diff --git a/src/json-crdt-extensions/mval/__tests__/extension.spec.ts b/src/json-crdt-extensions/mval/__tests__/extension.spec.ts
deleted file mode 100644
index 054f003820..0000000000
--- a/src/json-crdt-extensions/mval/__tests__/extension.spec.ts
+++ /dev/null
@@ -1,82 +0,0 @@
-import {ValueMvExt} from '..';
-import {konst} from '../../../json-crdt-patch/builder/Konst';
-import {Model} from '../../../json-crdt/model';
-
-test('can specify extension name', () => {
-  expect(ValueMvExt.name).toBe('mval');
-});
-
-test('can create a new multi-value register', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new(),
-  });
-  expect(model.view()).toEqual({
-    mv: [],
-  });
-});
-
-test('can provide initial value', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new({foo: 'bar'}),
-  });
-  expect(model.view()).toEqual({
-    mv: [{foo: 'bar'}],
-  });
-});
-
-test('does not treat ArrNode as extension if header is too long', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new(),
-  });
-  const buf = new Uint8Array(4);
-  buf.set(model.api.const(['mv', 0]).node.view() as Uint8Array, 0);
-  model.api.vec(['mv']).set([[0, buf]]);
-  expect(model.view()).toEqual({
-    mv: [buf, []],
-  });
-});
-
-test('does not treat ArrNode as extension if header sid is wrong', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new(),
-  });
-  const buf = model.api.const(['mv', 0]).node.view() as Uint8Array;
-  buf[1] += 1;
-  expect(model.view()).toEqual({
-    mv: [buf, []],
-  });
-});
-
-test('does not treat ArrNode as extension if header time is wrong', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new(),
-  });
-  const buf = model.api.const(['mv', 0]).node.view() as Uint8Array;
-  buf[2] += 1;
-  expect(model.view()).toEqual({
-    mv: [buf, []],
-  });
-});
-
-test('exposes API to edit extension data', () => {
-  const model = Model.withLogicalClock();
-  model.ext.register(ValueMvExt);
-  model.api.root({
-    mv: ValueMvExt.new(),
-  });
-  const nodeApi = model.api.in('mv').asExt(ValueMvExt);
-  nodeApi.set(konst('lol'));
-  expect(model.view()).toEqual({
-    mv: ['lol'],
-  });
-});
diff --git a/src/json-crdt/nodes/vec/VecNode.ts b/src/json-crdt/nodes/vec/VecNode.ts
index a374081da4..1c79e0e34f 100644
--- a/src/json-crdt/nodes/vec/VecNode.ts
+++ b/src/json-crdt/nodes/vec/VecNode.ts
@@ -71,6 +71,8 @@ export class VecNode<Value extends JsonNode[] = JsonNode[]> implements JsonNode<
 
   /**
    * @ignore
+   * @returns Returns the extension data node if this is an extension node,
+   *          otherwise `undefined`. The node is cached after the first access.
    */
   public ext(): JsonNode | undefined {
     if (this.__extNode) return this.__extNode;
@@ -92,6 +94,7 @@ export class VecNode<Value extends JsonNode[] = JsonNode[]> implements JsonNode<
 
   /**
    * @ignore
+   * @returns Returns extension ID if this is an extension node, otherwise -1.
    */
   public getExtId(): number {
     if (this.elements.length !== 2) return -1;
diff --git a/src/json-crdt/nodes/vec/__tests__/VecNode-extension.spec.ts b/src/json-crdt/nodes/vec/__tests__/VecNode-extension.spec.ts
new file mode 100644
index 0000000000..ac2c032f63
--- /dev/null
+++ b/src/json-crdt/nodes/vec/__tests__/VecNode-extension.spec.ts
@@ -0,0 +1,119 @@
+import {ITimestampStruct, delayed, s} from '../../../../json-crdt-patch';
+import {printTree} from '../../../../util/print/printTree';
+import {Printable} from '../../../../util/print/types';
+import {ext} from '../../../extensions';
+import {ExtensionApi, ExtensionDefinition, ExtensionJsonNode} from '../../../extensions/types';
+import {Model, NodeApi} from '../../../model';
+import {StrNode} from '../../nodes';
+import {JsonNode} from '../../types';
+
+test('treats a simple "vec" node as a vector', () => {
+  const model = Model.withLogicalClock();
+  model.api.root({
+    vec: s.vec(),
+  });
+  expect(model.view()).toEqual({
+    vec: [],
+  });
+  expect(model.api.vec(['vec']).node.isExt()).toBe(false);
+});
+
+test('does not treat "vec" node as extension, if extension is not registered in registry', () => {
+  const model = Model.withLogicalClock();
+  model.api.root({
+    vec: s.vec(),
+  });
+  const vec = model.api.vec(['vec']);
+  const buf = new Uint8Array(3);
+  buf[0] = 42;
+  buf[1] = vec.node.id.sid % 256;
+  buf[2] = vec.node.id.time % 256;
+  vec.push(buf, 123);
+  expect(vec.node.isExt()).toBe(false);
+});
+
+describe('sample extension', () => {
+  const DoubleConcatExt: ExtensionDefinition<StrNode, any, any> = {
+    id: 123,
+    name: 'double-concat',
+    new: (value: string = '') =>
+      ext(
+        123,
+        delayed((builder) => builder.json(value)),
+      ),
+    Node: class CntNode implements ExtensionJsonNode, Printable {
+      public readonly id: ITimestampStruct;
+
+      constructor(public readonly data: StrNode) {
+        this.id = data.id;
+      }
+
+      public name(): string {
