diff --git a/packages/editor/src/canvas-protocol/canvas-client.test.ts b/packages/editor/src/canvas-protocol/canvas-client.test.ts
new file mode 100644
index 0000000..b3101ad
--- /dev/null
+++ b/packages/editor/src/canvas-protocol/canvas-client.test.ts
@@ -0,0 +1,278 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { PROTOCOL_VERSION } from "@repo/core-runner-protocol";
+import {
+  createCanvasClient,
+  type CanvasClientHandlers,
+} from "@/canvas-protocol/canvas-client";
+import { ScriptedAgentDouble } from "@/canvas-protocol/scripted-agent.mock";
+import { useEditorStore } from "@/store/editor-store";
+
+const ADAPTER_ORIGIN = "https://adapter.veect.test:5199";
+const EVIL_ORIGIN = "https://evil.example.com";
+
+function setup(handlers: CanvasClientHandlers = {}) {
+  const agent = new ScriptedAgentDouble({ origin: ADAPTER_ORIGIN });
+  const client = createCanvasClient({
+    adapterOrigin: ADAPTER_ORIGIN,
+    getAgentWindow: () => agent.agentWindow,
+    editorWindow: agent.editorWindow,
+    handlers,
+  });
+  return { agent, client };
+}
+
+describe("createCanvasClient — origin pinning", () => {
+  it('rejects "*" as the adapter origin — wildcard defeats pinning', () => {
+    expect(() =>
+      createCanvasClient({
+        adapterOrigin: "*",
+        getAgentWindow: () => null,
+      }),
+    ).toThrow(TypeError);
+  });
+
+  it("rejects a non-URL origin", () => {
+    expect(() =>
+      createCanvasClient({
+        adapterOrigin: "not-an-origin",
+        getAgentWindow: () => null,
+      }),
+    ).toThrow(TypeError);
+  });
+
+  it("rejects an origin with a path — must be a bare origin", () => {
+    expect(() =>
+      createCanvasClient({
+        adapterOrigin: "https://adapter.veect.test:5199/frame",
+        getAgentWindow: () => null,
+      }),
+    ).toThrow(TypeError);
+  });
+
+  it("posts every outbound message with the pinned targetOrigin", () => {
+    const { agent, client } = setup();
+    client.start();
+    client.renderFrame("button");
+    client.requestGeometry();
+    expect(agent.posted).toHaveLength(2);
+    for (const post of agent.posted) {
+      expect(post.targetOrigin).toBe(ADAPTER_ORIGIN);
+    }
+  });
+
+  it("ignores inbound runtime.ready from a wrong origin", () => {
+    const onRuntimeReady = vi.fn();
+    const { agent, client } = setup({ onRuntimeReady });
+    client.start();
+    agent.postRuntimeReady({ origin: EVIL_ORIGIN });
+    expect(onRuntimeReady).not.toHaveBeenCalled();
+    agent.postRuntimeReady();
+    expect(onRuntimeReady).toHaveBeenCalledTimes(1);
+  });
+
+  it("ignores inbound click.target from a wrong origin", () => {
+    const onClick = vi.fn();
+    const { agent, client } = setup({ onClick });
+    client.start();
+    agent.postClick("el-1", { origin: EVIL_ORIGIN });
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("ignores events from a different source window on the right origin", () => {
+    const onClick = vi.fn();
+    const { agent, client } = setup({ onClick });
+    client.start();
+    agent.postClick("el-1", { source: { postMessage: vi.fn() } });
+    expect(onClick).not.toHaveBeenCalled();
+  });
+});
+
+describe("createCanvasClient — inbound message handling", () => {
+  it("dispatches runtime.ready to onRuntimeReady", () => {
+    const onRuntimeReady = vi.fn();
+    const { agent, client } = setup({ onRuntimeReady });
+    client.start();
+    agent.postRuntimeReady();
+    expect(onRuntimeReady).toHaveBeenCalledTimes(1);
+  });
+
+  it("dispatches geometry.report targets to onGeometry", () => {
+    const onGeometry = vi.fn();
+    const { agent, client } = setup({ onGeometry });
+    client.start();
+    const targets = [
+      { nodeId: "el-1", rect: { x: 10, y: 20, width: 300, height: 40 } },
+    ];
+    agent.postGeometry(targets);
+    expect(onGeometry).toHaveBeenCalledExactlyOnceWith(targets);
+  });
+
+  it("dispatches click.target to onClick, including null for background", () => {
+    const onClick = vi.fn();
+    const { agent, client } = setup({ onClick });
+    client.start();
+    agent.postClick("el-7");
+    agent.postClick(null);
+    expect(onClick).toHaveBeenNthCalledWith(1, "el-7");
+    expect(onClick).toHaveBeenNthCalledWith(2, null);
+  });
+
+  it("ignores malformed payloads: garbage, near-misses, and editor-direction messages", () => {
+    const onRuntimeReady = vi.fn();
+    const onClick = vi.fn();
+    const { agent, client } = setup({ onRuntimeReady, onClick });
+    client.start();
+    agent.postRaw("not an envelope");
+    agent.postRaw({ type: "runtime.ready" }); // bare message, no envelope
+    agent.postRaw({
+      protocolVersion: "999",
+      message: { type: "runtime.ready" },
+    });
+    agent.postRaw({
+      protocolVersion: PROTOCOL_VERSION,
+      message: { type: "render-frame", componentId: "button" },
+    });
+    expect(onRuntimeReady).not.toHaveBeenCalled();
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("survives handlers being omitted", () => {
