diff --git a/test/e2e/fixtures/workspace/docs/diffview.md b/test/e2e/fixtures/workspace/docs/diffview.md
new file mode 100644
index 0000000..3774a5c
--- /dev/null
+++ b/test/e2e/fixtures/workspace/docs/diffview.md
@@ -0,0 +1,7 @@
+# Diff view fixture
+
+The baseline opening paragraph stays put so the diff suite can anchor on it.
+
+A target sentence Claude will rewrite via the seam.
+
+A closing paragraph for the operator to edit by hand.
diff --git a/test/e2e/suite/diffView.test.ts b/test/e2e/suite/diffView.test.ts
new file mode 100644
index 0000000..2cc27e6
--- /dev/null
+++ b/test/e2e/suite/diffView.test.ts
@@ -0,0 +1,166 @@
+import * as assert from "assert";
+import * as fs from "fs";
+import * as path from "path";
+import * as vscode from "vscode";
+import type { CowritingApi } from "../../../src/extension";
+
+const WS = process.env.E2E_WORKSPACE!;
+const DOC_REL = "docs/diffview.md";
+
+async function openDoc(): Promise<vscode.TextDocument> {
+  const uri = vscode.Uri.file(path.join(WS, DOC_REL));
+  const doc = await vscode.workspace.openTextDocument(uri);
+  await vscode.window.showTextDocument(doc);
+  return doc;
+}
+async function getApi(): Promise<CowritingApi> {
+  const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
+  const api = (await ext.activate()) as CowritingApi;
+  assert.ok(api?.diffViewController, "extension exports diffViewController");
+  return api;
+}
+const settle = () => new Promise((r) => setTimeout(r, 300));
+
+// Order-dependent (F2–F4 pattern): later tests consume earlier state. Owns
+// docs/diffview.md exclusively.
+suite("F6 diff-view toggle (host E2E — programmatic seam ingress, no LLM)", () => {
+  const TARGET = "A target sentence Claude will rewrite via the seam.";
+  const REPLACEMENT = "A SENTENCE CLAUDE REWROTE via the seam.";
+
+  test("opening a tracked doc captures an `opened` baseline equal to the buffer (INV-18)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    // renderIfOpen on open already called ensureBaseline; assert it captured.
+    const baseline = api.diffViewController.getBaseline(DOC_REL);
+    assert.ok(baseline, "baseline captured on first sight");
+    assert.strictEqual(baseline!.reason, "opened");
+    assert.strictEqual(baseline!.text, doc.getText(), "baseline = open-time buffer");
+  });
+
+  test("toggle opens a diff tab (original scheme cowriting-baseline) over the live doc (PUC-1)", async () => {
+    const api = await getApi();
+    assert.strictEqual(api.diffViewController.isDiffOpen(DOC_REL), false, "no diff open yet");
+    await vscode.commands.executeCommand("cowriting.toggleDiffView");
+    await settle();
+    assert.strictEqual(api.diffViewController.isDiffOpen(DOC_REL), true, "diff tab open");
+    // The active tab is a TextDiff whose original is the baseline scheme.
+    const active = vscode.window.tabGroups.activeTabGroup.activeTab;
+    assert.ok(active && active.input instanceof vscode.TabInputTextDiff, "active tab is a diff");
+    assert.strictEqual(
+      (active!.input as vscode.TabInputTextDiff).original.scheme,
+      "cowriting-baseline",
+      "left side served by the baseline provider",
+    );
+  });
+
+  test("typing leaves the baseline unchanged while the buffer diverges", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    const before = api.diffViewController.getBaseline(DOC_REL)!.text;
+    const edit = new vscode.WorkspaceEdit();
+    edit.insert(doc.uri, new vscode.Position(0, 0), "OPERATOR ADDED LINE\n");
+    assert.ok(await vscode.workspace.applyEdit(edit), "operator edit applied");
+    await settle();
+    assert.strictEqual(api.diffViewController.getBaseline(DOC_REL)!.text, before, "baseline unchanged by typing");
+    assert.notStrictEqual(doc.getText(), before, "buffer diverged");
+  });
+
+  test("accepting a proposal advances the baseline past the landed text (PUC-2, INV-18)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    const start = doc.getText().indexOf(TARGET);
+    assert.ok(start >= 0, "fixture contains the target");
+    const id = await vscode.commands.executeCommand<string>("cowriting.proposeAgentEdit", {
+      uri: doc.uri.toString(),
+      start,
+      end: start + TARGET.length,
+      newText: REPLACEMENT,
+      model: "sonnet",
+      sessionId: "e2e-diff",
