: `interrupt()` + `Command(resume=...)` need a checkpointer; `RetryPolicy` for transient errors; stream modes `updates`, `values`, `messages`, `custom`. **Multi-agent**: start with one agent; supervisor, hand-offs (guard against ping-pong), sub-graphs and hierarchies; they cost more. **Ship**: scripted-model tests, outcome + trajectory + safety evaluation, durable checkpointer, tracing with redaction, least privilege, approvals, kill switch.

**Quiz:** Two parallel nodes both write to the same state key in one step. What do you need?

- [ ] A bigger model
- [x] A reducer that defines how to merge the updates
- [ ] A new checkpointer
- [ ] Nothing, it always works

*Answer:* A reducer that defines how to merge the updates. Without a reducer, concurrent writes to one key conflict.

**Quiz:** Your agent sometimes loops forever calling the same tool. Which combination helps most?

- [ ] Removing the checkpointer
- [ ] A higher temperature
- [ ] More tools
- [x] A step cap in state, a recursion limit, and a clear stop condition

*Answer:* A step cap in state, a recursion limit, and a clear stop condition. Layered limits bound the loop even when the model never stops.

**Quiz:** What lets a paused interrupt survive a server restart?

- [x] A durable checkpointer storing the thread state
- [ ] Keeping the process in memory only
- [ ] A larger context window
- [ ] A faster GPU

*Answer:* A durable checkpointer storing the thread state. The saved checkpoint is what is resumed later, even by another process.