diff --git a/docs/adr/custom-gateway.md b/docs/adr/custom-gateway.md
index 8b894baa..99d5bc0c 100644
--- a/docs/adr/custom-gateway.md
+++ b/docs/adr/custom-gateway.md
@@ -148,6 +148,9 @@ Key fields in the base schema:
 }
 ```
 
+Key fields in the outbound reply:
+- **`reply_to`**: the `event_id` of the inbound `GatewayEvent` that triggered this reply. The gateway can use this for reply correlation — e.g., looking up a cached LINE reply token to prefer the free Reply API over the quota-consuming Push API. Empty string if the reply is not associated with a specific inbound event (e.g., cron-triggered messages).
+
 ### Design Principles for the Schema
 
 - **Platform field is metadata, not routing logic** — OAB uses it for session key construction and sender context, but does not branch behavior on it
diff --git a/src/adapter.rs b/src/adapter.rs
index d64f1db1..a66b5aa9 100644
--- a/src/adapter.rs
+++ b/src/adapter.rs
@@ -22,7 +22,7 @@ use crate::reactions::StatusReactionController;
 /// Compare with `SenderContext`, which is **metadata for the agent**: there
 /// `channel_id` is the parent channel and `thread_id` is the thread,
 /// matching Slack's model for cross-platform consistency.
-#[derive(Clone, Debug, Hash, Eq, PartialEq)]
+#[derive(Clone, Debug)]
 pub struct ChannelRef {
     pub platform: String,
     pub channel_id: String,
@@ -31,6 +31,31 @@ pub struct ChannelRef {
     pub thread_id: Option<String>,
     /// Parent channel if this is a thread-as-channel (Discord).
     pub parent_id: Option<String>,
+    /// Originating gateway event ID, propagated back in `GatewayReply.reply_to`
+    /// so the gateway can correlate replies with inbound events (e.g. LINE reply tokens).
+    /// Excluded from Hash/Eq — two ChannelRefs pointing to the same channel are
+    /// equal regardless of which event they originated from.
+    pub origin_event_id: Option<String>,
+}
+
+impl PartialEq for ChannelRef {
+    fn eq(&self, other: &Self) -> bool {
+        self.platform == other.platform
+            && self.channel_id == other.channel_id
+            && self.thread_id == other.thread_id
+            && self.parent_id == other.parent_id
+    }
+}
+
+impl Eq for ChannelRef {}
+
+impl std::hash::Hash for ChannelRef {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.platform.hash(state);
+        self.channel_id.hash(state);
+        self.thread_id.hash(state);
+        self.parent_id.hash(state);
+    }
 }
 
 /// Identifies a message across platforms.
@@ -540,4 +565,66 @@ mod tests {
         // Verify the method is callable and returns the declared value
         assert!(!adapter.use_streaming(false));
     }
+
+    #[test]
+    fn origin_event_id_excluded_from_eq() {
+        let a = ChannelRef {
+            platform: "line".into(),
+            channel_id: "U123".into(),
+            thread_id: None,
+            parent_id: None,
+            origin_event_id: Some("evt_aaa".into()),
+        };
+        let b = ChannelRef {
+            platform: "line".into(),
+            channel_id: "U123".into(),
+            thread_id: None,
+            parent_id: None,
+            origin_event_id: Some("evt_bbb".into()),
+        };
+        assert_eq!(a, b, "same channel with different event IDs must be equal");
+    }
+
+    #[test]
+    fn origin_event_id_excluded_from_hash() {
+        use std::collections::HashMap;
+        let a = ChannelRef {
+            platform: "line".into(),
+            channel_id: "U123".into(),
+            thread_id: None,
+            parent_id: None,
+            origin_event_id: Some("evt_aaa".into()),
+        };
+        let b = ChannelRef {
+            platform: "line".into(),
+            channel_id: "U123".into(),
+            thread_id: None,
+            parent_id: None,
+            origin_event_id: Some("evt_bbb".into()),
+        };
+        let mut map = HashMap::new();
+        map.insert(a, "first");
+        // b should hit the same bucket and overwrite