+    const { agent, client } = setup();
+    client.start();
+    expect(() => {
+      agent.postRuntimeReady();
+      agent.postGeometry([]);
+      agent.postClick(null);
+    }).not.toThrow();
+  });
+});
+
+describe("createCanvasClient — outbound messages", () => {
+  it("renderFrame posts a valid editor envelope requesting one Element", () => {
+    const { agent, client } = setup();
+    client.start();
+    expect(client.renderFrame("button")).toBe(true);
+    expect(agent.posted[0]?.data).toEqual({
+      protocolVersion: PROTOCOL_VERSION,
+      message: { type: "render-frame", componentId: "button" },
+    });
+  });
+
+  it("renderFrame includes prop overrides when given", () => {
+    const { agent, client } = setup();
+    client.renderFrame("button", { label: "Save" });
+    expect(agent.posted[0]?.data).toEqual({
+      protocolVersion: PROTOCOL_VERSION,
+      message: {
+        type: "render-frame",
+        componentId: "button",
+        props: { label: "Save" },
+      },
+    });
+  });
+
+  it("requestGeometry posts a geometry.request envelope", () => {
+    const { agent, client } = setup();
+    client.requestGeometry();
+    expect(agent.posted[0]?.data).toEqual({
+      protocolVersion: PROTOCOL_VERSION,
+      message: { type: "geometry.request" },
+    });
+  });
+
+  it("returns false without posting when the agent window is unavailable", () => {
+    const agent = new ScriptedAgentDouble({ origin: ADAPTER_ORIGIN });
+    const client = createCanvasClient({
+      adapterOrigin: ADAPTER_ORIGIN,
+      getAgentWindow: () => null,
+      editorWindow: agent.editorWindow,
+    });
+    expect(client.renderFrame("button")).toBe(false);
+    expect(client.requestGeometry()).toBe(false);
+    expect(agent.posted).toHaveLength(0);
+  });
+});
+
+describe("createCanvasClient — lifecycle", () => {
+  it("start is idempotent — one listener no matter how many calls", () => {
+    const onRuntimeReady = vi.fn();
+    const { agent, client } = setup({ onRuntimeReady });
+    client.start();
+    client.start();
+    expect(agent.listenerCount).toBe(1);
+    agent.postRuntimeReady();
+    expect(onRuntimeReady).toHaveBeenCalledTimes(1);
+  });
+
+  it("stop detaches the listener and is idempotent", () => {
+    const onClick = vi.fn();
+    const { agent, client } = setup({ onClick });
+    client.start();
+    client.stop();
+    client.stop();
+    expect(agent.listenerCount).toBe(0);
+    agent.postClick("el-1");
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("does not hear messages before start", () => {
+    const onRuntimeReady = vi.fn();
+    const { agent } = setup({ onRuntimeReady });
+    agent.postRuntimeReady();
+    expect(onRuntimeReady).not.toHaveBeenCalled();
+  });
+});
+
+describe("createCanvasClient — default editor window", () => {
+  it("listens on the global window when editorWindow is not injected", () => {
+    const onRuntimeReady = vi.fn();
+    const client = createCanvasClient({
+      adapterOrigin: ADAPTER_ORIGIN,
+      getAgentWindow: () => null,
+      handlers: { onRuntimeReady },
+    });
+    client.start();
+    try {
+      window.dispatchEvent(
+        new MessageEvent("message", {
+          data: {
+            protocolVersion: PROTOCOL_VERSION,
+            message: { type: "runtime.ready" },
+          },
+          origin: ADAPTER_ORIGIN,
+        }),
+      );
+      expect(onRuntimeReady).toHaveBeenCalledTimes(1);
+    } finally {
+      client.stop();
+    }
+  });
+});
+
+describe("selection round-trip through the store", () => {
+  beforeEach(() => {
+    useEditorStore.setState({ registry: [], selectedNodeId: null });
+  });
+
+  it("agent click report drives store selection; background click deselects", () => {
+    const { agent, client } = setup({
+      onClick: (nodeId) => useEditorStore.getState().select(nodeId),
+    });
+    client.start();
+
+    agent.postClick("el-42");
+    expect(useEditorStore.getState().selectedNodeId).toBe("el-42");
+
+    agent.postClick(null);
+    expect(useEditorStore.getState().selectedNodeId).toBeNull();
+  });
+
+  it("a wrong-origin click never reaches the store", () => {
+    const { agent, client } = setup({
+      onClick: (nodeId) => useEditorStore.getState().select(nodeId),
+    });
+    client.start();
+    agent.postClick("el-42", { origin: EVIL_ORIGIN });
+    expect(useEditorStore.getState().selectedNodeId).toBeNull();
+  });
+});
diff --git a/packages/editor/src/canvas-protocol/canvas-client.ts b/packages/editor/src/canvas-protocol/canvas-client.ts
new file mode 100644
index 0000000..972763a
--- /dev/null
+++ b/packages/editor/src/canvas-protocol/canvas-client.ts
@@ -0,0 +1,170 @@
+import { PROTOCOL_VERSION } from "@repo/core-runner-protocol";
+import {
+  canvasAgentEnvelopeSchema,
+  canvasEditorEnvelopeSchema,
+  type CanvasEditorMessage,
+  type GeometryTarget,
+} from "./messages";
+
+/**
+ * Editor-side canvas-protocol client (ADR-028).
