diff --git a/Cargo.lock b/Cargo.lock
index 634cd3c22..f4c4d4c3f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1967,6 +1967,7 @@ dependencies = [
  "futures-util",
  "hive-sock-client",
  "matrix-sdk",
+ "matrix-sdk-base",
  "mime",
  "mime_guess",
  "reqwest",
@@ -1976,6 +1977,7 @@ dependencies = [
  "serde_json",
  "swarm-matrix-client",
  "swarm-secret-client",
+ "tempfile",
  "tokio",
  "tracing",
  "tracing-subscriber",
diff --git a/hive-matrix-mcp/Cargo.toml b/hive-matrix-mcp/Cargo.toml
index 31f3a9b39..96bd9cb72 100644
--- a/hive-matrix-mcp/Cargo.toml
+++ b/hive-matrix-mcp/Cargo.toml
@@ -14,6 +14,10 @@ clap.workspace = true
 futures-util.workspace = true
 hive-sock-client.workspace = true
 matrix-sdk.workspace = true
+# The crypto store trait and its `Changes`, for the to-device token reset in `sliding`.
+matrix-sdk-base = { version = "0.18", default-features = false, features = [
+  "e2e-encryption",
+] }
 mime = "0.3"
 mime_guess = "2"
 reqwest.workspace = true
@@ -35,4 +39,5 @@ name = "hive-matrix-daemon"
 path = "src/main.rs"
 
 [dev-dependencies]
+tempfile = "3"
 tokio = { workspace = true, features = ["test-util"] }
diff --git a/hive-matrix-mcp/src/client.rs b/hive-matrix-mcp/src/client.rs
index f128c9b6c..7436181bb 100644
--- a/hive-matrix-mcp/src/client.rs
+++ b/hive-matrix-mcp/src/client.rs
@@ -137,6 +137,7 @@ pub async fn build_and_restore(
     fs::create_dir_all(state_dir)
         .await
         .with_context(|| format!("mkdir matrix state dir {}", state_dir.display()))?;
+    crate::sliding::reset_classic_to_device_token(state_dir).await;
     let client = Client::builder()
         .homeserver_url(homeserver)
         .sqlite_store(state_dir, None)
diff --git a/hive-matrix-mcp/src/sliding.rs b/hive-matrix-mcp/src/sliding.rs
index fd62a9371..a48407765 100644
--- a/hive-matrix-mcp/src/sliding.rs
+++ b/hive-matrix-mcp/src/sliding.rs
@@ -11,11 +11,14 @@
 use std::future::Future;
 use std::time::Duration;
 
-use anyhow::{Result, bail};
+use std::path::Path;
+
+use anyhow::{Result, anyhow, bail};
 use futures_util::StreamExt as _;
-use matrix_sdk::Client;
 use matrix_sdk::ruma::events::StateEventType;
 use matrix_sdk::sliding_sync::{SlidingSyncList, SlidingSyncMode, Version};
+use matrix_sdk::{Client, SqliteCryptoStore};
+use matrix_sdk_base::crypto::store::{CryptoStore, types::Changes};
 
 /// `classic` forces `/sync`; anything else (or unset) lets the server's
 /// advertised support decide.
@@ -48,6 +51,53 @@ fn decide(forced_classic: bool, advertised: bool) -> bool {
     advertised && !forced_classic
 }
 
+/// The to-device token a classic `/sync` leaves in the crypto store cannot start
+/// a sliding sync: the sdk sends the store's token as `extensions.to_device.since`
+/// and the homeserver refuses anything but an integer (400, "should look like an
+/// int"), which sent every start back to `/sync`, whose own token is then in the
+/// store again. This puts `0` in its place (the server redelivers only the
+/// to-device events nobody acknowledged), so the first sliding poll is valid.
+/// A token that already is an integer, and a store with none, are left alone.
+/// Best effort: a failure is logged and the sliding sync tries as before.
+///
+/// Call it before the client opens the store.
+pub async fn reset_classic_to_device_token(state_dir: &Path) {
+    let result = async {
+        let store = SqliteCryptoStore::open(state_dir, None).await?;
+        reset_token(&store).await
+    }
+    .await;
+    match result {
+        Ok(true) => {
+            tracing::info!(
+                "reset the crypto store's /sync token so sliding sync can start from it"
+            );
+        }
+        Ok(false) => {}
+        Err(e) => tracing::warn!(error = %e, "could not check the crypto store's to-device token"),
+    }
+}
+
+/// Whether `store` held a token that is not an integer, now replaced by `0`.
+async fn reset_token(store: &impl CryptoStore) -> Result<bool> {
+    let held = store
+        .next_batch_token()
+        .await
+        .map_err(|e| anyhow!("{e:?}"))?;
+    if held.is_none_or(|t| !t.is_empty() && t.bytes().all(|b| b.is_ascii_digit())) {
+        return Ok(false);
+    }
+    let reset = Changes {
+        next_batch_token: Some("0".to_owned()),
+        ..Changes::default()
+    };
+    store
+        .save_changes(reset)
+        .await
+        .map_err(|e| anyhow!("{e:?}"))?;
+    Ok(true)
+}
+
 /// The state the daemon reads from a room: its name, whether it is
 /// encrypted, who is in it (lazily) and the power levels sends depend on.
 fn required_state() -> Vec<(StateEventType, String)> {
@@ -127,8 +177,47 @@ impl ErrorRun {
 
 #[cfg(test)]
 mod tests {
-    use super::{ErrorRun, MAX_CONSECUTIVE_ERRORS, decide, required_state};
+    use super::{
+        ErrorRun, MAX_CONSECUTIVE_ERRORS, decide, required_state, reset_classic_to_device_token,
+    };
+    use matrix_sdk::SqliteCryptoStore;
     use matrix_sdk::ruma::events::StateEventType;
+    use matrix_sdk_base::crypto::store::{CryptoStore, types::Changes};
+
+    async fn token_after_reset(before: Option<&str>) -> Option<String> {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let store = SqliteCryptoStore::open(dir.path(), None)
+            .await
+            .expect("open");
+        if let Some(token) = before {
+            let seeded = Changes {
+                next_batch_token: Some(token.to_owned()),
+                ..Changes::default()
+            };
+            store.save_changes(seeded).await.expect("seed");
+        }
+        drop(store);
+        reset_classic_to_device_token(dir.path()).await;
+        let store = SqliteCryptoStore::open(dir.path(), None)
+            .await
+            .expect("reopen");
+        store.next_batch_token().await.expect("read")
+    }
+
+    #[tokio::test]
+    async fn a_classic_sync_token_is_replaced_by_zero() {
+        let classic = "s251551370_223970929_3253937_0_41771_2_1_1_1";
+        assert_eq!(token_after_reset(Some(classic)).await.as_deref(), Some("0"));
+    }
+
+    #[tokio::test]
+    async fn an_integer_token_and_an_empty_store_are_left_alone() {
+        assert_eq!(
+            token_after_reset(Some("41771")).await.as_deref(),
+            Some("41771")
+        );
+        assert_eq!(token_after_reset(None).await, None);
+    }
 
     #[test]
     fn sliding_sync_needs_server_support_and_no_opt_out() {