diff --git a/tgpy/builtin_functions.py b/tgpy/builtin_functions.py
index ccadd35..a862c35 100644
--- a/tgpy/builtin_functions.py
+++ b/tgpy/builtin_functions.py
@@ -7,7 +7,7 @@
 from telethon.tl.custom import Message
 
 from tgpy import app
-from tgpy.message_design import get_code
+from tgpy.message_design import parse_message
 from tgpy.modules import (
     Module,
     delete_module_file,
@@ -94,9 +94,10 @@ async def add(self, name: str, code: Optional[str] = None) -> str:
             original: Message = await app.ctx.msg.get_reply_message()
             if original is None:
                 return 'Use this function in reply to a message'
-            code = get_code(original)
-            if not code:
+            message_data = parse_message(original)
+            if not message_data.is_tgpy_message:
                 return 'No code found in reply message'
+            code = message_data.code
 
         origin = f'{filename_prefix}module/{name}'
 
diff --git a/tgpy/handlers/__init__.py b/tgpy/handlers/__init__.py
index 657c031..ce9c117 100644
--- a/tgpy/handlers/__init__.py
+++ b/tgpy/handlers/__init__.py
@@ -2,48 +2,79 @@
 from telethon.tl.custom import Message
 from telethon.tl.types import Channel
 
-from tgpy import app, message_design
+from tgpy import app, message_design, reactions_fix
 from tgpy.handlers.utils import _handle_errors, outgoing_messages_filter
+from tgpy.reactions_fix import ReactionsFixResult
 from tgpy.run_code import eval_message, get_variable_names, parse_code
 
 
-async def handle_message(message: Message) -> None:
-    raw_text = message.raw_text
-
-    if not raw_text:
-        return
+async def handle_message(
+    message: Message, *, only_show_warning: bool = False
+) -> Message:
+    if not message.raw_text:
+        return message
 
     if message.text.startswith('//') and message.text[2:].strip():
-        await message.edit(message.text[2:])
-        return
+        return await message.edit(message.text[2:])
 
     locals_ = get_variable_names()
 
-    res = parse_code(raw_text, locals_)
+    res = parse_code(message.raw_text, locals_)
     if not res.is_code:
-        return
-
-    await eval_message(raw_text, message, uses_orig=res.uses_orig)
+        return message
+
+    if only_show_warning:
+        return await message_design.edit_message(
+            message,
+            message.raw_text,
+            'Edit message again to evaluate',
+        )
+    else:
+        return await eval_message(message.raw_text, message, res.uses_orig)
 
 
 @events.register(events.NewMessage(func=outgoing_messages_filter))
 @_handle_errors
 async def on_new_message(event: events.NewMessage.Event) -> None:
-    await handle_message(event.message)
+    message = await handle_message(event.message)
+    reactions_fix.update_hash(message)
 
 
 @events.register(events.MessageEdited(func=outgoing_messages_filter))
 @_handle_errors
-async def on_message_edited(event: events.NewMessage.Event) -> None:
-    if isinstance(event.message.chat, Channel) and event.message.chat.broadcast:
-        return
-    code = message_design.get_code(event.message)
-    if not code:
-        await handle_message(event.message)
+async def on_message_edited(event: events.MessageEdited.Event) -> None:
+    message: Message = event.message
+    if isinstance(message.chat, Channel) and message.chat.broadcast:
         return
-    await eval_message(
-        code, event.message, uses_orig=parse_code(code, get_variable_names()).uses_orig
-    )
+    message_data = message_design.parse_message(message)
+    reactions_fix_result = reactions_fix.check_hash(message)
+    try:
+        match reactions_fix_result:
+            case ReactionsFixResult.ignore:
+                return
+            case ReactionsFixResult.show_warning:
+                if message_data.is_tgpy_message:
+                    message = await message_design.edit_message(
+                        message, message_data.code, 'Edit message again to evaluate'
+                    )
+                else:
+                    message = await handle_message(message, only_show_warning=True)
+                return
+            case ReactionsFixResult.evaluate:
+                pass
+            case _:
+                raise ValueError(f'Bad reactions fix result: {reactions_fix_result}')
+
+        if not message_data.is_tgpy_message:
+            message = await handle_message(message)
+            return
+        message = await eval_message(
+            message_data.code,
+            message,
+            parse_code(message_data.code, get_variable_names()).uses_orig,
+        )
+    finally:
+        reactions_fix.update_hash(message)
 
