diff --git a/deltachat-rpc-client/tests/test_iroh_webxdc.py b/deltachat-rpc-client/tests/test_iroh_webxdc.py
index 0eb6c15a4..53217c922 100644
--- a/deltachat-rpc-client/tests/test_iroh_webxdc.py
+++ b/deltachat-rpc-client/tests/test_iroh_webxdc.py
@@ -24,6 +24,13 @@ def path_to_webxdc(request):
     return str(p)
 
 
+@pytest.fixture
+def path_to_large_webxdc(request):
+    p = request.path.parent.parent.parent.joinpath("test-data/webxdc/realtime-check.xdc")
+    assert p.exists()
+    return str(p)
+
+
 def log(msg):
     logging.info(msg)
 
@@ -227,3 +234,29 @@ def test_advertisement_after_chatting(acfactory, path_to_webxdc):
     ac2_webxdc_msg.send_webxdc_realtime_advertisement()
     event = ac1.wait_for_event(EventType.WEBXDC_REALTIME_ADVERTISEMENT_RECEIVED)
     assert event.msg_id == ac1_webxdc_msg.id
+
+
+def test_realtime_large_webxdc(acfactory, path_to_large_webxdc):
+    """Tests initializing realtime channel on a large webxdc.
+
+    This is a regression test for a bug that existed in version 2.42.0.
+    Large webxdc is split into pre- and post- message,
+    and this previously resulted in failure to initialize realtime.
+    """
+    ac1, ac2 = acfactory.get_online_accounts(2)
+    ac1.set_config("webxdc_realtime_enabled", "1")
+    ac2.set_config("webxdc_realtime_enabled", "1")
+
+    ac2.create_chat(ac1)
+    ac1_ac2_chat = ac1.create_chat(ac2)
+    ac1_webxdc_msg = ac1_ac2_chat.send_message(text="realtime check", file=path_to_large_webxdc)
+
+    # Receive pre-message.
+    ac2_webxdc_msg = ac2.wait_for_incoming_msg()
+
+    # Receive post-message.
+    ac2_webxdc_msg = ac2.wait_for_msg(EventType.MSGS_CHANGED)
+
+    ac2_webxdc_msg.send_webxdc_realtime_advertisement()
+    event = ac1.wait_for_event(EventType.WEBXDC_REALTIME_ADVERTISEMENT_RECEIVED)
+    assert event.msg_id == ac1_webxdc_msg.id
diff --git a/src/peer_channels.rs b/src/peer_channels.rs
index 31e089b8d..6fa166685 100644
--- a/src/peer_channels.rs
+++ b/src/peer_channels.rs
@@ -521,6 +521,18 @@ pub(crate) async fn create_iroh_header(ctx: &Context, msg_id: MsgId) -> Result<S
     Ok(topic_string)
 }
 
+/// Converts `Iroh-Gossip-Header` contents to iroh topic ID.
+pub(crate) fn iroh_topic_from_str(topic: &str) -> Result<TopicId> {
+    let mut topic_raw = [0u8; 32];
+    BASE32_NOPAD
+        .decode_mut(topic.to_ascii_uppercase().as_bytes(), &mut topic_raw)
+        .map_err(|e| e.error)
+        .context("Wrong gossip topic header")?;
+
+    let topic = TopicId::from_bytes(topic_raw);
+    Ok(topic)
+}
+
 async fn subscribe_loop(
     context: &Context,
     mut stream: iroh_gossip::net::GossipReceiver,
diff --git a/src/receive_imf.rs b/src/receive_imf.rs
index 906489c9b..adbc786d1 100644
--- a/src/receive_imf.rs
+++ b/src/receive_imf.rs
@@ -6,12 +6,10 @@ use std::iter;
 use std::sync::LazyLock;
 
 use anyhow::{Context as _, Result, ensure};
-use data_encoding::BASE32_NOPAD;
 use deltachat_contact_tools::{
     ContactAddress, addr_cmp, addr_normalize, may_be_valid_addr, sanitize_bidi_characters,
     sanitize_single_line,
 };
-use iroh_gossip::proto::TopicId;
 use mailparse::SingleInfo;
 use num_traits::FromPrimitive;
 use regex::Regex;
@@ -39,7 +37,7 @@ use crate::mimeparser::{
     AvatarAction, GossipedKey, MimeMessage, PreMessageMode, SystemMessage, parse_message_ids,
 };
 use crate::param::{Param, Params};
-use crate::peer_channels::{add_gossip_peer_from_header, insert_topic_stub};
+use crate::peer_channels::{add_gossip_peer_from_header, insert_topic_stub, iroh_topic_from_str};
 use crate::reaction::{Reaction, set_msg_reaction};
 use crate::rusqlite::OptionalExtension;
 use crate::securejoin::{
@@ -2298,21 +2296,12 @@ RETURNING id
 
     // Maybe set logging xdc and add gossip topics for webxdcs.
     for (part, msg_id) in mime_parser.parts.iter().zip(&created_db_entries) {
-        // check if any part contains a webxdc topic id
-        if part.typ == Viewtype::Webxdc {
-            if let Some(topic) = mime_parser.get_header(HeaderDef::IrohGossipTopic) {
-                // default encoding of topic ids is `hex`.
-                let mut topic_raw = [0u8; 32];
-                BASE32_NOPAD
-                    .decode_mut(topic.to_ascii_uppercase().as_bytes(), &mut topic_raw)
-                    .map_err(|e| e.error)
-                    .context("Wrong gossip topic header")?;
-
-                let topic = TopicId::from_bytes(topic_raw);
-                insert_topic_stub(context, *msg_id, topic).await?;
-            } else {
-                warn!(context, "webxdc doesn't have a gossip topic")
-            }
+        if mime_parser.pre_message != PreMessageMode::Post
+            && part.typ == Viewtype::Webxdc
+            && let Some(topic) = mime_parser.get_header(HeaderDef::IrohGossipTopic)
+        {
+            let topic = iroh_topic_from_str(topic)?;
+            insert_topic_stub(context, *msg_id, topic).await?;
         }
 
         maybe_set_logging_xdc_inner(
@@ -2517,6 +2506,13 @@ async fn handle_post_message(
         return Ok(());
     }
 
+    if part.typ == Viewtype::Webxdc
+        && let Some(topic) = mime_parser.get_header(HeaderDef::IrohGossipTopic)
+    {
+        let topic = iroh_topic_from_str(topic)?;
+        insert_topic_stub(context, msg_id, topic).await?;
+    }
+
     let mut new_params = original_msg.param.clone();
     new_params
         .merge_in_params(part.param.clone())
diff --git a/test-data/webxdc/realtime-check.xdc b/test-data/webxdc/realtime-check.xdc
new file mode 100644
index 000000000..254a728b4
Binary files /dev/null and b/test-data/webxdc/realtime-check.xdc differ