The entry node

The graph starts at a node, not at a field in the JSON header, and that node declares the pipeline's initial variables.

{
  "id": "start",
  "type": "entry",
  "variables": {
    "n": 5,
    "items": [1, 2, 3],
    "total.$": "vars.n * 2"
  },
  "next": "check"
}

• A name already present in the frame (session seed, a parent's inputs) is neither overwritten nor evaluated.
• Variables are bound in dependency order: an expression may reference a sibling variable of the same node, key order in the JSON is irrelevant, and a cycle is a validate() error.
• There is exactly one entry node per graph and jumping back into it is forbidden. The pipeline-level entry field is optional when such a node exists, and must point at it when given.
• Value types are checked statically, during validation.