+        map.insert(b, "second");
+        assert_eq!(map.len(), 1);
+        assert_eq!(map.values().next(), Some(&"second"));
+    }
+
+    #[test]
+    fn origin_event_id_survives_clone() {
+        let ch = ChannelRef {
+            platform: "line".into(),
+            channel_id: "U123".into(),
+            thread_id: None,
+            parent_id: None,
+            origin_event_id: Some("evt_abc".into()),
+        };
+        // Simulates create_thread propagation: clone preserves origin_event_id
+        let thread_ch = ChannelRef {
+            thread_id: Some("topic_1".into()),
+            origin_event_id: ch.origin_event_id.clone(),
+            ..ch.clone()
+        };
+        assert_eq!(thread_ch.origin_event_id.as_deref(), Some("evt_abc"));
+    }
 }
diff --git a/src/cron.rs b/src/cron.rs
index d0866ee9..cf922192 100644
--- a/src/cron.rs
+++ b/src/cron.rs
@@ -229,6 +229,7 @@ async fn fire_cronjob(
         channel_id: job.channel.clone(),
         thread_id: job.thread_id.clone(),
         parent_id: None,
+        origin_event_id: None,
     };
 
     // Send visible message first so users see what triggered
diff --git a/src/discord.rs b/src/discord.rs
index 3584786d..86e12990 100644
--- a/src/discord.rs
+++ b/src/discord.rs
@@ -99,6 +99,7 @@ impl ChatAdapter for DiscordAdapter {
             channel_id: thread.id.to_string(),
             thread_id: None,
             parent_id: Some(channel.channel_id.clone()),
+            origin_event_id: None,
         })
     }
 
@@ -582,6 +583,7 @@ impl EventHandler for Handler {
                 channel_id: msg.channel_id.get().to_string(),
                 thread_id: None,
                 parent_id: thread_parent_id.clone(),
+                origin_event_id: None,
             }
         } else {
             match get_or_create_thread(&ctx, &adapter, &msg, &prompt).await {
@@ -817,6 +819,7 @@ fn discord_msg_ref(msg: &Message) -> MessageRef {
             channel_id: msg.channel_id.get().to_string(),
             thread_id: None,
             parent_id: None,
+            origin_event_id: None,
         },
         message_id: msg.id.to_string(),
     }
@@ -837,6 +840,7 @@ async fn get_or_create_thread(
                 channel_id: msg.channel_id.get().to_string(),
                 thread_id: None,
                 parent_id: None,
+                origin_event_id: None,
             });
         }
     }
@@ -847,6 +851,7 @@ async fn get_or_create_thread(
         channel_id: msg.channel_id.get().to_string(),
         thread_id: None,
         parent_id: None,
+        origin_event_id: None,
     };
     let trigger_ref = discord_msg_ref(msg);
     match adapter.create_thread(&parent, &trigger_ref, &thread_name).await {
@@ -877,6 +882,7 @@ async fn get_or_create_thread(
                 channel_id: existing.id.to_string(),
                 thread_id: None,
                 parent_id: Some(msg.channel_id.get().to_string()),
+                origin_event_id: None,
             })
         }
         Err(e) => Err(e),
@@ -1446,6 +1452,7 @@ mod tests {
             channel_id: "111".into(),
             thread_id: None,
             parent_id: None,
+            origin_event_id: None,
         };
         assert_eq!(DiscordAdapter::resolve_channel(&ch), "111");
     }
@@ -1457,6 +1464,7 @@ mod tests {
             channel_id: "111".into(),
             thread_id: Some("222".into()),
             parent_id: None,
+            origin_event_id: None,
         };
         assert_eq!(DiscordAdapter::resolve_channel(&ch), "222");
     }
diff --git a/src/gateway.rs b/src/gateway.rs
index a1721c4c..da51e4e1 100644
--- a/src/gateway.rs
+++ b/src/gateway.rs
@@ -15,7 +15,6 @@ use tracing::{error, info, warn};
 struct GatewayEvent {
     #[allow(dead_code)]
     schema: String,
-    #[allow(dead_code)]
     event_id: String,
     #[allow(dead_code)]
     timestamp: String,
@@ -139,7 +138,7 @@ impl ChatAdapter for GatewayAdapter {
