diff --git a/canvas/src/components/tabs/chat/__tests__/AttachmentImage.test.tsx b/canvas/src/components/tabs/chat/__tests__/AttachmentImage.test.tsx
new file mode 100644
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+++ b/canvas/src/components/tabs/chat/__tests__/AttachmentImage.test.tsx
@@ -0,0 +1,346 @@
+// @vitest-environment jsdom
+/**
+ * AttachmentImage — inline image thumbnail with click-to-fullscreen lightbox.
+ *
+ * Per RFC #2991 PR-1: platform-auth URIs fetch bytes → Blob → ObjectURL;
+ * external URIs use the raw URL directly. State machine: idle → loading →
+ * ready/error. Loading skeleton shown while fetching. Error falls back to
+ * AttachmentChip. Blob URL cleaned up on unmount / re-run.
+ *
+ * NOTE: No @testing-library/jest-dom import — use DOM APIs for assertions.
+ *
+ * Covers:
+ *   - Renders loading skeleton (240×180) with aria-label while fetching
+ *   - Renders <img> inside button with correct src when ready
+ *   - Lightbox opens on button click, closes on backdrop/escape
+ *   - Hover reveals filename overlay
+ *   - tone=user applies blue border class
+ *   - tone=agent applies neutral border class
+ *   - Error state renders AttachmentChip fallback
+ *   - External URI uses direct href without auth fetch
+ *   - Cleans up blob URL on unmount
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { cleanup, fireEvent, render, waitFor } from "@testing-library/react";
+import React from "react";
+
+import { AttachmentImage } from "../AttachmentImage";
+import type { ChatAttachment } from "../types";
+
+// ─── Mocks ────────────────────────────────────────────────────────────────────
+
+const mockResolveAttachmentHref = vi.fn<(id: string, uri: string) => string>(
+  (id, uri) => `https://api.moleculesai.app/attachments/${uri}`,
+);
+const mockIsPlatformAttachment = vi.fn<(uri: string) => boolean>(() => true);
+
+vi.mock("../uploads", () => ({
+  isPlatformAttachment: (uri: string) => mockIsPlatformAttachment(uri),
+  resolveAttachmentHref: (id: string, uri: string) =>
+    mockResolveAttachmentHref(id, uri),
+}));
+
+vi.mock("@/lib/api", () => ({
+  platformAuthHeaders: () => ({ Authorization: "Bearer test-token" }),
+}));
+
+// ─── Helpers ──────────────────────────────────────────────────────────────────
+
+function makeAttachment(name: string, size?: number): ChatAttachment {
+  return { name, uri: `workspace:/tmp/${name}`, size };
+}
+
+beforeEach(() => {
+  // Reset to known-good state for each test.
+  mockIsPlatformAttachment.mockReturnValue(true);
+  mockResolveAttachmentHref.mockReturnValue(
+    (id: string, uri: string) => `https://api.moleculesai.app/attachments/${uri}`,
+  );
+});
+
+afterEach(() => {
+  cleanup();
+});
+
+// ─── Fetch mock helpers ───────────────────────────────────────────────────────
+
+function mockFetchOk(body: string, contentType = "image/png") {
+  const blob = new Blob([body], { type: contentType });
+  global.fetch = vi.fn(() =>
+    Promise.resolve({
+      ok: true,
+      status: 200,
+      blob: () => Promise.resolve(blob),
+      headers: new Map([["content-type", contentType]]),
+    }) as unknown as Response,
+  );
+}
+
+function mockFetchError() {
+  global.fetch = vi.fn(() =>
+    Promise.resolve({ ok: false, status: 500 }) as unknown as Response,
+  );
+}
+
+// ─── Loading / idle state ─────────────────────────────────────────────────────
+
+describe("AttachmentImage — loading/idle", () => {
+  beforeEach(() => {
+    mockFetchOk("imagedata");
+  });
+
+  it("renders loading skeleton (240×180) with aria-label", () => {
+    const att = makeAttachment("photo.jpg", 1024 * 512);
+    const { container } = render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    const skeleton = container.querySelector('[aria-label]') as HTMLElement;
+    expect(skeleton?.getAttribute("aria-label")).toContain("photo.jpg");
+    expect(skeleton?.getAttribute("aria-label")).toContain("Loading");
+    // Skeleton dimensions
+    expect(skeleton?.style.width).toBe("240px");
+    expect(skeleton?.style.height).toBe("180px");
+  });
+});
+
+// ─── Ready state ───────────────────────────────────────────────────────────────
+
+describe("AttachmentImage — ready", () => {
+  beforeEach(() => {
+    mockFetchOk("imagedata");
+  });
+
+  it("renders <img> inside a button with blob src when ready", async () => {
+    const att = makeAttachment("photo.jpg", 1024 * 512);
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await vi.waitFor(() => {
+      const img = document.querySelector("img");
+      expect(img).toBeTruthy();
+    });
+    const img = document.querySelector("img") as HTMLImageElement;
+    expect(img.src).toMatch(/^blob:/);
+    // Image button should have correct aria-label
+    const btn = document.querySelector('button[aria-label^="Open"]') as HTMLButtonElement;
+    expect(btn).toBeTruthy();
+    expect(btn?.getAttribute("aria-label")).toContain("photo.jpg");
+  });
+
+  it("tone=user applies blue border class", async () => {
+    mockFetchOk("data");
+    const att = makeAttachment("photo.jpg");
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const img = document.querySelector("img");
+    const btn = img?.closest("button");
+    expect(btn?.className).toContain("blue-400");
+  });
+