 
 @events.register(
@@ -55,14 +86,14 @@ async def cancel(message: Message):
         async for msg in app.client.iter_messages(
             message.chat_id, max_id=message.id, limit=10
         ):
-            if msg.out and message_design.get_code(msg):
+            if msg.out and message_design.parse_message(msg).is_tgpy_message:
                 prev = msg
                 break
         else:
             return
     # noinspection PyBroadException
     try:
-        await prev.edit(message_design.get_code(prev))
+        await prev.edit(message_design.parse_message(prev).code)
     except Exception:
         pass
     else:
diff --git a/tgpy/message_design.py b/tgpy/message_design.py
index 2509464..8db597d 100644
--- a/tgpy/message_design.py
+++ b/tgpy/message_design.py
@@ -1,5 +1,6 @@
 import sys
 import traceback as tb
+from dataclasses import dataclass
 
 from telethon.tl.custom import Message
 from telethon.tl.types import MessageEntityBold, MessageEntityCode, MessageEntityTextUrl
@@ -11,57 +12,96 @@
 FORMATTED_ERROR_HEADER = f'<b><a href="{TITLE_URL}">TGPy error&gt;</a></b>'
 
 
-def utf16_codepoints_len(s: str):
-    return len(s.encode('utf-16-le')) // 2
+class Utf16CodepointsWrapper(str):
+    def __len__(self):
+        return len(self.encode('utf-16-le')) // 2
 
-
-def utf16_codepoints_prefix(s: str, length: int):
-    s = s.encode('utf-16-le')
-    s = s[: length * 2]
-    return s.decode('utf-16-le')
+    def __getitem__(self, item):
+        s = self.encode('utf-16-le')
+        if isinstance(item, slice):
+            item = slice(
+                item.start * 2 if item.start else None,
+                item.stop * 2 if item.stop else None,
+                item.step * 2 if item.step else None,
+            )
+            s = s[item]
+        elif isinstance(item, int):
+            s = s[item * 2 : item * 2 + 2]
+        else:
+            raise TypeError(f'{type(item)} is not supported')
+        return s.decode('utf-16-le')
 
 
 async def edit_message(
-    message: Message, code: str, result, traceback: str = '', output: str = ''
-) -> None:
+    message: Message,
+    code: str,
+    result: str,
+    traceback: str = '',
+    output: str = '',
+) -> Message:
     if result is None and output:
         result = output
         output = ''
 
-    parts = [code.strip(), f'{TITLE} {str(result).strip()}']
-    parts += [part for part in (output.strip(), traceback.strip()) if part]
-    text = '\n\n'.join(parts)
+    title = Utf16CodepointsWrapper(TITLE)
+    parts = [
+        Utf16CodepointsWrapper(code.strip()),
+        Utf16CodepointsWrapper(f'{title} {str(result).strip()}'),
+    ]
+    parts += [
+        Utf16CodepointsWrapper(part)
+        for part in (output.strip(), traceback.strip())
+        if part
+    ]
 
     entities = []
     offset = 0
     for p in parts:
-        entities.append(MessageEntityCode(offset, utf16_codepoints_len(p)))
-        offset += utf16_codepoints_len(p) + 2
+        entities.append(MessageEntityCode(offset, len(p)))
+        offset += len(p) + 2
 
-    entities[1].offset += utf16_codepoints_len(TITLE) + 1
-    entities[1].length -= utf16_codepoints_len(TITLE) + 1
+    entities[1].offset += len(title) + 1
+    entities[1].length -= len(title) + 1
     entities[1:1] = [
         MessageEntityBold(
-            utf16_codepoints_len(parts[0]) + 2,
-            utf16_codepoints_len(TITLE),
+            len(parts[0]) + 2,
+            len(title),
         ),
         MessageEntityTextUrl(
-            utf16_codepoints_len(parts[0]) + 2,
-            utf16_codepoints_len(TITLE),
+            len(parts[0]) + 2,
+            len(title),
             TITLE_URL,
         ),
     ]
 
+    text = str('\n\n'.join(parts))
     if len(text) > 4096:
         text = text[:4095] + '…'
-    await message.edit(text, formatting_entities=entities, link_preview=False)
+    return await message.edit(text, formatting_entities=entities, link_preview=False)
+
+
+@dataclass
+class MessageParseResult:
+    is_tgpy_message: bool
+    code: str | None
+    result: str | None
 
