diff --git a/packages/core/src/api/clipboard/toClipboard/copyExtension.browser.test.ts b/packages/core/src/api/clipboard/toClipboard/copyExtension.browser.test.ts
new file mode 100644
index 0000000000..7cf7e39473
--- /dev/null
+++ b/packages/core/src/api/clipboard/toClipboard/copyExtension.browser.test.ts
@@ -0,0 +1,79 @@
+import { Mark } from "@tiptap/core";
+import { NodeSelection, TextSelection } from "prosemirror-state";
+import { describe, expect, it } from "vite-plus/test";
+
+import { BlockNoteEditor } from "../../../editor/BlockNoteEditor.js";
+import { createExtension } from "../../../editor/BlockNoteExtension.js";
+import { BLOCK_LEVEL_SUGGESTION_GROUP } from "../../../pm-nodes/suggestionMarks.js";
+import { selectedFragmentToHTML } from "./copyExtension.js";
+
+describe("selectedFragmentToHTML", () => {
+  it.each(["comment", "insertion", "deletion", "modification", "custom"])(
+    "strips ignored %s marks while preserving content, formatting, and the source",
+    (name) => {
+      const annotation = Mark.create({
+        name,
+        group: BLOCK_LEVEL_SUGGESTION_GROUP,
+        renderHTML() {
+          return ["span", { "data-annotation": name }, 0];
+        },
+        extendMarkSchema(extension) {
+          return extension.name === name ? { blocknoteIgnore: true } : {};
+        },
+      });
+      const editor = BlockNoteEditor.create({
+        initialContent: [
+          {
+            type: "paragraph",
+            content: [
+              {
+                type: "link",
+                href: "https://example.com",
+                content: [
+                  { type: "text", text: "Hello", styles: { bold: true } },
+                ],
+              },
+            ],
+          },
+        ],
+        trailingBlock: false,
+        extensions: [
+          createExtension({
+            key: "test-annotation",
+            tiptapExtensions: [annotation],
+          }),
+        ],
+      });
+      editor.mount(document.createElement("div"));
+      try {
+        editor.transact((tr) => {
+          const mark = editor.pmSchema.marks[name].create();
+          tr.addMark(3, 8, mark);
+          tr.addNodeMark(2, mark);
+        });
+
+        // Exercise both an open text slice and a complete block with nested marks.
+        for (const selection of [
+          TextSelection.create(editor.prosemirrorState.doc, 3, 8),
+          NodeSelection.create(editor.prosemirrorState.doc, 1),
+        ]) {
+          editor.transact((tr) => tr.setSelection(selection));
+          const before = editor.prosemirrorState.doc.toJSON();
+          const { clipboardHTML, externalHTML, markdown } =
+            selectedFragmentToHTML(editor.prosemirrorView, editor);
+
+          for (const html of [clipboardHTML, externalHTML]) {
+            expect(html).toContain("Hello");
+            expect(html).toContain("https://example.com");
+            expect(html).toMatch(/<(strong|b)[ >]/);
+            expect(html).not.toContain("data-annotation");
+          }
+          expect(markdown).toContain("Hello");
+          expect(editor.prosemirrorState.doc.toJSON()).toEqual(before);
+        }
+      } finally {
+        editor.unmount();
+      }
+    },
+  );
+});
diff --git a/packages/core/src/api/clipboard/toClipboard/copyExtension.ts b/packages/core/src/api/clipboard/toClipboard/copyExtension.ts
index d284d251bb..4a19b035be 100644
--- a/packages/core/src/api/clipboard/toClipboard/copyExtension.ts
+++ b/packages/core/src/api/clipboard/toClipboard/copyExtension.ts
@@ -1,5 +1,5 @@
 import { Extension } from "@tiptap/core";
-import { Fragment, Node } from "prosemirror-model";
+import { Fragment, Node, Slice } from "prosemirror-model";
 import { NodeSelection, Plugin } from "prosemirror-state";
 import { CellSelection } from "prosemirror-tables";
 import type { EditorView } from "prosemirror-view";
@@ -127,16 +127,36 @@ export function selectedFragmentToHTML<
     );
   }
 
+  // Strip internal annotations from the copied content, not the source document.
+  function removeIgnoredMarks(fragment: Fragment): Fragment {
+    const children: Node[] = [];
+    fragment.forEach((child) => {
+      const unmarked = child.mark(
+        child.marks.filter((mark) => !mark.type.spec.blocknoteIgnore),
+      );
+      children.push(
+        unmarked.content.size > 0
+          ? unmarked.copy(removeIgnoredMarks(unmarked.content))
+          : unmarked,
+      );
+    });
+    return Fragment.fromArray(children);
+  }
+
+  const selectedSlice = view.state.selection.content();
+
   // Uses default ProseMirror clipboard serialization.
   const clipboardHTML: string = view.serializeForClipboard(
-    view.state.selection.content(),
+    new Slice(
+      removeIgnoredMarks(selectedSlice.content),
+      selectedSlice.openStart,
+      selectedSlice.openEnd,
+    ),
   ).dom.innerHTML;
 