+        return 'double-concat';
+      }
+
+      public view(): string {
+        const str = String(this.data.view());
+        return str + str;
+      }
+
+      public children(callback: (node: JsonNode) => void): void {}
+
+      public child?(): JsonNode | undefined {
+        return this.data;
+      }
+
+      public container(): JsonNode | undefined {
+        return this.data.container();
+      }
+
+      public api: undefined | unknown = undefined;
+
+      public toString(tab?: string): string {
+        return `${this.name()} (${this.view()})` + printTree(tab, [(tab) => this.data.toString(tab)]);
+      }
+    },
+    Api: class CntApi extends NodeApi<any> implements ExtensionApi<any> {
+      public ins(index: number, text: string): this {
+        const {api, node} = this;
+        const dataApi = api.wrap(node.data as StrNode);
+        dataApi.ins(index, text);
+        return this;
+      }
+    },
+  };
+
+  test('can run an extension', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(DoubleConcatExt);
+    model.api.root({
+      str: DoubleConcatExt.new('foo'),
+    });
+    expect(model.view()).toEqual({
+      str: 'foofoo',
+    });
+  });
+
+  test('can use extension node', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(DoubleConcatExt);
+    model.api.root({
+      str: DoubleConcatExt.new('abc'),
+    });
+    const extNode = model.api.in(['str']).asExt(DoubleConcatExt).node;
+    expect(extNode.view()).toBe('abcabc');
+  });
+
+  test('can use extension API node', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(DoubleConcatExt);
+    model.api.root({
+      str: DoubleConcatExt.new('abc'),
+    });
+    const extStr = model.api.in(['str']).asExt(DoubleConcatExt);
+    extStr.ins(1, '.');
+    expect(model.view()).toEqual({
+      str: 'a.bca.bc',
+    });
+  });
+});
diff --git a/src/json-crdt/nodes/vec/__tests__/extension.spec.ts b/src/json-crdt/nodes/vec/__tests__/extension.spec.ts
new file mode 100644
index 0000000000..8e6ae482cc
--- /dev/null
+++ b/src/json-crdt/nodes/vec/__tests__/extension.spec.ts
@@ -0,0 +1,98 @@
+import {ValueMvExt} from '../../../../json-crdt-extensions/mval';
+import {konst} from '../../../../json-crdt-patch/builder/Konst';
+import {Model} from '../../../../json-crdt/model';
+
+test('can specify extension name', () => {
+  expect(ValueMvExt.name).toBe('mval');
+});
+
+test('can create a new multi-value register', () => {
+  const model = Model.withLogicalClock();
+  model.ext.register(ValueMvExt);
+  model.api.root({
+    mv: ValueMvExt.new(),
+  });
+  expect(model.view()).toEqual({
+    mv: [],
+  });
+});
+
+test('can provide initial value', () => {
+  const model = Model.withLogicalClock();
+  model.ext.register(ValueMvExt);
+  model.api.root({
+    mv: ValueMvExt.new({foo: 'bar'}),
+  });
+  expect(model.view()).toEqual({
+    mv: [{foo: 'bar'}],
+  });
+});
+
+test('can read view from node or API node', () => {
+  const model = Model.withLogicalClock();
+  model.ext.register(ValueMvExt);
+  model.api.root({
+    mv: ValueMvExt.new('foo'),
+  });
+  const api = model.api.in('mv').asExt(ValueMvExt);
+  expect(api.view()).toEqual(['foo']);
+  expect(api.node.view()).toEqual(['foo']);
+});
+
+test('exposes API to edit extension data', () => {
+  const model = Model.withLogicalClock();
+  model.ext.register(ValueMvExt);
+  model.api.root({
+    mv: ValueMvExt.new(),
+  });
+  const nodeApi = model.api.in('mv').asExt(ValueMvExt);
+  nodeApi.set(konst('lol'));
+  expect(model.view()).toEqual({
+    mv: ['lol'],
+  });
+});
+
+describe('extension validity checks', () => {
+  test('does not treat ArrNode as extension if header is too long', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(ValueMvExt);
+    model.api.root({
+      mv: ValueMvExt.new(),
+    });
+    const buf = new Uint8Array(4);
+    buf.set(model.api.const(['mv', 0]).node.view() as Uint8Array, 0);
+    model.api.vec(['mv']).set([[0, buf]]);
+    expect(model.view()).toEqual({
+      mv: [buf, []],
+    });
+    expect(model.api.vec(['mv']).node.isExt()).toBe(false);
+  });
+
+  test('does not treat ArrNode as extension if header sid is wrong', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(ValueMvExt);
+    model.api.root({
+      mv: ValueMvExt.new(),
+    });
+    const buf = model.api.const(['mv', 0]).node.view() as Uint8Array;
+    buf[1] += 1;
+    expect(model.view()).toEqual({
+      mv: [buf, []],
+    });
+    expect(model.api.vec(['mv']).node.isExt()).toBe(false);
+  });
+
+  test('does not treat ArrNode as extension if header time is wrong', () => {
+    const model = Model.withLogicalClock();
+    model.ext.register(ValueMvExt);
+    model.api.root({
+      mv: ValueMvExt.new(),
+    });
+    const buf = model.api.const(['mv', 0]).node.view() as Uint8Array;
+    buf[2] += 1;
+    expect(model.view()).toEqual({
+      mv: [buf, []],
+    });
+    expect(model.api.vec(['mv']).node.isExt()).toBe(false);
+  });
+});