diff --git a/pkg/connector/capabilities_test.go b/pkg/connector/capabilities_test.go
new file mode 100644
index 0000000..4620ad8
--- /dev/null
+++ b/pkg/connector/capabilities_test.go
@@ -0,0 +1,28 @@
+package connector
+
+import (
+	"context"
+	"testing"
+
+	"maunium.net/go/mautrix/event"
+)
+
+func TestCapabilitiesAdvertiseFileSizeLimit(t *testing.T) {
+	const wantMaxFileSize int64 = 100 * 1024 * 1024
+	caps := (&LineClient{}).GetCapabilities(context.Background(), nil)
+	for _, messageType := range []event.CapabilityMsgType{
+		event.MsgImage,
+		event.MsgFile,
+		event.MsgVideo,
+		event.MsgAudio,
+		event.CapMsgVoice,
+	} {
+		features := caps.File[messageType]
+		if features == nil {
+			t.Fatalf("%s file features are missing", messageType)
+		}
+		if features.MaxSize != wantMaxFileSize {
+			t.Errorf("%s MaxSize = %d, want %d", messageType, features.MaxSize, wantMaxFileSize)
+		}
+	}
+}
diff --git a/pkg/connector/connector.go b/pkg/connector/connector.go
index 54cd1cf..cae3a39 100644
--- a/pkg/connector/connector.go
+++ b/pkg/connector/connector.go
@@ -50,7 +50,7 @@ func (lc *LineConnector) Start(ctx context.Context) error {
 }
 
 func (lc *LineConnector) GetBridgeInfoVersion() (info, capabilities int) {
-	return 1, 2
+	return 1, 3
 }
 
 func (lc *LineConnector) GetCapabilities() *bridgev2.NetworkGeneralCapabilities {
diff --git a/pkg/connector/handlers/audio.go b/pkg/connector/handlers/audio.go
index 1e2236b..a0f7ec8 100644
--- a/pkg/connector/handlers/audio.go
+++ b/pkg/connector/handlers/audio.go
@@ -15,6 +15,10 @@ import (
 
 // ConvertAudio converts a LINE audio message to a Matrix audio message.
 func (h *Handler) ConvertAudio(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, decryptedBody string, relatesTo *event.RelatesTo) (*bridgev2.ConvertedMessage, error) {
+	if oversized := h.oversizedMediaNoticeFromMetadata(data.ContentMetadata, relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	client := h.NewClient()
 	oid := data.ContentMetadata["OID"]
 	isPlainMedia := oid == ""
@@ -95,6 +99,10 @@ func (h *Handler) ConvertAudio(ctx context.Context, portal *bridgev2.Portal, int
 		}
 	}
 
+	if oversized := h.oversizedMediaNotice(int64(len(audioData)), "downloaded", relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	var duration int
 	if durationStr := data.ContentMetadata["DURATION"]; durationStr != "" {
 		if d, err := strconv.Atoi(durationStr); err == nil {
diff --git a/pkg/connector/handlers/file.go b/pkg/connector/handlers/file.go
index e2eac47..5ab1b16 100644
--- a/pkg/connector/handlers/file.go
+++ b/pkg/connector/handlers/file.go
@@ -14,6 +14,10 @@ import (
 
 // ConvertFile converts a LINE file message to a Matrix file message.
 func (h *Handler) ConvertFile(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, decryptedBody string, relatesTo *event.RelatesTo) (*bridgev2.ConvertedMessage, error) {
+	if oversized := h.oversizedMediaNoticeFromMetadata(data.ContentMetadata, relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	client := h.NewClient()
 	oid := data.ContentMetadata["OID"]
 	isPlainMedia := oid == ""
@@ -89,6 +93,10 @@ func (h *Handler) ConvertFile(ctx context.Context, portal *bridgev2.Portal, inte
 		}
 	}
 
+	if oversized := h.oversizedMediaNotice(int64(len(fileData)), "downloaded", relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	if fileName == "" {
 		fileName = data.ContentMetadata["FILE_NAME"]
 	}
diff --git a/pkg/connector/handlers/image.go b/pkg/connector/handlers/image.go
index 83f315b..4c9792a 100644
--- a/pkg/connector/handlers/image.go
+++ b/pkg/connector/handlers/image.go
@@ -15,6 +15,10 @@ import (
 