-  const selectedFragment = view.state.selection.content().content;
-
   const externalHTML = fragmentToExternalHTML<BSchema, I, S>(
     view,
-    selectedFragment,
+    selectedSlice.content,
     editor,
   );
 
diff --git a/packages/core/src/comments/extension.test.ts b/packages/core/src/comments/extension.test.ts
new file mode 100644
index 0000000000..52cc1eb270
--- /dev/null
+++ b/packages/core/src/comments/extension.test.ts
@@ -0,0 +1,114 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { TextSelection } from "prosemirror-state";
+import { afterEach, describe, expect, it } from "vite-plus/test";
+
+import { selectedFragmentToHTML } from "../api/clipboard/toClipboard/copyExtension.js";
+import { BlockNoteEditor } from "../editor/BlockNoteEditor.js";
+import { CommentsExtension } from "./extension.js";
+import type { ThreadStore } from "./threadstore/ThreadStore.js";
+
+const editors: BlockNoteEditor[] = [];
+
+afterEach(() => {
+  for (const editor of editors) {
+    editor.unmount();
+  }
+  editors.length = 0;
+});
+
+// Creates an editor whose first paragraph ("Hello world") has a comment on
+// "Hello", and returns the clipboard HTML of copying that comment.
+function createEditorWithComment() {
+  const threadStore = {
+    subscribe: () => () => {},
+    getThreads: () => new Map([["thread-1", { id: "thread-1" }]]),
+  } as unknown as ThreadStore;
+  const editor = BlockNoteEditor.create({
+    initialContent: [
+      { type: "paragraph", content: "Hello world" },
+      { type: "paragraph", content: "target" },
+    ],
+    trailingBlock: false,
+    extensions: [
+      CommentsExtension({ threadStore, resolveUsers: async () => [] }),
+    ],
+  });
+  editors.push(editor);
+  editor.mount(document.createElement("div"));
+  editor.transact((tr) =>
+    tr.addMark(
+      3,
+      8,
+      editor.pmSchema.marks.comment.create({
+        threadId: "thread-1",
+        orphan: false,
+      }),
+    ),
+  );
+  editor.transact((tr) => tr.setSelection(TextSelection.create(tr.doc, 3, 8)));
+  const clipboard = selectedFragmentToHTML(editor.prosemirrorView!, editor);
+  return { editor, ...clipboard };
+}
+
+function pasteAtEndOfTarget(editor: BlockNoteEditor, html: string) {
+  let end = 0;
+  editor.prosemirrorState.doc.descendants((node, pos) => {
+    if (node.isText && node.text === "target") {
+      end = pos + node.nodeSize;
+    }
+  });
+  editor.transact((tr) => tr.setSelection(TextSelection.create(tr.doc, end)));
+  editor.pasteHTML(html, true);
+}
+
+function commentedText(editor: BlockNoteEditor) {
+  const texts: string[] = [];
+  editor.prosemirrorState.doc.descendants((node) => {
+    if (node.marks.some((mark) => mark.type.name === "comment")) {
+      texts.push(node.text!);
+    }
+  });
+  return texts;
+}
+
+describe("CommentsExtension", () => {
+  it("doesn't put comment marks or thread IDs on the clipboard", () => {
+    const { editor, clipboardHTML, externalHTML, markdown } =
+      createEditorWithComment();
+
+    for (const html of [clipboardHTML, externalHTML]) {
+      expect(html).toContain("Hello");
+      expect(html).not.toContain("bn-thread-mark");
+      expect(html).not.toContain("data-bn-thread-id");
+      expect(html).not.toContain("thread-1");
+    }
+    expect(markdown).toBe("Hello\n");
+    // Serializing the selection must not change the original anchor.
+    expect(commentedText(editor)).toEqual(["Hello"]);
+  });
+
+  it("doesn't paste the comment of copied text", () => {
+    const { editor, clipboardHTML } = createEditorWithComment();
+
+    pasteAtEndOfTarget(editor, clipboardHTML);
+
+    expect(editor.document[1].content).toMatchObject([
+      { type: "text", text: "targetHello" },
+    ]);
+    // The comment stays on the original text only.
+    expect(commentedText(editor)).toEqual(["Hello"]);
+  });
+
+  it("doesn't strip comment marks from incoming HTML", () => {
+    const { editor } = createEditorWithComment();
+
+    pasteAtEndOfTarget(
+      editor,
+      '<p><span class="bn-thread-mark" data-bn-thread-id="thread-1">Hello</span></p>',
+    );
+
+    expect(commentedText(editor)).toEqual(["Hello", "Hello"]);
+  });
+});