diff --git a/canvas/src/components/tabs/chat/__tests__/AttachmentTextPreview.test.tsx b/canvas/src/components/tabs/chat/__tests__/AttachmentTextPreview.test.tsx
new file mode 100644
index 00000000..b29ae975
--- /dev/null
+++ b/canvas/src/components/tabs/chat/__tests__/AttachmentTextPreview.test.tsx
@@ -0,0 +1,299 @@
+// @vitest-environment jsdom
+/**
+ * Tests for AttachmentTextPreview — inline <pre><code> text file renderer.
+ *
+ * Per RFC #2991 PR-3. Manages its own fetch cycle (idle → loading →
+ * ready/error). Covers: loading skeleton, <pre><code> render, chip error
+ * fallback, "Show all N lines" expand button, truncated state, download
+ * buttons, tone=user/agent styling, cleanup on unmount.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { render, screen, fireEvent, cleanup, waitFor, act } from "@testing-library/react";
+import React from "react";
+
+import { AttachmentTextPreview } from "../AttachmentTextPreview";
+import type { ChatAttachment } from "../types";
+
+// ─── Setup ────────────────────────────────────────────────────────────────────
+
+vi.stubEnv("NEXT_PUBLIC_ADMIN_TOKEN", "test-token");
+
+const fetchMock = vi.fn();
+beforeEach(() => {
+  fetchMock.mockReset();
+  vi.stubGlobal("fetch", fetchMock);
+});
+
+afterEach(() => {
+  cleanup();
+  vi.restoreAllMocks();
+});
+
+// ─── Fixtures ────────────────────────────────────────────────────────────────
+
+const makeAtt = (name = "log.txt"): ChatAttachment =>
+  ({ name, uri: "workspace:/workspace/tmp/" + name });
+
+function renderTextPreview(
+  att: ChatAttachment,
+  tone: "user" | "agent" = "agent",
+) {
+  return render(
+    <AttachmentTextPreview
+      workspaceId="ws-1"
+      attachment={att}
+      onDownload={vi.fn()}
+      tone={tone}
+    />,
+  );
+}
+
+// ─── Tests ───────────────────────────────────────────────────────────────────
+
+describe("AttachmentTextPreview", () => {
+  // ── idle / loading ───────────────────────────────────────────────────────
+
+  it("renders loading skeleton (idle state)", () => {
+    fetchMock.mockReturnValue(new Promise(() => {})); // hangs forever
+    renderTextPreview(makeAtt());
+    const skeleton = screen.getByLabelText(/Loading log\.txt/i);
+    expect(skeleton).toBeTruthy();
+    expect(skeleton.className).toContain("animate-pulse");
+  });
+
+  it("renders loading skeleton (loading state)", async () => {
+    // Never-resolving fetch → stays in loading state.
+    fetchMock.mockReturnValue(new Promise(() => {}));
+    renderTextPreview(makeAtt("data.json"));
+    await waitFor(() => {
+      expect(screen.getByLabelText(/Loading data\.json/i)).toBeTruthy();
+    });
+  });
+
+  // ── error fallback ───────────────────────────────────────────────────────
+
+  it("renders AttachmentChip when fetch fails (404)", async () => {
+    fetchMock.mockResolvedValue({ ok: false, status: 404 });
+    renderTextPreview(makeAtt("missing.txt"));
+    await waitFor(() => {
+      expect(screen.getByTitle(/Download missing\.txt/i)).toBeTruthy();
+    });
+    // <pre> must NOT appear — proved we fell back to the chip.