+ *
+ * Speaks the postMessage seam with the agent script inside one adapter
+ * iframe. `targetOrigin` is pinned in BOTH directions:
+ *
+ *   - outbound: every `postMessage` passes the adapter origin, never `"*"`;
+ *   - inbound: events whose `origin` is not the adapter origin are dropped
+ *     before parsing, as are events from a different source window and
+ *     payloads that fail the strict agent-envelope schema.
+ */
+
+/** The minimal surface of the iframe's content window the client posts to. */
+export type AgentWindowLike = {
+  postMessage: (message: unknown, targetOrigin: string) => void;
+};
+
+/** The minimal surface of the editor window the client listens on. */
+export type EditorWindowLike = {
+  addEventListener: (
+    type: "message",
+    listener: (event: MessageEvent) => void,
+  ) => void;
+  removeEventListener: (
+    type: "message",
+    listener: (event: MessageEvent) => void,
+  ) => void;
+};
+
+export type CanvasClientHandlers = {
+  /** Agent runtime is up — safe to send `render-frame` (spec §9). */
+  onRuntimeReady?: () => void;
+  /** Fresh geometry report — reposition editor-side overlays. */
+  onGeometry?: (targets: readonly GeometryTarget[]) => void;
+  /** Click-target resolution; `null` means background (deselect). */
+  onClick?: (nodeId: string | null) => void;
+};
+
+export type CanvasClientOptions = {
+  /** The adapter's origin — the pinned `targetOrigin` for both directions. */
+  adapterOrigin: string;
+  /**
+   * Late-bound accessor for the iframe's content window; `null` while the
+   * frame is not mounted. Late binding lets the client outlive iframe
+   * remounts.
+   */
+  getAgentWindow: () => AgentWindowLike | null;
+  /** Defaults to the global `window` — injectable for tests. */
+  editorWindow?: EditorWindowLike;
+  handlers?: CanvasClientHandlers;
+};
+
+export type CanvasClient = {
+  /** Attach the message listener. Idempotent. */
+  start: () => void;
+  /** Detach the message listener. Idempotent. */
+  stop: () => void;
+  /**
+   * Ask the adapter's frame host to render one discovered component
+   * (schema from `@repo/core-runner-protocol`). Returns `false` when the
+   * agent window is not available yet.
+   */
+  renderFrame: (
+    componentId: string,
+    props?: Record<string, unknown>,
+  ) => boolean;
+  /** Ask the agent for a fresh `geometry.report`. */
+  requestGeometry: () => boolean;
+};
+
+/**
+ * Validates that `origin` is a well-formed, pinnable web origin. `"*"` is
+ * rejected outright — the wildcard would defeat the ADR-028 pinning
+ * guarantee on the outbound direction.
+ */
+function assertPinnableOrigin(origin: string): void {
+  if (origin === "*") {
+    throw new TypeError(
+      'adapterOrigin must be a concrete origin — "*" defeats targetOrigin pinning (ADR-028)',
+    );
+  }
+  let parsed: URL;
+  try {
+    parsed = new URL(origin);
+  } catch {
+    throw new TypeError(`adapterOrigin is not a valid origin: "${origin}"`);
+  }
+  if (parsed.origin !== origin) {
+    throw new TypeError(
+      `adapterOrigin must be a bare origin (no path/trailing slash): "${origin}"`,
+    );
+  }
+}
+
+export function createCanvasClient(options: CanvasClientOptions): CanvasClient {
+  const { adapterOrigin, getAgentWindow, handlers = {} } = options;
+  assertPinnableOrigin(adapterOrigin);
+  const editorWindow: EditorWindowLike =
+    options.editorWindow ?? (globalThis.window as unknown as EditorWindowLike);
+
+  let listening = false;
+
+  const onMessage = (event: MessageEvent): void => {
+    // Inbound pin: only the adapter origin may speak to this client.