 // ConvertImage converts a LINE image message to a Matrix image message.
 func (h *Handler) ConvertImage(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, decryptedBody string, relatesTo *event.RelatesTo) (*bridgev2.ConvertedMessage, error) {
+	if oversized := h.oversizedMediaNoticeFromMetadata(data.ContentMetadata, relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	client := h.NewClient()
 	oid := data.ContentMetadata["OID"]
 	isPlainMedia := oid == ""
@@ -95,6 +99,10 @@ func (h *Handler) ConvertImage(ctx context.Context, portal *bridgev2.Portal, int
 		}
 	}
 
+	if oversized := h.oversizedMediaNotice(int64(len(imgData)), "downloaded", relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	// Upload to Matrix
 	uploadStart := time.Now()
 	mxc, file, err := intent.UploadMedia(ctx, portal.MXID, imgData, "image.jpg", "image/jpeg")
diff --git a/pkg/connector/handlers/media_size.go b/pkg/connector/handlers/media_size.go
new file mode 100644
index 0000000..3f9966e
--- /dev/null
+++ b/pkg/connector/handlers/media_size.go
@@ -0,0 +1,48 @@
+package handlers
+
+import (
+	"strconv"
+	"strings"
+
+	"maunium.net/go/mautrix/bridgev2"
+	"maunium.net/go/mautrix/event"
+)
+
+const (
+	BeeperMaxFileSize          = 100 * 1024 * 1024
+	encryptedMediaSizeOverhead = 32
+	oversizedFileBody          = "This file exceeds Beeper's 100MB file size limit. Open LINE to view it."
+)
+
+func (h *Handler) oversizedMediaNoticeFromMetadata(metadata map[string]string, relatesTo *event.RelatesTo) *bridgev2.ConvertedMessage {
+	size, err := strconv.ParseInt(strings.TrimSpace(metadata["FILE_SIZE"]), 10, 64)
+	// E2EE media stored in OBS may include a 32-byte HMAC in FILE_SIZE. Keep
+	// borderline values on the authoritative post-decryption size-check path.
+	if err != nil || size <= BeeperMaxFileSize+encryptedMediaSizeOverhead {
+		return nil
+	}
+	return h.oversizedMediaNotice(size, "metadata", relatesTo)
+}
+
+func (h *Handler) oversizedMediaNotice(size int64, sizeSource string, relatesTo *event.RelatesTo) *bridgev2.ConvertedMessage {
+	if size <= BeeperMaxFileSize {
+		return nil
+	}
+	h.Log.Warn().
+		Int64("size_bytes", size).
+		Int("limit_bytes", BeeperMaxFileSize).
+		Str("size_source", sizeSource).
+		Msg("Skipping oversized LINE media upload")
+	return &bridgev2.ConvertedMessage{
+		Parts: []*bridgev2.ConvertedMessagePart{
+			{
+				Type: event.EventMessage,
+				Content: &event.MessageEventContent{
+					MsgType:   event.MsgNotice,
+					Body:      oversizedFileBody,
+					RelatesTo: relatesTo,
+				},
+			},
+		},
+	}
+}
diff --git a/pkg/connector/handlers/media_size_test.go b/pkg/connector/handlers/media_size_test.go
new file mode 100644
index 0000000..7b50b38
--- /dev/null
+++ b/pkg/connector/handlers/media_size_test.go
@@ -0,0 +1,108 @@
+package handlers
+
+import (
+	"context"
+	"testing"
+
+	"github.com/rs/zerolog"
+	"maunium.net/go/mautrix/bridgev2"
+	"maunium.net/go/mautrix/event"
+
+	"github.com/highesttt/matrix-line-messenger/pkg/line"
+)
+
+func TestOversizedMediaNotice(t *testing.T) {
+	h := &Handler{Log: zerolog.Nop()}
+	relatesTo := &event.RelatesTo{}
+	converted := h.oversizedMediaNotice(BeeperMaxFileSize+1, "downloaded", relatesTo)
+	if converted == nil || len(converted.Parts) != 1 {
+		t.Fatalf("converted = %#v, want one notice part", converted)
+	}
+
+	part := converted.Parts[0]
+	if part.Type != event.EventMessage {
+		t.Fatalf("event type = %v, want %v", part.Type, event.EventMessage)
+	}
+	if part.Content == nil {
+		t.Fatal("notice content is nil")
+	}
+	if part.Content.MsgType != event.MsgNotice {
+		t.Fatalf("message type = %v, want %v", part.Content.MsgType, event.MsgNotice)