+    expect(document.querySelector("pre")).toBeNull();
+  });
+
+  it("renders chip on network error", async () => {
+    fetchMock.mockRejectedValue(new Error("network down"));
+    renderTextPreview(makeAtt("offline.json"));
+    await waitFor(() => {
+      expect(screen.getByTitle(/Download offline\.json/i)).toBeTruthy();
+    });
+  });
+
+  // ── ready / <pre><code> ──────────────────────────────────────────────────
+
+  it("renders <pre><code> with text content when fetch succeeds", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "line1\nline2\nline3",
+    });
+    renderTextPreview(makeAtt("report.txt"));
+    await waitFor(() => {
+      const code = document.querySelector("pre code");
+      expect(code).not.toBeNull();
+      expect(code?.textContent).toBe("line1\nline2\nline3");
+    });
+  });
+
+  it("renders filename header span", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "hello",
+    });
+    renderTextPreview(makeAtt("notes.md"));
+    await waitFor(() => {
+      expect(screen.getByText("notes.md")).toBeTruthy();
+    });
+  });
+
+  it("renders exactly one <pre> element when ready", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "content",
+    });
+    renderTextPreview(makeAtt("code.js"));
+    await waitFor(() => {
+      expect(document.querySelectorAll("pre")).toHaveLength(1);
+    });
+  });
+
+  // ── show all lines button ─────────────────────────────────────────────────
+
+  it("shows 'Show all N lines' button when file has >10 lines", async () => {
+    const body = Array.from({ length: 25 }, (_, i) => `line ${i + 1}`).join("\n");
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => body,
+    });
+    renderTextPreview(makeAtt("big.log"));
+    await waitFor(() => {
+      expect(
+        screen.getByRole("button", { name: /show all 25 lines/i }),
+      ).toBeTruthy();
+    });
+    // First 10 lines only in preview
+    const code = document.querySelector("pre code");
+    expect(code?.textContent).toContain("line 10");
+    expect(code?.textContent).not.toContain("line 11");
+  });
+
+  it("expand button is NOT shown when file has ≤10 lines", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "a\nb\nc",
+    });
+    renderTextPreview(makeAtt("short.txt"));
+    await waitFor(() => {
+      expect(document.querySelector("pre code")).not.toBeNull();
+    });
+    expect(screen.queryByRole("button", { name: /show all/i })).toBeNull();
+  });
+
+  it("clicking 'Show all' expands to full content", async () => {
+    const body = Array.from({ length: 25 }, (_, i) => `line ${i + 1}`).join("\n");
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => body,
+    });
+    renderTextPreview(makeAtt("expand.txt"));
+    await waitFor(() => {
+      expect(screen.getByRole("button", { name: /show all 25 lines/i })).toBeTruthy();
+    });
+    fireEvent.click(screen.getByRole("button", { name: /show all 25 lines/i }));
+    const code = document.querySelector("pre code");
+    expect(code?.textContent).toContain("line 25");
+  });
+
+  // ── download buttons ──────────────────────────────────────────────────────
+
+  it("header download button fires onDownload with attachment", async () => {
+    const onDownload = vi.fn();
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "hello",
+    });
+    const { rerender } = render(
+      <AttachmentTextPreview
+        workspaceId="ws-1"
+        attachment={makeAtt("readme.md")}
+        onDownload={onDownload}
+        tone="agent"
+      />,
+    );
+    await waitFor(() => {
+      expect(document.querySelector("pre code")).not.toBeNull();
+    });
+    const downloadBtn = screen.getByLabelText(/download readme\.md/i);
+    downloadBtn.click();
+    expect(onDownload).toHaveBeenCalledWith(
+      expect.objectContaining({ name: "readme.md" }),
+    );
+  });
+
+  it("onDownload is NOT called during loading or ready states", async () => {
+    const onDownload = vi.fn();
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "hello world",
+    });
+    render(
+      <AttachmentTextPreview
+        workspaceId="ws-1"
+        attachment={makeAtt("quiet.txt")}
+        onDownload={onDownload}
+        tone="agent"
+      />,
+    );
+    await waitFor(() => {
+      expect(document.querySelector("pre code")).not.toBeNull();
+    });
+    expect(onDownload).not.toHaveBeenCalled();
+  });
+
+  // ── tone styling ─────────────────────────────────────────────────────────
+
+  it("tone=user applies blue border class on container", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "hello",
+    });
+    const { container } = render(
+      <AttachmentTextPreview
+        workspaceId="ws-1"
+        attachment={makeAtt("blue.txt")}
+        onDownload={vi.fn()}
+        tone="user"
+      />,
+    );
+    await waitFor(() => {
+      expect(document.querySelector("pre code")).not.toBeNull();
+    });
+    const blueDiv = Array.from(container.querySelectorAll("div")).find((d) =>
+      d.className.includes("blue-400"),
+    );
+    expect(blueDiv).toBeTruthy();
+  });
+
+  it("tone=agent does not apply blue border class", async () => {
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: async () => "hello",
+    });
+    const { container } = render(
+      <AttachmentTextPreview
+        workspaceId="ws-1"
+        attachment={makeAtt("gray.txt")}
+        onDownload={vi.fn()}
+        tone="agent"
+      />,
+    );
+    await waitFor(() => {
+      expect(document.querySelector("pre code")).not.toBeNull();
+    });
+    const blueDivs = Array.from(container.querySelectorAll("div")).filter((d) =>
+      d.className.includes("blue-400"),
+    );
+    expect(blueDivs).toHaveLength(0);
+  });
+
+  // ── cleanup ─────────────────────────────────────────────────────────────
+
+  it("no state update after unmount (cancelled flag prevents setState)", async () => {
+    // The component sets cancelled=true in cleanup, which prevents setState
+    // from firing after the pending read() resolves. We verify no crash
+    // and no error element appears (since the pending read eventually resolves
+    // but the component ignores it due to cancelled=true).
+    fetchMock.mockResolvedValue({
+      ok: true,
+      body: null,
+      text: () => new Promise<string>((resolve) => setTimeout(() => resolve("delayed"), 100)),
+    });
+    const { unmount } = renderTextPreview(makeAtt("cleanup.txt"));
+    await act(async () => {
+      unmount();
+    });
+    // No crash, no error state rendered (chip would appear on error)
+    expect(document.querySelector("pre code")).toBeNull();
+    expect(document.querySelector('[aria-label*="Download cleanup.txt"]')).toBeNull();
+  });
+});