+    if (event.origin !== adapterOrigin) return;
+    // Source pin: when both sides are known, they must be the same window.
+    const agentWindow = getAgentWindow();
+    if (
+      agentWindow !== null &&
+      event.source !== null &&
+      event.source !== agentWindow
+    ) {
+      return;
+    }
+    const parsed = canvasAgentEnvelopeSchema.safeParse(event.data);
+    if (!parsed.success) return;
+    const message = parsed.data.message;
+    switch (message.type) {
+      case "runtime.ready":
+        handlers.onRuntimeReady?.();
+        return;
+      case "geometry.report":
+        handlers.onGeometry?.(message.targets);
+        return;
+      case "click.target":
+        handlers.onClick?.(message.nodeId);
+        return;
+    }
+  };
+
+  const send = (message: CanvasEditorMessage): boolean => {
+    const agentWindow = getAgentWindow();
+    if (agentWindow === null) return false;
+    // Self-check outbound shape, then post with the pinned targetOrigin.
+    const envelope = canvasEditorEnvelopeSchema.parse({
+      protocolVersion: PROTOCOL_VERSION,
+      message,
+    });
+    agentWindow.postMessage(envelope, adapterOrigin);
+    return true;
+  };
+
+  return {
+    start: () => {
+      if (listening) return;
+      editorWindow.addEventListener("message", onMessage);
+      listening = true;
+    },
+    stop: () => {
+      if (!listening) return;
+      editorWindow.removeEventListener("message", onMessage);
+      listening = false;
+    },
+    renderFrame: (componentId, props) =>
+      send({
+        type: "render-frame",
+        componentId,
+        ...(props === undefined ? {} : { props }),
+      }),
+    requestGeometry: () => send({ type: "geometry.request" }),
+  };
+}
diff --git a/packages/editor/src/canvas-protocol/messages.test.ts b/packages/editor/src/canvas-protocol/messages.test.ts
new file mode 100644
index 0000000..fdf0a3e
--- /dev/null
+++ b/packages/editor/src/canvas-protocol/messages.test.ts
@@ -0,0 +1,147 @@
+import { describe, it, expect } from "vitest";
+import { PROTOCOL_VERSION } from "@repo/core-runner-protocol";
+import {
+  canvasAgentEnvelopeSchema,
+  canvasAgentMessageSchema,
+  canvasEditorEnvelopeSchema,
+  canvasEditorMessageSchema,
+  geometryTargetSchema,
+  rectSchema,
+} from "@/canvas-protocol/messages";
+
+const agentEnvelope = (message: unknown) => ({
+  protocolVersion: PROTOCOL_VERSION,
+  message,
+});
+
+describe("canvasAgentMessageSchema", () => {
+  it("round-trips runtime.ready", () => {
+    const msg = { type: "runtime.ready" } as const;
+    expect(canvasAgentMessageSchema.parse(msg)).toEqual(msg);
+  });
+
+  it("round-trips a geometry.report with targets", () => {
+    const msg = {
+      type: "geometry.report",
+      targets: [
+        { nodeId: "el-1", rect: { x: 0, y: 12.5, width: 320, height: 48 } },
+      ],
+    } as const;
+    expect(canvasAgentMessageSchema.parse(msg)).toEqual(msg);
+  });
+
+  it("round-trips click.target with a node id and with null (background)", () => {
+    expect(
+      canvasAgentMessageSchema.parse({ type: "click.target", nodeId: "el-1" }),
+    ).toEqual({ type: "click.target", nodeId: "el-1" });
+    expect(
+      canvasAgentMessageSchema.parse({ type: "click.target", nodeId: null }),
+    ).toEqual({ type: "click.target", nodeId: null });
+  });
+
+  it("rejects unknown message types", () => {
+    expect(
+      canvasAgentMessageSchema.safeParse({ type: "hover.target" }).success,
+    ).toBe(false);
+  });
+
+  it("rejects editor-direction messages — direction unions do not overlap", () => {
+    expect(
+      canvasAgentMessageSchema.safeParse({
+        type: "render-frame",
+        componentId: "button",
+      }).success,
+    ).toBe(false);
+    expect(
+      canvasEditorMessageSchema.safeParse({ type: "runtime.ready" }).success,
+    ).toBe(false);
+  });
+
+  it("is strict: unknown fields are rejected", () => {
+    expect(
+      canvasAgentMessageSchema.safeParse({ type: "runtime.ready", extra: 1 })
+        .success,
