LangGraph: Graph-Based State Machines

The loop IS the graph — the largest architectural divergence in the roster. Every other harness hides its loop in an implicit while-true. LangGraph makes the loop a declared, visible, testable, editable graph you draw before you run it. Module 1 (Loop): 5/5 — the reference. This is the framework for when the process is the product.

Super-step checkpoints are the Module 8 reference at 5/5. State is serialized at every node boundary — the granularity is the STEP, not the session. A crash resumes from the last completed node. For multi-day, multi-human workflows this is the difference between "resume from where you were" and "resume from approximately where you were."

interrupt() is the cleanest HITL primitive in the roster — structural, not behavioral. The edge from propose to execute goes through an interrupt node. The model cannot route around the approval because the model does not control the edges — the graph definition does. Same principle as NemoClaw (DD-09): enforcement outside the agent's reach.

Subgraphs provide declared multi-agent coordination — the C4 E09 pole. A multi-agent system is a graph of subgraphs: each subagent is a node, edges between subagents are declared transitions. LangGraph is the declared-coordination pole; CrewAI (DD-12) is the emergent-coordination pole. The contrast defines the multi-agent axis the way NemoClaw-vs-Tau defines the governance axis.