diff --git a/src/diffViewController.ts b/src/diffViewController.ts
index 11285fb..1d61fb7 100644
--- a/src/diffViewController.ts
+++ b/src/diffViewController.ts
@@ -143,9 +143,77 @@ export class DiffViewController implements vscode.Disposable {
     return this.store?.baselinePath(docPath);
   }
 
-  // Replaced by the real implementation in Task 4 (toggle UX, SLICE-3).
-  private toggle(_editor: vscode.TextEditor | undefined): void {
-    /* stub */
+  // ---- toggle UX (§6.5 PUC-1) --------------------------------------------------------
+
+  /**
+   * If this doc's baseline diff is the active/open tab → close it and reveal the
+   * normal editor; if the active editor is a tracked doc with no diff open →
+   * open vscode.diff (baseline left, the live document right). Untracked → warn,
+   * no diff.
+   */
+  private async toggle(editor: vscode.TextEditor | undefined): Promise<void> {
+    if (!editor || !this.isTracked(editor.document)) {
+      void vscode.window.showWarningMessage(
+        "Cowriting: open a tracked workspace document to toggle its diff view.",
+      );
+      return;
+    }
+    const document = editor.document;
+    const docPath = this.docPathOf(document.uri);
+    const openTab = this.findDiffTab(document.uri);
+    if (openTab) {
+      await vscode.window.tabGroups.close(openTab);
+      await vscode.window.showTextDocument(document, { preview: false });
+      return;
+    }
+    this.ensureBaseline(document);
+    const baseline = this.baselines.get(docPath)!;
+    const title = `${path.basename(docPath)} — my changes since ${this.epochLabel(baseline)}`;
+    await vscode.commands.executeCommand(
+      "vscode.diff",
+      this.baselineUri(docPath),
+      document.uri,
+      title,
+      { preview: false },
+    );
+  }
+
+  /** The open baseline-diff tab for this document, if any. */
+  private findDiffTab(modified: vscode.Uri): vscode.Tab | undefined {
+    for (const group of vscode.window.tabGroups.all) {
+      for (const tab of group.tabs) {
+        const input = tab.input;
+        if (
+          input instanceof vscode.TabInputTextDiff &&
+          input.original.scheme === BASELINE_SCHEME &&
+          input.modified.toString() === modified.toString()
+        ) {
+          return tab;
+        }
+      }
+    }
+    return undefined;
+  }
+
+  /** Human-readable epoch for the diff tab title (§5 / §6.5). */
+  private epochLabel(baseline: Baseline): string {
+    const time = new Date(baseline.capturedAt).toLocaleTimeString();
+    switch (baseline.reason) {
+      case "opened":
+        return `opened ${time}`;
+      case "machine-landing":
+        return `Claude landed ${time}`;
+      case "pinned":
+        return `pinned ${time}`;
+    }
+  }
+
+  /**
+   * Test-facing (§6.4): is this doc's baseline diff currently open in any tab
+   * group? The diff's `modified` side is the document's own file: URI.
+   */
+  isDiffOpen(docPath: string): boolean {
+    return this.findDiffTab(vscode.Uri.file(path.join(this.rootDir, docPath))) !== undefined;
   }
 
   dispose(): void {