diff --git a/chat.go b/chat.go
index 18a6375..4a170df 100644
--- a/chat.go
+++ b/chat.go
@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 
+	chatclasses "git.golem15.com/golem15/sm-chat-plugin/classes"
 	chatcontracts "git.golem15.com/golem15/sm-chat-plugin/contracts"
 	chanclasses "git.golem15.com/golem15/sm-chatchannels-plugin/classes"
 	"git.golem15.com/golem15/summercms/modules/backpack"
@@ -42,9 +43,27 @@ func (p *Plugin) registerChat(app *backpack.App) error {
 	if err := reg.Add(p.ID(), chatcontracts.Registration{Alias: classes.ChatContextType, Loader: p.loadChatContext}); err != nil {
 		return err
 	}
+	if err := p.installChatLimits(app); err != nil {
+		return err
+	}
 	return p.installChatAccess(app)
 }
 
+// installChatLimits makes golem15.chat enforce jz.bm.chat.max_upload_bytes
+// as the total size of one message's attachments, the value the bootstrap
+// advertises to the student app. golem15.chat publishes its service from
+// its own database hook, which runs before this one.
+func (p *Plugin) installChatLimits(app *backpack.App) error {
+	return lagoon.OnDatabase(app, func(_ *sql.DB, _ *gorm.DB) error {
+		svc, ok := app.Lookup[*chatclasses.Service]()
+		if !ok || svc == nil {
+			return errors.New("jz.bm: golem15.chat must be listed before jz.bm in summer.yaml")
+		}
+		svc.SetMaxMessageBytes(int64(p.chatMaxUploadBytes()))
+		return nil
+	})
+}
+
 // loadChatContext is the loader of the bm alias. The only context is id 1;
 // any other id is unknown, which golem15.chat answers with its not-a-member
 // error.
diff --git a/config/config.yaml b/config/config.yaml
index 2f99da9..88bf9c9 100644
--- a/config/config.yaml
+++ b/config/config.yaml
@@ -19,7 +19,9 @@ public_url: ""
 chat:
   group: studenci
   moderator_group: moderatorzy
-  # Total size of one message's attachments (60 MiB). It stays under nginx's
+  # Total size of one message's attachments (60 MiB), enforced by
+  # golem15.chat when the message is sent (Service.SetMaxMessageBytes) and
+  # checked by the student app before upload. It stays under nginx's
   # client_max_body_size 64m so the multipart envelope still fits.
   max_upload_bytes: 62914560
   # Entity embeds (deadline, class, reading) per message.