+	}
+	const expectedBody = "This file exceeds Beeper's 100MB file size limit. Open LINE to view it."
+	if part.Content.Body != expectedBody {
+		t.Fatalf("body = %q, want %q", part.Content.Body, expectedBody)
+	}
+	if part.Content.RelatesTo != relatesTo {
+		t.Fatalf("relates_to = %#v, want original pointer %#v", part.Content.RelatesTo, relatesTo)
+	}
+}
+
+func TestOversizedMediaNoticeAllowsLimitAndBelow(t *testing.T) {
+	h := &Handler{Log: zerolog.Nop()}
+	for _, size := range []int64{0, BeeperMaxFileSize - 1, BeeperMaxFileSize} {
+		if converted := h.oversizedMediaNotice(size, "downloaded", nil); converted != nil {
+			t.Fatalf("size %d converted = %#v, want nil", size, converted)
+		}
+	}
+}
+
+func TestOversizedMediaMetadataShortCircuitsAllMediaHandlers(t *testing.T) {
+	h := &Handler{
+		Log: zerolog.Nop(),
+		NewClient: func() *line.Client {
+			t.Fatal("NewClient was called for media declared over the size limit")
+			return nil
+		},
+	}
+	message := line.Message{
+		ID: "message-id",
+		ContentMetadata: map[string]string{
+			"FILE_SIZE": "104857633",
+		},
+	}
+	relatesTo := &event.RelatesTo{}
+
+	tests := map[string]func() (*bridgev2.ConvertedMessage, error){
+		"image": func() (*bridgev2.ConvertedMessage, error) {
+			return h.ConvertImage(context.Background(), nil, nil, message, "", relatesTo)
+		},
+		"video": func() (*bridgev2.ConvertedMessage, error) {
+			return h.ConvertVideo(context.Background(), nil, nil, message, "", relatesTo)
+		},
+		"audio and voice": func() (*bridgev2.ConvertedMessage, error) {
+			return h.ConvertAudio(context.Background(), nil, nil, message, "", relatesTo)
+		},
+		"file": func() (*bridgev2.ConvertedMessage, error) {
+			return h.ConvertFile(context.Background(), nil, nil, message, "", relatesTo)
+		},
+	}
+	for name, convert := range tests {
+		t.Run(name, func(t *testing.T) {
+			converted, err := convert()
+			if err != nil {
+				t.Fatal(err)
+			}
+			if converted == nil || len(converted.Parts) != 1 || converted.Parts[0].Content.Body != oversizedFileBody {
+				t.Fatalf("converted = %#v, want oversized notice", converted)
+			}
+		})
+	}
+}
+
+func TestOversizedMediaMetadataFallsBackForMissingInvalidAndAllowedSizes(t *testing.T) {
+	h := &Handler{Log: zerolog.Nop()}
+	for name, metadata := range map[string]map[string]string{
+		"missing":                     nil,
+		"invalid":                     {"FILE_SIZE": "not-a-number"},
+		"negative":                    {"FILE_SIZE": "-1"},
+		"at limit":                    {"FILE_SIZE": "104857600"},
+		"possible encrypted overhead": {"FILE_SIZE": "104857632"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			if converted := h.oversizedMediaNoticeFromMetadata(metadata, nil); converted != nil {
+				t.Fatalf("converted = %#v, want normal download fallback", converted)
+			}
+		})
+	}
+}
diff --git a/pkg/connector/handlers/video.go b/pkg/connector/handlers/video.go
index 1aca568..a6f95f7 100644
--- a/pkg/connector/handlers/video.go
+++ b/pkg/connector/handlers/video.go
@@ -16,6 +16,10 @@ import (
 
 // ConvertVideo converts a LINE video message to a Matrix video message.
 func (h *Handler) ConvertVideo(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, decryptedBody string, relatesTo *event.RelatesTo) (*bridgev2.ConvertedMessage, error) {
+	if oversized := h.oversizedMediaNoticeFromMetadata(data.ContentMetadata, relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	client := h.NewClient()
 	oid := data.ContentMetadata["OID"]
 	isPlainMedia := oid == ""
@@ -109,6 +113,10 @@ func (h *Handler) ConvertVideo(ctx context.Context, portal *bridgev2.Portal, int
 		}
 	}
 
