diff --git a/src/model/factory.ts b/src/model/factory.ts
new file mode 100644
index 0000000..f66bf3c
--- /dev/null
+++ b/src/model/factory.ts
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+/**
+ * Pure constructors for Model pieces: id generation, a blank Model, and the
+ * auto-naming that lets F1 place a node with zero dialogs. No store, no Vue —
+ * just data in, data out, so it stays unit-testable.
+ */
+import {
+  type CloudNode,
+  type ConverterNode,
+  type FlowNode,
+  type Model,
+  MODEL_VERSION,
+  type ModelNode,
+  type NodeKind,
+  type Position,
+  type StockNode,
+} from "./types"
+
+/** Short, readable, collision-resistant id. `crypto.randomUUID` is built in. */
+export function newId(prefix: string): string {
+  return `${prefix}-${crypto.randomUUID().slice(0, 8)}`
+}
+
+/** Human label per kind, used for auto-naming. Clouds are unnamed boundaries. */
+const LABEL: Record<Exclude<NodeKind, "cloud">, string> = {
+  stock: "Stock",
+  flow: "Flow",
+  converter: "Converter",
+}
+
+export function emptyModel(name = "Untitled model"): Model {
+  return { version: MODEL_VERSION, id: newId("model"), name, nodes: [], infoLinks: [] }
+}
+
+/**
+ * Next free auto-name for `kind`, e.g. "Stock 3". Picks one past the highest
+ * trailing number already in use so it stays stable across deletes/renames.
+ */
+export function nextName(nodes: ModelNode[], kind: Exclude<NodeKind, "cloud">): string {
+  const label = LABEL[kind]
+  const pattern = new RegExp(`^${label} (\\d+)$`)
+  let max = 0
+  for (const node of nodes) {
+    if (node.kind !== kind || !("name" in node)) continue
+    const match = pattern.exec(node.name)
+    if (match) max = Math.max(max, Number(match[1]))
+  }
+  return `${label} ${max + 1}`
+}
+
+export function makeStock(position: Position, name: string): StockNode {
+  return { id: newId("stock"), kind: "stock", name, position }
+}
+
+export function makeConverter(position: Position, name: string): ConverterNode {
+  return { id: newId("conv"), kind: "converter", name, position }
+}
+
+export function makeFlow(
+  position: Position,
+  name: string,
+  source: string,
+  target: string,
+): FlowNode {
+  return { id: newId("flow"), kind: "flow", name, source, target, position }
+}
+
+export function makeCloud(position: Position): CloudNode {
+  return { id: newId("cloud"), kind: "cloud", position }
+}
+
+/** Midpoint between two positions — where a Flow's valve sits on its pipe. */
+export function midpoint(a: Position, b: Position): Position {
+  return { x: (a.x + b.x) / 2, y: (a.y + b.y) / 2 }
+}
diff --git a/src/model/types.ts b/src/model/types.ts
new file mode 100644
index 0000000..89db7ff
--- /dev/null
+++ b/src/model/types.ts
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+/**
+ * The domain Model — the single source of truth for a meadows document.
+ *
+ * The Vue Flow graph is a derived projection of this (ADR-0002); the future
+ * simulator reads this and knows nothing of the view layer. Everything here is
+ * plain, serialisable data (no class instances, no functions) so it round-trips
+ * as JSON (F8) and deep-clones cheaply for undo snapshots (F9).
+ *
+ * Vocabulary follows CONTEXT.md. Structural rules follow the ADRs:
+ *  - A Flow is a *node* with `source`/`target` references, not an edge (ADR-0003).
+ *  - Source/Sink boundaries are materialised Cloud nodes, so no end is ever null.
+ *  - The only stored edge type is the Information Link; loops are detected, never
+ *    stored (ADR-0001).
+ */
+
+/** Schema version stamped on every Model for round-trip migrations (F8). */
+export const MODEL_VERSION = 1
+
+/** The four node kinds. A `kind` field discriminates the `ModelNode` union. */
+export type NodeKind = "stock" | "flow" | "converter" | "cloud"
+
+/**
+ * The sign on an Information Link (and inherent to a Flow): `+` when more cause
+ * means more effect, `−` when more cause means less effect. The input that lets
+ * loops be classified Reinforcing/Balancing.
+ */
+export type Polarity = "+" | "-"
+
+export interface Position {
+  x: number
+  y: number
+}
+
+interface BaseNode {
+  id: string
+  position: Position
+}
+
+/**
+ * An accumulation that holds a quantity over time — the system's memory and the
+ * only stateful element. Changes only through Flows.
+ */
+export interface StockNode extends BaseNode {
+  kind: "stock"
+  name: string
+  /** Initial accumulated quantity. Optional in the diagram phase; the simulator reads it. */
+  initialValue?: number
+}
+
+/**
+ * A rate that moves quantity between two ends. A Flow is a node (ADR-0003): the
+ * pipe-with-valve you see is rendered from `source`/`target`. Inflow/outflow is
+ * relative to a given Stock, not a separate type — a stock→stock Flow is an
+ * outflow for `source` and an inflow for `target`.
+ */
+export interface FlowNode extends BaseNode {
+  kind: "flow"
+  name: string
+  /** Node id of the Stock or Cloud the Flow draws from. */
+  source: string
+  /** Node id of the Stock or Cloud the Flow feeds into. */
+  target: string
+  /** Rate expression, recomputed each instant. Optional in the diagram phase. */
+  equation?: string
+}
+
+/**
+ * A stateless helper value, recomputed each instant from its inputs (or a fixed
+ * constant). Feeds Flows or other Converters via Information Links.
+ */
+export interface ConverterNode extends BaseNode {
+  kind: "converter"
+  name: string
+  /** Expression or constant. Optional in the diagram phase. */
+  equation?: string
+}
+
+/**
+ * A model boundary (Source/Sink) on an open Flow end — drawn as a cloud.
+ * Auto-managed: spawned when a Flow is left open-ended, removed when the Flow
+ * attaches to a Stock (ADR-0003). Carries no name and no value.
+ */
+export interface CloudNode extends BaseNode {
+  kind: "cloud"
+}
+
+export type ModelNode = StockNode | FlowNode | ConverterNode | CloudNode
+
+/**
+ * Shows that one element's value influences a Flow's or Converter's value.
+ * Carries a Polarity; carries no quantity and never targets a Stock (a Stock
+ * changes only via Flows).
+ */
+export interface InformationLink {
+  id: string
+  /** Source node id — a Stock, Flow, or Converter (the value being read). */
+  source: string
+  /** Target node id — a Flow or Converter, never a Stock (ADR-0001). */
+  target: string
+  polarity: Polarity
+}
+
+/** One saved document: the unit of save / export / reopen. */
+export interface Model {
+  version: number
+  id: string
+  name: string
+  nodes: ModelNode[]
+  infoLinks: InformationLink[]
+}