+    ).toBe(false);
+  });
+});
+
+describe("rectSchema / geometryTargetSchema", () => {
+  it("rejects negative dimensions", () => {
+    expect(
+      rectSchema.safeParse({ x: 0, y: 0, width: -1, height: 10 }).success,
+    ).toBe(false);
+  });
+
+  it("rejects non-finite coordinates", () => {
+    expect(
+      rectSchema.safeParse({ x: Infinity, y: 0, width: 1, height: 1 }).success,
+    ).toBe(false);
+    expect(
+      rectSchema.safeParse({ x: 0, y: NaN, width: 1, height: 1 }).success,
+    ).toBe(false);
+  });
+
+  it("rejects an empty nodeId", () => {
+    expect(
+      geometryTargetSchema.safeParse({
+        nodeId: "",
+        rect: { x: 0, y: 0, width: 1, height: 1 },
+      }).success,
+    ).toBe(false);
+  });
+});
+
+describe("canvasEditorMessageSchema", () => {
+  it("accepts render-frame (shared schema from @repo/core-runner-protocol)", () => {
+    const msg = {
+      type: "render-frame",
+      componentId: "button",
+      props: { label: "Save" },
+    } as const;
+    expect(canvasEditorMessageSchema.parse(msg)).toEqual(msg);
+  });
+
+  it("accepts render-frame without props (default props)", () => {
+    const msg = { type: "render-frame", componentId: "button" } as const;
+    expect(canvasEditorMessageSchema.parse(msg)).toEqual(msg);
+  });
+
+  it("accepts geometry.request", () => {
+    expect(
+      canvasEditorMessageSchema.parse({ type: "geometry.request" }),
+    ).toEqual({ type: "geometry.request" });
+  });
+});
+
+describe("envelopes", () => {
+  it("agent envelope round-trips a valid message", () => {
+    const envelope = agentEnvelope({ type: "runtime.ready" });
+    expect(canvasAgentEnvelopeSchema.parse(envelope)).toEqual(envelope);
+  });
+
+  it("rejects a foreign protocol version", () => {
+    expect(
+      canvasAgentEnvelopeSchema.safeParse({
+        protocolVersion: "999",
+        message: { type: "runtime.ready" },
+      }).success,
+    ).toBe(false);
+  });
+
+  it("rejects unknown envelope fields (no token at this seam — the origin pair is the trust boundary)", () => {
+    expect(
+      canvasAgentEnvelopeSchema.safeParse({
+        ...agentEnvelope({ type: "runtime.ready" }),
+        token: "smuggled",
+      }).success,
+    ).toBe(false);
+  });
+
+  it("editor envelope rejects agent-direction messages", () => {
+    expect(
+      canvasEditorEnvelopeSchema.safeParse(
+        agentEnvelope({ type: "click.target", nodeId: "el-1" }),
+      ).success,
+    ).toBe(false);
+  });
+});
diff --git a/packages/editor/src/canvas-protocol/messages.ts b/packages/editor/src/canvas-protocol/messages.ts
new file mode 100644
index 0000000..428caa6
--- /dev/null
+++ b/packages/editor/src/canvas-protocol/messages.ts
@@ -0,0 +1,145 @@
+import { z } from "zod";
+import {
+  PROTOCOL_VERSION,
+  renderFrameMessageSchema,
+} from "@repo/core-runner-protocol";
+
+/**
+ * Canvas-protocol message set, editor side (ADR-028).
+ *
+ * The canvas protocol is the postMessage seam between the editor and the
+ * Veect agent script injected into the adapter-served iframe. Chrome
+ * (selection/hover rings) renders as editor-side overlays from geometry the
+ * agent reports — nothing Veect-authored renders inside the customer's
+ * document, and `targetOrigin` is pinned in BOTH directions.
+ *
+ * These schemas are local to `@repo/editor` for now: the adapter's agent
+ * script (walking-skeleton story 05) delivers the other side of this seam,
+ * and the shapes reconcile into `@repo/core-runner-protocol` when the two
+ * meet in story 10. They are deliberately small. `render-frame` is already
+ * shared — it comes verbatim from `@repo/core-runner-protocol`, which is
+ * why its `type` literal is kebab-case while the agent events use the tech
+ * spec's dot-namespaced names (`runtime.ready`, spec §9).
+ *
+ * There is no auth token at this seam — the pinned origin pair is the trust
+ * boundary (the workspace-scoped token lives on the runner WS envelope).
+ * All object schemas are `.strict()` so unknown fields are rejected.