     async fn send_message(&self, channel: &ChannelRef, content: &str) -> Result<MessageRef> {
         let reply = GatewayReply {
             schema: "openab.gateway.reply.v1".into(),
-            reply_to: String::new(),
+            reply_to: channel.origin_event_id.clone().unwrap_or_default(),
             platform: channel.platform.clone(),
             channel: ReplyChannel {
                 id: channel.channel_id.clone(),
@@ -196,6 +195,7 @@ impl ChatAdapter for GatewayAdapter {
                 channel_id: channel.channel_id.clone(),
                 thread_id: resp.thread_id,
                 parent_id: None,
+                origin_event_id: channel.origin_event_id.clone(),
             }),
             Ok(Ok(resp)) => {
                 warn!(err = ?resp.error, "create_topic failed, falling back to same channel");
@@ -364,6 +364,7 @@ pub async fn run_gateway_adapter(
                                         channel_id: event.channel.id.clone(),
                                         thread_id: event.channel.thread_id.clone(),
                                         parent_id: None,
+                                        origin_event_id: Some(event.event_id.clone()),
                                     };
 
                                     let sender_ctx = SenderContext {
diff --git a/src/slack.rs b/src/slack.rs
index c0b43b93..7619255f 100644
--- a/src/slack.rs
+++ b/src/slack.rs
@@ -350,6 +350,7 @@ impl ChatAdapter for SlackAdapter {
                 channel_id: channel.channel_id.clone(),
                 thread_id: channel.thread_id.clone(),
                 parent_id: None,
+                origin_event_id: None,
             },
             message_id: ts.to_string(),
         })
@@ -368,6 +369,7 @@ impl ChatAdapter for SlackAdapter {
             channel_id: channel.channel_id.clone(),
             thread_id: Some(trigger_msg.message_id.clone()),
             parent_id: None,
+            origin_event_id: None,
         })
     }
 
@@ -692,6 +694,7 @@ pub async fn run_slack_adapter(
                                                                         channel_id: channel_id.to_string(),
                                                                         thread_id: event["thread_ts"].as_str().map(|s| s.to_string()),
                                                                         parent_id: None,
+                                                                        origin_event_id: None,
                                                                     };
                                                                     let _ = adapter.send_message(&warn_channel, &user_message).await;
                                                                 }
@@ -956,6 +959,7 @@ async fn handle_message(
                 channel_id: channel_id.clone(),
                 thread_id: thread_ts.clone(),
                 parent_id: None,
+                origin_event_id: None,
             },
             message_id: ts.clone(),
         };
@@ -1021,6 +1025,7 @@ async fn handle_message(
                             channel_id: channel_id.clone(),
                             thread_id: thread_ts.clone(),
                             parent_id: None,
+                            origin_event_id: None,
                         },
                         message_id: ts.clone(),
                     };
@@ -1096,6 +1101,7 @@ async fn handle_message(
             channel_id: channel_id.clone(),
             thread_id: thread_ts.clone(),
             parent_id: None,
+            origin_event_id: None,
         },
         message_id: ts.clone(),
     };
@@ -1106,6 +1112,7 @@ async fn handle_message(
         channel_id: channel_id.clone(),
         thread_id: Some(thread_ts.unwrap_or(ts)),
         parent_id: None,
+        origin_event_id: None,
     };
 
     // Serialize sender context with Slack-native key names so agents calling