diff --git a/pkg/connector/send_message_test.go b/pkg/connector/send_message_test.go
index 2b6511d..071d3cd 100644
--- a/pkg/connector/send_message_test.go
+++ b/pkg/connector/send_message_test.go
@@ -123,6 +123,14 @@ func TestLineGroupE2EEFetchFailureErrorAllowsNoUsableGroupKeyFallback(t *testing
 	}
 }
 
+func TestLineGroupE2EEFetchFailureErrorAllowsOversizedGroupFallback(t *testing.T) {
+	registerErr := errors.New(`registerE2EEGroupKey failed: API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":100,"reason":"exceed max member","parameterMap":{}}}`)
+	err := lineGroupE2EEFetchFailureError(fmt.Errorf("auto-register group key: %w", registerErr))
+	if err != nil {
+		t.Fatalf("err = %v, want nil to allow plaintext fallback", err)
+	}
+}
+
 func TestLineGroupE2EEFetchFailureErrorWrapsMissingPrivateKeyStatus(t *testing.T) {
 	err := lineGroupE2EEFetchFailureError(fmt.Errorf("failed to unwrap group key: %w", e2ee.ErrMissingOwnPrivateKey))
 	var status bridgev2.MessageStatus
diff --git a/pkg/line/errors.go b/pkg/line/errors.go
index 8956d37..17c50b3 100644
--- a/pkg/line/errors.go
+++ b/pkg/line/errors.go
@@ -129,11 +129,14 @@ func IsNoUsableE2EEGroupKey(err error) bool {
 		return true
 	}
 	// Detect TalkException codes in raw API error strings (HTTP 400 with code 10051).
-	// Code 98 = member has LS off; Code 1 = auth failed.
+	// Code 98 = member has LS off; Code 1 = auth failed;
+	// Code 100 "exceed max member" = the group is too large for key registration.
 	// NOTE: Code 5 "not found" is handled by IsGroupKeyNotFound (auto-register), NOT here.
-	if strings.Contains(msg, "\"code\":10051") && strings.Contains(msg, "talkexception") {
+	if hasResponseErrorCode(msg) && strings.Contains(msg, "talkexception") {
 		if strings.Contains(msg, "\"code\":98,") || strings.Contains(msg, "\"code\":98}") ||
-			strings.Contains(msg, "\"code\":1,") || strings.Contains(msg, "\"code\":1}") {
+			strings.Contains(msg, "\"code\":1,") || strings.Contains(msg, "\"code\":1}") ||
+			(hasJSONCode(msg, 100) && (strings.Contains(msg, `"reason":"exceed max member"`) ||
+				strings.Contains(msg, `"reason": "exceed max member"`))) {
 			return true
 		}
 	}
@@ -222,8 +225,10 @@ func isNoUsableE2EEGroupKeyTalkException(message string, data talkExceptionData)
 	}
 	// Error 5 "not found" = no group shared key exists
 	// Error 98 "member settings off" = at least one member has LS disabled
+	// Error 100 "exceed max member" = the group is too large for key registration
 	return (data.Code == 5 && strings.EqualFold(data.Reason, "not found")) ||
-		(data.Code == 98 && strings.Contains(strings.ToLower(data.Reason), "member settings off"))
+		(data.Code == 98 && strings.Contains(strings.ToLower(data.Reason), "member settings off")) ||
+		(data.Code == 100 && strings.EqualFold(strings.TrimSpace(data.Reason), "exceed max member"))
 }
 
 func parseTalkExceptionData(raw json.RawMessage) talkExceptionData {
diff --git a/pkg/line/errors_test.go b/pkg/line/errors_test.go
index 345326d..a268a57 100644
--- a/pkg/line/errors_test.go
+++ b/pkg/line/errors_test.go
@@ -121,3 +121,30 @@ func TestIsTalkExceptionNotFound(t *testing.T) {
 		t.Fatal("nil should not be classified as not-found")
 	}
 }
+
+func TestIsNoUsableE2EEGroupKeyExceedMaxMember(t *testing.T) {
+	err := errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":100,"reason":"exceed max member","parameterMap":{}}}`)
+	if !IsNoUsableE2EEGroupKey(err) {
+		t.Fatal("expected exceed max member to disable group E2EE")
+	}
+
+	otherCode100 := errors.New(`API error 400: {"code":10051,"message":"RESPONSE_ERROR","data":{"name":"TalkException","message":"TalkException","code":100,"reason":"different reason","parameterMap":{}}}`)
+	if IsNoUsableE2EEGroupKey(otherCode100) {
+		t.Fatal("unrelated TalkException code 100 must not disable group E2EE")
+	}
+
+	phraseOutsideReason := errors.New(`API error 400: {"code":10051,"message":"exceed max member","data":{"name":"TalkException","code":100,"reason":"different reason"}}`)
+	if IsNoUsableE2EEGroupKey(phraseOutsideReason) {
+		t.Fatal("exceed max member outside the reason field must not disable group E2EE")
+	}
+}
+
+func TestNoUsableE2EEGroupKeyTalkExceptionExceedMaxMember(t *testing.T) {
+	if !isNoUsableE2EEGroupKeyTalkException("RESPONSE_ERROR", talkExceptionData{
+		Name:   "TalkException",
+		Code:   100,
+		Reason: "exceed max member",
+	}) {
+		t.Fatal("expected structured exceed max member response to disable group E2EE")
+	}
+}