+ */
+
+// --- Geometry ------------------------------------------------------------
+
+/** A rectangle in the iframe document's CSS pixel space. */
+export const rectSchema = z
+  .object({
+    x: z.number().finite(),
+    y: z.number().finite(),
+    width: z.number().finite().nonnegative(),
+    height: z.number().finite().nonnegative(),
+  })
+  .strict();
+
+export type Rect = z.infer<typeof rectSchema>;
+
+/** One measured node: the agent-assigned node id and its bounding rect. */
+export const geometryTargetSchema = z
+  .object({
+    nodeId: z.string().min(1),
+    rect: rectSchema,
+  })
+  .strict();
+
+export type GeometryTarget = z.infer<typeof geometryTargetSchema>;
+
+// --- Agent -> editor -----------------------------------------------------
+
+/**
+ * The injected agent script announces the iframe runtime is up and the
+ * agent is listening (spec §9: `runtime.ready` queue-drain — the editor
+ * sends `render-frame` only after this).
+ */
+export const runtimeReadyMessageSchema = z
+  .object({
+    type: z.literal("runtime.ready"),
+  })
+  .strict();
+
+export type RuntimeReadyMessage = z.infer<typeof runtimeReadyMessageSchema>;
+
+/**
+ * Geometry report: bounding rects for the rendered nodes, measured by the
+ * agent post-`fonts.loaded` (ADR-028). The editor renders selection/hover
+ * overlays from these — it never measures across the origin boundary.
+ */
+export const geometryReportMessageSchema = z
+  .object({
+    type: z.literal("geometry.report"),
+    targets: z.array(geometryTargetSchema),
+  })
+  .strict();
+
+export type GeometryReportMessage = z.infer<typeof geometryReportMessageSchema>;
+
+/**
+ * Click-target report: the agent hit-tests clicks inside the iframe and
+ * reports the resolved node id. `null` means background — deselect.
+ */
+export const clickTargetMessageSchema = z
+  .object({
+    type: z.literal("click.target"),
+    nodeId: z.string().min(1).nullable(),
+  })
+  .strict();
+
+export type ClickTargetMessage = z.infer<typeof clickTargetMessageSchema>;
+
+export const canvasAgentMessageSchema = z.discriminatedUnion("type", [
+  runtimeReadyMessageSchema,
+  geometryReportMessageSchema,
+  clickTargetMessageSchema,
+]);
+
+export type CanvasAgentMessage = z.infer<typeof canvasAgentMessageSchema>;
+
+// --- Editor -> agent -----------------------------------------------------
+
+/** Ask the agent to (re-)measure and send a fresh `geometry.report`. */
+export const geometryRequestMessageSchema = z
+  .object({
+    type: z.literal("geometry.request"),
+  })
+  .strict();
+
+export type GeometryRequestMessage = z.infer<
+  typeof geometryRequestMessageSchema
+>;
+
+export const canvasEditorMessageSchema = z.discriminatedUnion("type", [
+  renderFrameMessageSchema,
+  geometryRequestMessageSchema,
+]);
+
+export type CanvasEditorMessage = z.infer<typeof canvasEditorMessageSchema>;
+
+// --- Envelopes (direction-specific) --------------------------------------
+
+/**
+ * Direction-specific envelopes: an agent envelope can never smuggle an
+ * editor command and vice versa — each side parses only its inbound union.
+ */
+export const canvasAgentEnvelopeSchema = z
+  .object({
+    protocolVersion: z.literal(PROTOCOL_VERSION),
+    message: canvasAgentMessageSchema,
+  })
+  .strict();
+
+export type CanvasAgentEnvelope = z.infer<typeof canvasAgentEnvelopeSchema>;
+
+export const canvasEditorEnvelopeSchema = z
+  .object({
+    protocolVersion: z.literal(PROTOCOL_VERSION),
+    message: canvasEditorMessageSchema,
+  })
+  .strict();
+
+export type CanvasEditorEnvelope = z.infer<typeof canvasEditorEnvelopeSchema>;
diff --git a/packages/editor/src/canvas-protocol/scripted-agent.mock.ts b/packages/editor/src/canvas-protocol/scripted-agent.mock.ts
new file mode 100644
index 0000000..40f49a3
--- /dev/null
+++ b/packages/editor/src/canvas-protocol/scripted-agent.mock.ts
@@ -0,0 +1,97 @@
+import { PROTOCOL_VERSION } from "@repo/core-runner-protocol";
+import type { AgentWindowLike, EditorWindowLike } from "./canvas-client";
+import type { CanvasAgentMessage, GeometryTarget } from "./messages";
+
+/**
+ * Scripted agent double (PRD Testing decisions): stands in for the adapter
+ * iframe's injected agent script in unit and component tests.
+ *
+ * One double = one fake origin pair. It owns:
+ *   - `agentWindow` — records every editor→agent post with its
+ *     `targetOrigin`, so tests can assert outbound pinning;
+ *   - `editorWindow` — a fake message target the client listens on, so the
+ *     double can script agent→editor events with any origin/source
+ *     (including hostile ones) without jsdom `MessageEvent` restrictions.