 
-def get_code(message: Message) -> str:
+def get_title_entity(message: Message) -> MessageEntityTextUrl | None:
     for e in message.entities or []:
         if isinstance(e, MessageEntityTextUrl) and e.url == TITLE_URL:
-            return utf16_codepoints_prefix(message.raw_text, length=e.offset).strip()
-    return ''
+            return e
+    return None
+
+
+def parse_message(message: Message) -> MessageParseResult:
+    e = get_title_entity(message)
+    if not e:
+        return MessageParseResult(False, None, None)
+    msg_text = Utf16CodepointsWrapper(message.raw_text)
+    code = msg_text[: e.offset].strip()
+    result = msg_text[e.offset + e.length :].strip()
+    return MessageParseResult(True, code, result)
 
 
 async def send_error(chat) -> None:
@@ -71,3 +111,11 @@ async def send_error(chat) -> None:
     await app.client.send_message(
         chat, f'{FORMATTED_ERROR_HEADER}\n\n<code>{exc}</code>', link_preview=False
     )
+
+
+__all__ = [
+    'edit_message',
+    'MessageParseResult',
+    'parse_message',
+    'send_error',
+]
diff --git a/tgpy/reactions_fix.py b/tgpy/reactions_fix.py
new file mode 100644
index 0000000..0522403
--- /dev/null
+++ b/tgpy/reactions_fix.py
@@ -0,0 +1,43 @@
+"""
+This module tries to fix Telegram bug/undocumented feature where
+setting/removing reaction sometimes triggers message edit event.
+This bug/feature introduces a security vulnerability in TGPy,
+because message reevaluation can be triggered by other users.
+"""
+import json
+from enum import Enum
+from hashlib import sha256
+
+from telethon.tl.custom import Message
+
+content_hashes: dict[tuple[int, int], bytes] = {}
+
+
+def get_content_hash(message: Message) -> bytes:
+    entities = [json.dumps(e.to_dict()) for e in message.entities or []]
+    data = str(len(entities)) + '\n' + '\n'.join(entities) + message.raw_text
+    return sha256(data.encode('utf-8')).digest()
+
+
+class ReactionsFixResult(Enum):
+    ignore = 1
+    evaluate = 2
+    show_warning = 3
+
+
+def check_hash(message: Message) -> ReactionsFixResult:
+    message_uid = (message.chat_id, message.id)
+    content_hash = get_content_hash(message)
+    if message_uid not in content_hashes:
+        return ReactionsFixResult.show_warning
+    if content_hashes[message_uid] == content_hash:
+        return ReactionsFixResult.ignore
+    return ReactionsFixResult.evaluate
+
+
+def update_hash(message: Message) -> None:
+    message_uid = (message.chat_id, message.id)
+    content_hashes[message_uid] = get_content_hash(message)
+
+
+__all__ = ['ReactionsFixResult', 'check_hash', 'update_hash']
diff --git a/tgpy/run_code/__init__.py b/tgpy/run_code/__init__.py
index 6fab24a..613fbc9 100644
--- a/tgpy/run_code/__init__.py
+++ b/tgpy/run_code/__init__.py
@@ -1,4 +1,3 @@
-from telethon.errors import MessageIdInvalidError
 from telethon.tl.custom import Message
 
 from tgpy import app, message_design
@@ -17,7 +16,7 @@ def get_variable_names(include_orig=True):
     # fmt: on
 
 
-async def eval_message(code: str, message: Message, uses_orig=False) -> None:
+async def eval_message(code: str, message: Message, uses_orig: bool) -> Message:
     await message_design.edit_message(message, code, 'Running...')
 
     app.ctx.msg = message
@@ -48,10 +47,11 @@ async def eval_message(code: str, message: Message, uses_orig=False) -> None:
         result = convert_result(result)
         exc = ''
 
-    try:
-        # noinspection PyProtectedMember
-        await message_design.edit_message(
-            message, code, result, traceback=exc, output=app.ctx._print_output
-        )
-    except MessageIdInvalidError:
-        pass
+    # noinspection PyProtectedMember
+    return await message_design.edit_message(
+        message,
+        code,
+        result,
+        traceback=exc,
+        output=app.ctx._print_output,
+    )
diff --git a/tgpy/utils.py b/tgpy/utils.py
index 8d66713..6bd9e77 100644
--- a/tgpy/utils.py
+++ b/tgpy/utils.py
@@ -9,6 +9,7 @@
 from subprocess import PIPE, Popen
 
 import appdirs
+from telethon.tl import types
 
 from tgpy import version
 
@@ -109,6 +110,17 @@ def get_version():
     return 'unknown'
 
 
+def peer_to_id(peer: types.TypePeer):
+    if isinstance(peer, types.PeerUser):
+        return peer.user_id
+    elif isinstance(peer, types.PeerChat):
+        return peer.chat_id
+    elif isinstance(peer, types.PeerChannel):
+        return peer.channel_id
+    else:
+        raise TypeError(f'Unknown peer type: {type(peer)}')
+
+
 __all__ = [
     'DATA_DIR',
     'MODULES_DIR',
@@ -126,4 +138,5 @@ def get_version():
     'get_user',
     'get_hostname',
     'running_in_docker',
+    'peer_to_id',
 ]