diff --git a/app/src/lib/graph-interface/graph-state.svelte.test.ts b/app/src/lib/graph-interface/graph-state.svelte.test.ts
new file mode 100644
index 0000000..7e5f175
--- /dev/null
+++ b/app/src/lib/graph-interface/graph-state.svelte.test.ts
@@ -0,0 +1,285 @@
+import { assert, beforeEach, describe, expect, it } from 'vitest';
+import { GraphManager } from './graph-manager.svelte';
+import { GraphState } from './graph-state.svelte';
+import {
+  createMockNodeRegistry,
+  mockFloatInputNode,
+  mockFloatOutputNode
+} from './test-utils';
+
+// GraphState constructor reads localStorage synchronously — mock before any instantiation
+Object.defineProperty(globalThis, 'localStorage', {
+  value: {
+    getItem: () => null,
+    setItem: () => {},
+    removeItem: () => {},
+    clear: () => {},
+    length: 0,
+    key: () => null
+  } as Storage,
+  writable: true,
+  configurable: true
+});
+
+function createFixture() {
+  const registry = createMockNodeRegistry([mockFloatOutputNode, mockFloatInputNode]);
+  const manager = new GraphManager(registry);
+  const state = new GraphState(manager);
+  return { manager, state };
+}
+
+describe('clearSelection', () => {
+  it('empties selectedNodes', () => {
+    const { state } = createFixture();
+    state.selectedNodes.add(1);
+    state.selectedNodes.add(2);
+    state.clearSelection();
+    expect(state.selectedNodes.size).toBe(0);
+  });
+});
+
+describe('projectScreenToWorld', () => {
+  it('maps the viewport centre to the camera position', () => {
+    const { state } = createFixture();
+    // cameraPosition default: [140, 100, 3.5], width=100, height=100
+    state.width = 100;
+    state.height = 100;
+    state.cameraPosition = [140, 100, 3.5];
+    const [wx, wy] = state.projectScreenToWorld(50, 50);
+    expect(wx).toBeCloseTo(140);
+    expect(wy).toBeCloseTo(100);
+  });
+
+  it('offsets correctly for a point not at centre', () => {
+    const { state } = createFixture();
+    state.width = 100;
+    state.height = 100;
+    state.cameraPosition = [0, 0, 2];
+    const [wx, wy] = state.projectScreenToWorld(100, 50);
+    // x: 0 + (100 - 50) / 2 = 25
+    expect(wx).toBeCloseTo(25);
+    expect(wy).toBeCloseTo(0);
+  });
+});
+
+describe('groupSelectedNodes', () => {
+  it('delegates to graph.groupNodes with selected IDs and activeNodeId', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    const nodeB = manager.createNode({ type: 'test/node/input', position: [100, 0], props: {} });
+    assert.isDefined(nodeA);
+    assert.isDefined(nodeB);
+
+    state.selectedNodes.add(nodeA!.id);
+    state.activeNodeId = nodeB!.id;
+
+    const groupNode = state.groupSelectedNodes();
+    assert.isDefined(groupNode);
+
+    const graph = manager.serialize();
+    expect(graph.groups.length).toBe(1);
+    expect(graph.nodes.map(n => n.id)).toContain(groupNode!.id);
+  });
+
+  it('works when only activeNodeId is set with no extra selection', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(nodeA);
+
+    state.activeNodeId = nodeA!.id;
+    const groupNode = state.groupSelectedNodes();
+    assert.isDefined(groupNode);
+
+    expect(manager.graph.groups.length).toBe(1);
+  });
+});
+
+describe('enterGroupNode', () => {
+  it('does nothing when activeNodeId is -1', () => {
+    const { manager, state } = createFixture();
+    state.activeNodeId = -1;
+    state.enterGroupNode();
+    expect(manager.graphStack.length).toBe(0);
+  });
+
+  it('does nothing when the active node is not a group instance', () => {
+    const { manager, state } = createFixture();
+    const node = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(node);
+    state.activeNodeId = node!.id;
+    state.enterGroupNode();
+    expect(manager.graphStack.length).toBe(0);
+  });
+
+  it('enters the group, pushes graphStack, and clears UI state', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(nodeA);
+    const groupNode = manager.groupNodes([nodeA!.id]);
+    assert.isDefined(groupNode);
+
+    state.selectedNodes.add(nodeA!.id);
+    state.activeNodeId = groupNode!.id;
+    state.cameraPosition = [10, 20, 5];
+
+    state.enterGroupNode();
+
+    expect(manager.graphStack.length).toBe(1);
+    expect(state.activeNodeId).toBe(-1);
+    expect(state.selectedNodes.size).toBe(0);
+    expect(manager.isInsideGroup).toBe(true);
+  });
+});
+
+describe('exitGroupNode', () => {
+  it('does nothing when not inside a group', () => {
+    const { manager, state } = createFixture();
+    const before = [...state.cameraPosition];
+    state.exitGroupNode();