+ */
+
+export type PostedMessage = {
+  data: unknown;
+  targetOrigin: string;
+};
+
+type PostOverrides = {
+  /** Origin the event claims to come from. Defaults to the double's origin. */
+  origin?: string;
+  /** Source window of the event. Defaults to the double's agentWindow. */
+  source?: unknown;
+};
+
+export class ScriptedAgentDouble {
+  readonly origin: string;
+  /** Every editor→agent post, in order, with the targetOrigin used. */
+  readonly posted: PostedMessage[] = [];
+  readonly agentWindow: AgentWindowLike;
+  readonly editorWindow: EditorWindowLike;
+
+  private readonly listeners = new Set<(event: MessageEvent) => void>();
+
+  constructor(options: { origin: string }) {
+    this.origin = options.origin;
+    this.agentWindow = {
+      postMessage: (data: unknown, targetOrigin: string) => {
+        this.posted.push({ data, targetOrigin });
+      },
+    };
+    this.editorWindow = {
+      addEventListener: (_type, listener) => {
+        this.listeners.add(listener);
+      },
+      removeEventListener: (_type, listener) => {
+        this.listeners.delete(listener);
+      },
+    };
+  }
+
+  /** Number of attached message listeners (asserts start/stop symmetry). */
+  get listenerCount(): number {
+    return this.listeners.size;
+  }
+
+  postRuntimeReady(overrides?: PostOverrides): void {
+    this.postMessage({ type: "runtime.ready" }, overrides);
+  }
+
+  postGeometry(
+    targets: readonly GeometryTarget[],
+    overrides?: PostOverrides,
+  ): void {
+    this.postMessage(
+      { type: "geometry.report", targets: [...targets] },
+      overrides,
+    );
+  }
+
+  postClick(nodeId: string | null, overrides?: PostOverrides): void {
+    this.postMessage({ type: "click.target", nodeId }, overrides);
+  }
+
+  /** Script a well-formed agent envelope. */
+  postMessage(message: CanvasAgentMessage, overrides?: PostOverrides): void {
+    this.postRaw({ protocolVersion: PROTOCOL_VERSION, message }, overrides);
+  }
+
+  /** Script an arbitrary payload — for malformed-message tests. */
+  postRaw(data: unknown, overrides?: PostOverrides): void {
+    const event = {
+      data,
+      origin: overrides?.origin ?? this.origin,
+      source:
+        overrides && "source" in overrides
+          ? overrides.source
+          : this.agentWindow,
+    } as MessageEvent;
+    for (const listener of [...this.listeners]) {
+      listener(event);
+    }
+  }
+}
diff --git a/packages/editor/src/index.ts b/packages/editor/src/index.ts
index 658c9ed..5ac3cff 100644
--- a/packages/editor/src/index.ts
+++ b/packages/editor/src/index.ts
@@ -1,4 +1,35 @@
-// Public contract surface of @repo/editor (types, schemas, manifest).
-// UI artifacts (board, frame node, overlay, store hook) live behind
-// `@repo/editor/ui` — see src/ui/index.ts.
+// Public contract surface of @repo/editor (types, schemas, the canvas
+// protocol seam, the manifest). UI artifacts (board, frame node, overlay,
+// store hook) live behind `@repo/editor/ui` — see src/ui/index.ts.
 export { editorManifest, type EditorManifest } from "./feature.manifest";
+export {
+  rectSchema,
+  type Rect,
+  geometryTargetSchema,
+  type GeometryTarget,
+  runtimeReadyMessageSchema,
+  type RuntimeReadyMessage,
+  geometryReportMessageSchema,
+  type GeometryReportMessage,
+  clickTargetMessageSchema,
+  type ClickTargetMessage,
+  canvasAgentMessageSchema,
+  type CanvasAgentMessage,
+  geometryRequestMessageSchema,
+  type GeometryRequestMessage,
+  canvasEditorMessageSchema,
+  type CanvasEditorMessage,
+  canvasAgentEnvelopeSchema,
+  type CanvasAgentEnvelope,
+  canvasEditorEnvelopeSchema,
+  type CanvasEditorEnvelope,
+} from "./canvas-protocol/messages";
+export {
+  createCanvasClient,
+  type CanvasClient,
+  type CanvasClientHandlers,
+  type CanvasClientOptions,
+  type AgentWindowLike,
+  type EditorWindowLike,
+} from "./canvas-protocol/canvas-client";
+export type { RegistryComponent } from "./store/editor-store";
diff --git a/packages/editor/src/store/editor-store.test.ts b/packages/editor/src/store/editor-store.test.ts
new file mode 100644
index 0000000..dee9d0c
--- /dev/null
+++ b/packages/editor/src/store/editor-store.test.ts
@@ -0,0 +1,59 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { useEditorStore, type RegistryComponent } from "@/store/editor-store";
+
+beforeEach(() => {
+  useEditorStore.setState({ registry: [], selectedNodeId: null });
