[feat][graph_workflow][sub-workflow composition]

Problem

GraphWorkflow nodes are always individual agents. There is no way to use one GraphWorkflow as a node inside another, which rules out reusable graph modules and multi-level orchestration.

Proposed feature

A node can itself be a GraphWorkflow. The outer graph treats the inner graph as a single black-box node whose input is the node's upstream payload and whose output is the inner graph's final result.

Design sketch

• GraphWorkflow.add_node(x) accepts either an Agent or another GraphWorkflow.
• Inner graph's inputs are merged upstream outputs; inner graph's final layer output becomes the node's output.
• Topology serialization (save_spec() / from_topology_spec()) is extended to support nested specs — the inner graph's spec is embedded as {"type": "subgraph", "spec": {...}}.
• Checkpoint format stores inner-graph state under the parent node's ID for clean restart.

Tradeoff

Checkpoint format has to handle nesting. Nested topology serialization adds a small amount of parsing complexity at load time.

Files

• swarms/structs/graph_workflow.py

Why

Enables reusable graph modules — a "research pipeline" subgraph can be dropped into multiple outer workflows. Also pairs with topology serialization for shareable, composable graph libraries.