+    expect(manager.graphStack.length).toBe(0);
+    expect(state.cameraPosition).toEqual(before);
+  });
+
+  it('restores the camera position from before entry', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(nodeA);
+    const groupNode = manager.groupNodes([nodeA!.id]);
+    assert.isDefined(groupNode);
+
+    state.activeNodeId = groupNode!.id;
+    state.cameraPosition = [77, 88, 4];
+
+    state.enterGroupNode();
+    // Simulate camera moving inside the group
+    state.cameraPosition = [0, 0, 1];
+
+    state.exitGroupNode();
+
+    expect(state.cameraPosition).toEqual([77, 88, 4]);
+  });
+
+  it('clears activeNodeId and selection after exit', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(nodeA);
+    const groupNode = manager.groupNodes([nodeA!.id]);
+    assert.isDefined(groupNode);
+
+    state.activeNodeId = groupNode!.id;
+    state.enterGroupNode();
+    state.activeNodeId = 99;
+    state.selectedNodes.add(99);
+
+    state.exitGroupNode();
+
+    expect(state.activeNodeId).toBe(-1);
+    expect(state.selectedNodes.size).toBe(0);
+  });
+
+  it('restores outer nodes to manager after exit', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    const nodeB = manager.createNode({ type: 'test/node/input', position: [100, 0], props: {} });
+    assert.isDefined(nodeA);
+    assert.isDefined(nodeB);
+
+    manager.createEdge(nodeA!, 0, nodeB!, 'value');
+    const groupNode = manager.groupNodes([nodeA!.id]);
+    assert.isDefined(groupNode);
+
+    state.activeNodeId = groupNode!.id;
+    state.enterGroupNode();
+
+    // Inside the group: nodeA is an internal node so it IS active; the outer
+    // nodes (nodeB, groupNode) are saved and no longer in the active Map.
+    expect(manager.nodes.has(nodeA!.id)).toBe(true);
+    expect(manager.nodes.has(nodeB!.id)).toBe(false);
+
+    state.exitGroupNode();
+
+    // After exit: outer nodes are restored
+    expect(manager.nodes.has(nodeB!.id)).toBe(true);
+    expect(manager.nodes.has(groupNode!.id)).toBe(true);
+    expect(manager.isInsideGroup).toBe(false);
+  });
+
+  it('isInsideGroup is false after exiting the only group level', () => {
+    const { manager, state } = createFixture();
+
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    assert.isDefined(nodeA);
+    const groupNode = manager.groupNodes([nodeA!.id]);
+    assert.isDefined(groupNode);
+
+    state.activeNodeId = groupNode!.id;
+    state.enterGroupNode();
+    expect(manager.isInsideGroup).toBe(true);
+
+    state.exitGroupNode();
+    expect(manager.isInsideGroup).toBe(false);
+  });
+});
+
+describe('copyNodes / pasteNodes', () => {
+  it('copies the active node into the clipboard', () => {
+    const { manager, state } = createFixture();
+    const node = manager.createNode({ type: 'test/node/output', position: [10, 20], props: {} });
+    assert.isDefined(node);
+
+    state.activeNodeId = node!.id;
+    state.mousePosition = [0, 0];
+    state.copyNodes();
+
+    assert.isNotNull(state.clipboard);
+    expect(state.clipboard!.nodes.map(n => n.id)).toContain(node!.id);
+  });
+
+  it('includes edges between copied nodes', () => {
+    const { manager, state } = createFixture();
+    const nodeA = manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    const nodeB = manager.createNode({ type: 'test/node/input', position: [100, 0], props: {} });
+    assert.isDefined(nodeA);
+    assert.isDefined(nodeB);
+
+    manager.createEdge(nodeA!, 0, nodeB!, 'value');
+
+    state.activeNodeId = nodeA!.id;
+    state.selectedNodes.add(nodeB!.id);
+    state.mousePosition = [0, 0];
+    state.copyNodes();
+
+    assert.isNotNull(state.clipboard);
+    expect(state.clipboard!.edges.length).toBe(1);
+  });
+
+  it('pastes nodes and adds them to the graph', () => {
+    const { manager, state } = createFixture();
+    const node = manager.createNode({ type: 'test/node/output', position: [10, 20], props: {} });
+    assert.isDefined(node);
+
+    state.activeNodeId = node!.id;
+    state.mousePosition = [0, 0];
+    state.copyNodes();
+
+    const countBefore = manager.nodes.size;
+    state.mousePosition = [50, 50];
+    state.pasteNodes();
+
+    expect(manager.nodes.size).toBe(countBefore + 1);
+  });
+
+  it('does nothing when clipboard is empty', () => {
+    const { manager, state } = createFixture();
+    manager.createNode({ type: 'test/node/output', position: [0, 0], props: {} });
+    const countBefore = manager.nodes.size;
+    state.pasteNodes();
+    expect(manager.nodes.size).toBe(countBefore);
+  });
+});