+});
+
+describe("useEditorStore", () => {
+  it("starts with an empty registry and no selection", () => {
+    expect(useEditorStore.getState().registry).toEqual([]);
+    expect(useEditorStore.getState().selectedNodeId).toBeNull();
+  });
+
+  it("setRegistry replaces the registry", () => {
+    const registry: RegistryComponent[] = [
+      { id: "button", name: "Button" },
+      { id: "card", name: "Card" },
+    ];
+    useEditorStore.getState().setRegistry(registry);
+    expect(useEditorStore.getState().registry).toEqual(registry);
+
+    useEditorStore.getState().setRegistry([{ id: "badge", name: "Badge" }]);
+    expect(useEditorStore.getState().registry).toEqual([
+      { id: "badge", name: "Badge" },
+    ]);
+  });
+
+  it("setRegistry copies the input — later caller mutation does not leak in", () => {
+    const input: RegistryComponent[] = [{ id: "button", name: "Button" }];
+    useEditorStore.getState().setRegistry(input);
+    input.push({ id: "rogue", name: "Rogue" });
+    expect(useEditorStore.getState().registry).toEqual([
+      { id: "button", name: "Button" },
+    ]);
+  });
+
+  it("select sets and clears the selected node id", () => {
+    useEditorStore.getState().select("el-1");
+    expect(useEditorStore.getState().selectedNodeId).toBe("el-1");
+    useEditorStore.getState().select(null);
+    expect(useEditorStore.getState().selectedNodeId).toBeNull();
+  });
+
+  it("selection does not touch the registry and vice versa", () => {
+    useEditorStore.getState().setRegistry([{ id: "button", name: "Button" }]);
+    useEditorStore.getState().select("el-1");
+    useEditorStore.getState().setRegistry([]);
+    expect(useEditorStore.getState().selectedNodeId).toBe("el-1");
+  });
+
+  it("holds registry + selection ONLY — no document tree until DesignDoc v1 (ADR-029)", () => {
+    expect(Object.keys(useEditorStore.getState()).sort()).toEqual([
+      "registry",
+      "select",
+      "selectedNodeId",
+      "setRegistry",
+    ]);
+  });
+});
diff --git a/packages/editor/src/store/editor-store.ts b/packages/editor/src/store/editor-store.ts
new file mode 100644
index 0000000..b92526c
--- /dev/null
+++ b/packages/editor/src/store/editor-store.ts
@@ -0,0 +1,36 @@
+import { create } from "zustand";
+
+/**
+ * The minimal editor store (walking-skeleton story 09): registry +
+ * selection ONLY. There is deliberately no document tree here — DesignDoc
+ * v1 and the view-model mapping layer arrive with the
+ * `design-doc-and-editor-foundation` epic (ADR-029).
+ *
+ * Geometry is NOT store state: agent-reported rects are frame-local
+ * (each frame node keeps its own report), keeping this store honest to
+ * its two concerns.
+ */
+
+/** One discovered component, as the runner's scan reports it (ADR-027). */
+export type RegistryComponent = {
+  /** Registry id the frame's `render-frame` request references. */
+  id: string;
+  /** Human-readable component name (e.g. "Button"). */
+  name: string;
+};
+
+export type EditorStoreState = {
+  /** Discovered components available to place on the board. */
+  registry: readonly RegistryComponent[];
+  /** Agent-assigned node id of the selected element; `null` = none. */
+  selectedNodeId: string | null;
+  setRegistry: (registry: readonly RegistryComponent[]) => void;
+  select: (nodeId: string | null) => void;
+};
+
+export const useEditorStore = create<EditorStoreState>((set) => ({
+  registry: [],
+  selectedNodeId: null,
+  setRegistry: (registry) => set({ registry: [...registry] }),
+  select: (nodeId) => set({ selectedNodeId: nodeId }),
+}));
diff --git a/packages/editor/src/ui/index.ts b/packages/editor/src/ui/index.ts
index 1a7fc86..995d33f 100644
--- a/packages/editor/src/ui/index.ts
+++ b/packages/editor/src/ui/index.ts
@@ -1,5 +1,5 @@
-// Public UI surface of @repo/editor: the React Flow board, iframe frame
-// node, selection overlay, and the editor store hook land here as the
-// walking-skeleton slices ship. Apps import components/hooks from
-// `@repo/editor/ui` and contracts from `@repo/editor`.
-export {};
+// Public UI surface of @repo/editor: apps import components/hooks from
+// `@repo/editor/ui` and contracts from `@repo/editor`. The React Flow
+// board, iframe frame node, and selection overlay land here with the
+// board-shell slice.
+export { useEditorStore, type EditorStoreState } from "../store/editor-store";