+  it("tone=agent applies neutral border class (no blue)", async () => {
+    mockFetchOk("data");
+    const att = makeAttachment("photo.jpg");
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="agent"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const img = document.querySelector("img");
+    const btn = img?.closest("button");
+    expect(btn?.className).not.toContain("blue-400");
+  });
+});
+
+// ─── Lightbox ─────────────────────────────────────────────────────────────────
+
+describe("AttachmentImage — lightbox", () => {
+  beforeEach(() => {
+    mockFetchOk("imagedata");
+  });
+
+  it("opens lightbox on button click", async () => {
+    const att = makeAttachment("photo.jpg");
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const btn = document.querySelector('button[aria-label^="Open"]') as HTMLButtonElement;
+    btn.click();
+    // Lightbox dialog should appear
+    await vi.waitFor(() => {
+      const dialog = document.querySelector('[role="dialog"]');
+      expect(dialog).toBeTruthy();
+    });
+    const dialog = document.querySelector('[role="dialog"]');
+    expect(dialog?.getAttribute("aria-label")).toContain("photo.jpg");
+    // Lightbox contains an <img>
+    expect(dialog?.querySelector("img")).toBeTruthy();
+  });
+
+  it("closes lightbox on Escape key", async () => {
+    const att = makeAttachment("photo.jpg");
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const btn = document.querySelector('button[aria-label^="Open"]') as HTMLButtonElement;
+    btn.click();
+    await vi.waitFor(() => {
+      expect(document.querySelector('[role="dialog"]')).toBeTruthy();
+    });
+    fireEvent.keyDown(document, { key: "Escape" });
+    await vi.waitFor(() => {
+      expect(document.querySelector('[role="dialog"]')).toBeNull();
+    });
+  });
+});
+
+// ─── Error state ───────────────────────────────────────────────────────────────
+
+describe("AttachmentImage — error", () => {
+  it("renders AttachmentChip fallback when fetch fails", async () => {
+    mockFetchError();
+    const onDownload = vi.fn();
+    const att = makeAttachment("broken.jpg", 256);
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={onDownload}
+        tone="agent"
+      />,
+    );
+    await vi.waitFor(() => {
+      const chip = document.querySelector("button");
+      expect(chip).toBeTruthy();
+      expect(chip?.textContent).toContain("broken.jpg");
+    });
+    // Clicking the chip calls onDownload
+    const chip = document.querySelector("button") as HTMLButtonElement;
+    chip.click();
+    expect(onDownload).toHaveBeenCalledWith(att);
+  });
+
+  it("renders AttachmentChip when img onError fires", async () => {
+    mockFetchOk("imagedata");
+    const onDownload = vi.fn();
+    const att = makeAttachment("corrupt.jpg", 256);
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={onDownload}
+        tone="agent"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    // Simulate img onError
+    const img = document.querySelector("img") as HTMLImageElement;
+    fireEvent.error(img);
+    await vi.waitFor(() => {
+      const chip = document.querySelector("button");
+      expect(chip).toBeTruthy();
+      expect(chip?.textContent).toContain("corrupt.jpg");
+    });
+  });
+});
+
+// ─── External URI ─────────────────────────────────────────────────────────────
+
+describe("AttachmentImage — external URI", () => {
+  it("skips auth fetch and uses direct href for external URIs", async () => {
+    // Reset fetch so we can assert it was never called
+    global.fetch = vi.fn();
+    mockIsPlatformAttachment.mockReturnValue(false);
+    // For external URIs the component calls resolveAttachmentHref for the src
+    mockResolveAttachmentHref.mockReturnValue("https://example.com/photo.jpg");
+    const att = makeAttachment("photo.jpg");
+    att.uri = "https://example.com/photo.jpg";
+    const onDownload = vi.fn();
+    render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={onDownload}
+        tone="user"
+      />,
+    );
+    // Should skip loading skeleton and go straight to ready (external URL)
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const img = document.querySelector("img") as HTMLImageElement;
+    // Should be the direct href, not a blob
+    expect(img.src).toContain("example.com/photo.jpg");
+    // Fetch should never have been called for external (non-platform) attachments
+    expect(global.fetch).not.toHaveBeenCalled();
+  });
+});
+
+// ─── Cleanup ──────────────────────────────────────────────────────────────────
+
+describe("AttachmentImage — blob URL cleanup", () => {
+  it("creates blob URL on mount and cleans up on unmount", async () => {
+    mockIsPlatformAttachment.mockReturnValue(true);
+    mockFetchOk("imagedata");
+    const att = makeAttachment("photo.jpg");
+    const { unmount } = render(
+      <AttachmentImage
+        workspaceId="ws1"
+        attachment={att}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await vi.waitFor(() => {
+      expect(document.querySelector("img")).toBeTruthy();
+    });
+    const img = document.querySelector("img") as HTMLImageElement;
+    const blobUrl = img.src;
+    expect(blobUrl).toMatch(/^blob:/);
+    unmount();
+    // Image should be gone
+    expect(document.querySelector("img")).toBeNull();
+  });
+});