+	if oversized := h.oversizedMediaNotice(int64(len(videoData)), "downloaded", relatesTo); oversized != nil {
+		return oversized, nil
+	}
+
 	fileName := data.ContentMetadata["FILE_NAME"]
 
 	if fileName == "" && decryptedBody != "" && strings.Contains(decryptedBody, "fileName") {
diff --git a/pkg/connector/reaction_test.go b/pkg/connector/reaction_test.go
index 5bb86ea..6a0eeed 100644
--- a/pkg/connector/reaction_test.go
+++ b/pkg/connector/reaction_test.go
@@ -20,8 +20,8 @@ import (
 
 func TestCapabilitiesAdvertiseSupportedReactions(t *testing.T) {
 	infoVersion, capabilityVersion := (&LineConnector{}).GetBridgeInfoVersion()
-	if infoVersion != 1 || capabilityVersion != 2 {
-		t.Fatalf("bridge info/capability versions = %d/%d, want 1/2", infoVersion, capabilityVersion)
+	if infoVersion != 1 || capabilityVersion != 3 {
+		t.Fatalf("bridge info/capability versions = %d/%d, want 1/3", infoVersion, capabilityVersion)
 	}
 
 	caps := (&LineClient{}).GetCapabilities(context.Background(), nil)
diff --git a/pkg/connector/userinfo.go b/pkg/connector/userinfo.go
index 1d0f2b1..8ba7aeb 100644
--- a/pkg/connector/userinfo.go
+++ b/pkg/connector/userinfo.go
@@ -15,6 +15,7 @@ import (
 	"maunium.net/go/mautrix/bridgev2/networkid"
 	"maunium.net/go/mautrix/event"
 
+	"github.com/highesttt/matrix-line-messenger/pkg/connector/handlers"
 	"github.com/highesttt/matrix-line-messenger/pkg/line"
 )
 
@@ -64,6 +65,7 @@ func (lc *LineClient) GetCapabilities(ctx context.Context, portal *bridgev2.Port
 		File: event.FileFeatureMap{
 			event.MsgImage: {
 				Caption: event.CapLevelRejected,
+				MaxSize: handlers.BeeperMaxFileSize,
 				MimeTypes: map[string]event.CapabilitySupportLevel{
 					"image/jpeg":    event.CapLevelFullySupported,
 					"image/png":     event.CapLevelFullySupported,
@@ -80,6 +82,7 @@ func (lc *LineClient) GetCapabilities(ctx context.Context, portal *bridgev2.Port
 			},
 			event.MsgFile: {
 				Caption: event.CapLevelRejected,
+				MaxSize: handlers.BeeperMaxFileSize,
 				MimeTypes: map[string]event.CapabilitySupportLevel{
 					"image/gif": event.CapLevelFullySupported,
 					"*/*":       event.CapLevelFullySupported,
@@ -87,6 +90,7 @@ func (lc *LineClient) GetCapabilities(ctx context.Context, portal *bridgev2.Port
 			},
 			event.MsgVideo: {
 				Caption: event.CapLevelRejected,
+				MaxSize: handlers.BeeperMaxFileSize,
 				MimeTypes: map[string]event.CapabilitySupportLevel{
 					"video/mp4":        event.CapLevelFullySupported,
 					"video/webm":       event.CapLevelFullySupported,
@@ -100,6 +104,7 @@ func (lc *LineClient) GetCapabilities(ctx context.Context, portal *bridgev2.Port
 			},
 			event.MsgAudio: {
 				Caption: event.CapLevelRejected,
+				MaxSize: handlers.BeeperMaxFileSize,
 				MimeTypes: map[string]event.CapabilitySupportLevel{
 					"audio/mpeg":  event.CapLevelFullySupported,
 					"audio/ogg":   event.CapLevelFullySupported,
@@ -117,6 +122,7 @@ func (lc *LineClient) GetCapabilities(ctx context.Context, portal *bridgev2.Port
 			},
 			event.CapMsgVoice: {
 				Caption: event.CapLevelRejected,
+				MaxSize: handlers.BeeperMaxFileSize,
 				MimeTypes: map[string]event.CapabilitySupportLevel{
 					"audio/ogg":   event.CapLevelFullySupported,
 					"audio/mp4":   event.CapLevelFullySupported,