diff --git a/pkg/connector/send_message.go b/pkg/connector/send_message.go
index 3b98017..05e75cf 100644
--- a/pkg/connector/send_message.go
+++ b/pkg/connector/send_message.go
@@ -783,6 +783,20 @@ func (lc *LineClient) HandleMatrixMessage(ctx context.Context, msg *bridgev2.Mat
 		}
 	}
 
+	if shouldRetrySendWithoutReplyRelation(lineMsg, err) {
+		relatedMessageID := lineMsg.RelatedMessageID
+		clearReplyRelation(lineMsg)
+		lc.UserLogin.Bridge.Log.Warn().
+			Err(err).
+			Str("chat_mid", portalMid).
+			Str("related_message_id", relatedMessageID).
+			Msg("SendMessage failed because LINE could not find reply target, retrying without reply relation")
+
+		retryReqSeq := int(time.Now().UnixMilli() % 1_000_000_000)
+		lc.trackReqSeq(retryReqSeq)
+		sentMsg, err = sendLineMessage(retryReqSeq, lineMsg)
+	}
+
 	if err != nil {
 		return nil, err
 	}
@@ -862,6 +876,21 @@ func contentTypeForMsgType(msgType event.MessageType) int {
 	}
 }
 
+func shouldRetrySendWithoutReplyRelation(msg *line.Message, err error) bool {
+	return msg != nil && msg.RelatedMessageID != "" && line.IsTalkExceptionNotFound(err)
+}
+
+func clearReplyRelation(msg *line.Message) {
+	msg.RelatedMessageID = ""
+	msg.MessageRelationType = 0
+	msg.RelatedMessageServiceCode = 0
+	if msg.ContentMetadata != nil {
+		delete(msg.ContentMetadata, "message_relation_server_message_id")
+		delete(msg.ContentMetadata, "message_relation_type")
+		delete(msg.ContentMetadata, "message_relation_service_code")
+	}
+}
+
 func (lc *LineClient) HandleMatrixMessageRemove(ctx context.Context, msg *bridgev2.MatrixMessageRemove) error {
 	reqSeq := int(time.Now().UnixMilli() % 1_000_000_000)
 	lc.trackReqSeq(reqSeq)
diff --git a/pkg/connector/send_message_test.go b/pkg/connector/send_message_test.go
index 7b4a502..3bb308e 100644
--- a/pkg/connector/send_message_test.go
+++ b/pkg/connector/send_message_test.go
@@ -67,3 +67,48 @@ func TestLineGroupE2EEFetchFailureErrorWrapsMissingPrivateKeyStatus(t *testing.T
 		t.Fatalf("status = %#v, want reconnect failure notice", status)
 	}
 }
+
+func TestShouldRetrySendWithoutReplyRelation(t *testing.T) {
+	err := errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":5,"reason":"not found","parameterMap":null}}`)
+	msg := &line.Message{RelatedMessageID: "1234567890"}
+
+	if !shouldRetrySendWithoutReplyRelation(msg, err) {
+		t.Fatal("expected related-message not found error to trigger retry")
+	}
+
+	msg.RelatedMessageID = ""
+	if shouldRetrySendWithoutReplyRelation(msg, err) {
+		t.Fatal("message without reply relation should not retry")
+	}
+
+	msg.RelatedMessageID = "1234567890"
+	if shouldRetrySendWithoutReplyRelation(msg, errors.New("other error")) {
+		t.Fatal("non-LINE not found error should not retry")
+	}
+}
+
+func TestClearReplyRelation(t *testing.T) {
+	msg := &line.Message{
+		RelatedMessageID:          "1234567890",
+		MessageRelationType:       3,
+		RelatedMessageServiceCode: 1,
+		ContentMetadata: map[string]string{
+			"message_relation_server_message_id": "1234567890",
+			"message_relation_type":              "3",
+			"message_relation_service_code":      "1",
+			"keep":                               "value",
+		},
+	}
+
+	clearReplyRelation(msg)
+
+	if msg.RelatedMessageID != "" || msg.MessageRelationType != 0 || msg.RelatedMessageServiceCode != 0 {
+		t.Fatalf("reply relation was not cleared: %#v", msg)
+	}
+	if _, ok := msg.ContentMetadata["message_relation_server_message_id"]; ok {
+		t.Fatal("content metadata relation ID was not cleared")
+	}
+	if got := msg.ContentMetadata["keep"]; got != "value" {
+		t.Fatalf("unrelated content metadata = %q, want value", got)
+	}
+}
diff --git a/pkg/line/errors.go b/pkg/line/errors.go
index 74fb9f3..c99ccf9 100644
--- a/pkg/line/errors.go
+++ b/pkg/line/errors.go
@@ -145,6 +145,23 @@ func IsGroupKeyNotRegisteredError(err error) bool {
 		strings.Contains(msg, "group key is not registered")
 }
 
+// IsTalkExceptionNotFound returns true when LINE wraps a TalkException code 5
+// "not found" response. Callers must interpret the method context themselves:
+// the same code can mean different missing resources for different Talk APIs.
+func IsTalkExceptionNotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	return hasResponseErrorCode(msg) &&
+		strings.Contains(msg, "talkexception") &&
+		(strings.Contains(msg, "\"code\":5,") ||
+			strings.Contains(msg, "\"code\":5}") ||
+			strings.Contains(msg, "\"code\": 5,")) &&
+		(strings.Contains(msg, "\"reason\":\"not found\"") ||
+			strings.Contains(msg, "\"reason\": \"not found\""))
+}
+
 // IsNotAMemberError returns true when the API reports the user is not a member of a chat.
 func IsNotAMemberError(err error) bool {
 	if err == nil {
@@ -167,7 +184,9 @@ func IsInvalidPaidReactionType(err error) bool {
 }
 
 func hasResponseErrorCode(msg string) bool {
-	return strings.Contains(msg, "\"code\":10051") || strings.Contains(msg, "code 10051")
+	return strings.Contains(msg, "\"code\":10051") ||
+		strings.Contains(msg, "\"code\": 10051") ||
+		strings.Contains(msg, "code 10051")
 }
 
 func isNoUsableE2EEGroupKeyTalkException(message string, data talkExceptionData) bool {
diff --git a/pkg/line/errors_test.go b/pkg/line/errors_test.go
index 652f128..372e850 100644
--- a/pkg/line/errors_test.go
+++ b/pkg/line/errors_test.go
@@ -94,3 +94,19 @@ func TestIsUnauthorizedStatus(t *testing.T) {
 		}
 	}
 }
+
+func TestIsTalkExceptionNotFound(t *testing.T) {
+	err := errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":5,"reason":"not found","parameterMap":null}}`)
+	if !IsTalkExceptionNotFound(err) {
+		t.Fatal("expected TalkException code 5 not found to be detected")
+	}
+	if IsTalkExceptionNotFound(errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":10,"reason":"not a member","parameterMap":null}}`)) {
+		t.Fatal("not-a-member should not be classified as not-found")
+	}
+	if IsTalkExceptionNotFound(errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":5,"reason":"different","parameterMap":null}}`)) {
+		t.Fatal("code 5 with a different reason should not be classified as not-found")
+	}
+	if IsTalkExceptionNotFound(nil) {
+		t.Fatal("nil should not be classified as not-found")
+	}
+}