+      turnId: "turn-d1",
+    });
+    assert.ok(id, "propose returns an id");
+    assert.ok(await api.proposalController.acceptById(DOC_REL, id!), "accept applies via the seam");
+    await settle();
+    const baseline = api.diffViewController.getBaseline(DOC_REL)!;
+    assert.strictEqual(baseline.reason, "machine-landing", "baseline advanced on the landing");
+    assert.ok(baseline.text.includes(REPLACEMENT), "landed text is in the baseline (won't show as a change)");
+    assert.ok(!baseline.text.includes(TARGET), "old target gone from the baseline too");
+    assert.strictEqual(baseline.text, doc.getText(), "baseline == buffer right after the landing");
+  });
+
+  test("an operator edit after the landing makes baseline ≠ buffer (the operator delta)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    const edit = new vscode.WorkspaceEdit();
+    edit.insert(doc.uri, new vscode.Position(0, 0), "POST-LANDING OPERATOR LINE\n");
+    assert.ok(await vscode.workspace.applyEdit(edit));
+    await settle();
+    assert.notStrictEqual(
+      api.diffViewController.getBaseline(DOC_REL)!.text,
+      doc.getText(),
+      "operator changes show against the advanced baseline",
+    );
+  });
+
+  test("pin resets the baseline to now: baseline == buffer, reason pinned (PUC-3)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    await vscode.commands.executeCommand("cowriting.pinDiffBaseline");
+    await settle();
+    const baseline = api.diffViewController.getBaseline(DOC_REL)!;
+    assert.strictEqual(baseline.reason, "pinned");
+    assert.strictEqual(baseline.text, doc.getText(), "pinned baseline == current buffer (diff empties)");
+  });
+
+  test("the baseline is persisted on disk under the storage dir with the expected content (PUC-4)", async () => {
+    const api = await getApi();
+    const p = api.diffViewController.baselineFilePath(DOC_REL);
+    assert.ok(p, "storage-backed baseline path is available");
+    assert.ok(fs.existsSync(p!), `baseline file exists at ${p}`);
+    const onDisk = JSON.parse(fs.readFileSync(p!, "utf8"));
+    assert.strictEqual(onDisk.docPath, DOC_REL);
+    assert.strictEqual(onDisk.reason, "pinned", "last epoch (pin) persisted");
+    assert.strictEqual(onDisk.text, api.diffViewController.getBaseline(DOC_REL)!.text, "on-disk == in-memory");
+    // INV-19: nothing leaked into the repo's .threads sidecar tree.
+    assert.ok(!p!.includes(`${path.sep}.threads${path.sep}`), "baseline is NOT in the sidecar tree");
+  });
+
+  test("toggle again closes the diff tab and reveals the normal editor (PUC-1)", async () => {
+    const api = await getApi();
+    if (!api.diffViewController.isDiffOpen(DOC_REL)) {
+      await vscode.commands.executeCommand("cowriting.toggleDiffView");
+      await settle();
+    }
+    assert.strictEqual(api.diffViewController.isDiffOpen(DOC_REL), true, "diff open before close");
+    await vscode.commands.executeCommand("cowriting.toggleDiffView");
+    await settle();
+    assert.strictEqual(api.diffViewController.isDiffOpen(DOC_REL), false, "diff tab closed");
+    // "Normal editor is back": the file is the active text editor, and no
+    // baseline-diff tab for it remains in any group.
+    assert.strictEqual(
+      vscode.window.activeTextEditor?.document.uri.toString(),
+      vscode.Uri.file(path.join(WS, DOC_REL)).toString(),
+      "the normal editor for the doc is active after closing the diff",
+    );
+  });
+
+  test("toggling on an untracked doc warns and opens no diff (PUC-5)", async () => {
+    await getApi();
+    const untracked = await vscode.workspace.openTextDocument({ content: "scratch", language: "markdown" });
+    await vscode.window.showTextDocument(untracked);
+    await settle();
+    await vscode.commands.executeCommand("cowriting.toggleDiffView");
+    await settle();
+    // No baseline-diff tab for an untitled doc anywhere.
+    const anyDiff = vscode.window.tabGroups.all.some((g) =>
+      g.tabs.some(
+        (t) =>
+          t.input instanceof vscode.TabInputTextDiff &&
+          t.input.original.scheme === "cowriting-baseline" &&
+          t.input.modified.toString() === untracked.uri.toString(),
+      ),
+    );
+    assert.strictEqual(anyDiff, false, "no diff opened for the untracked doc");
+  });
+});