backend/internal/server/dto.go

@@ -20,6 +20,7 @@ type sessionResponse struct {
 	UserID          string `json:"user_id"`
 	IsGuest         bool   `json:"is_guest"`
 	DisplayName     string `json:"display_name"`
+	ServiceLanguage string `json:"service_language"`
 }
 
 // okResponse is a simple success acknowledgement.
@@ -163,6 +164,7 @@ func sessionResponseFor(token string, acc account.Account) sessionResponse {
 		UserID:          acc.ID.String(),
 		IsGuest:         acc.IsGuest,
 		DisplayName:     acc.DisplayName,
+		ServiceLanguage: acc.ServiceLanguage,
 	}
 }
 

backend/internal/server/handlers_auth.go

@@ -48,6 +48,7 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
+	acc.ServiceLanguage = req.ServiceLanguage // reflect this login's bot in the session response
 	s.mintSession(c, acc)
 }
 

gateway/internal/backendclient/api.go

@@ -18,6 +18,7 @@ type SessionResp struct {
 	UserID          string `json:"user_id"`
 	IsGuest         bool   `json:"is_guest"`
 	DisplayName     string `json:"display_name"`
+	ServiceLanguage string `json:"service_language"`
 }
 
 // ProfileResp is an account's own profile.

gateway/internal/transcode/encode.go

@@ -36,6 +36,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	token := b.CreateString(s.Token)
 	uid := b.CreateString(s.UserID)
 	name := b.CreateString(s.DisplayName)
+	svcLang := b.CreateString(s.ServiceLanguage)
 	langs := buildSupportedLanguagesVector(b, supportedLangs)
 	fb.SessionStart(b)
 	fb.SessionAddToken(b, token)
@@ -43,6 +44,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	fb.SessionAddIsGuest(b, s.IsGuest)
 	fb.SessionAddDisplayName(b, name)
 	fb.SessionAddSupportedLanguages(b, langs)
+	fb.SessionAddServiceLanguage(b, svcLang)
 	b.Finish(fb.SessionEnd(b))
 	return b.FinishedBytes()
 }

gateway/internal/transcode/transcode_telegram_test.go

@@ -43,7 +43,7 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
 		_ = json.NewDecoder(r.Body).Decode(&gotBody)
-		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван"}`))
+		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван","service_language":"ru"}`))
 	})
 	defer cleanup()
 
@@ -67,6 +67,11 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 	if got := sessionLanguages(sess); len(got) != 1 || got[0] != "ru" {
 		t.Errorf("session supported_languages = %v, want [ru]", got)
 	}
+	// The bot's service language rides the Session so the UI can build a share link to
+	// the same bot.
+	if got := string(sess.ServiceLanguage()); got != "ru" {
+		t.Errorf("session service_language = %q, want ru", got)
+	}
 	// The validated launch fields are forwarded so the backend can seed a new account;
 	// service_language is recorded to route the account's later out-of-app push.
 	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" || gotBody["service_language"] != "ru" {

pkg/fbs/scrabble.fbs

@@ -123,6 +123,10 @@ table Session {
   is_guest:bool;
   display_name:string;
   supported_languages:[string];
+  // service_language is the language tag (en/ru) of the Telegram bot the session was
+  // minted through; empty for a non-Telegram login. The UI uses it to build a share
+  // link that points at the same bot.
+  service_language:string;
 }
 
 // Ack is a simple success acknowledgement (e.g. an email-code request).

pkg/fbs/scrabblefb/Session.go

@@ -94,8 +94,16 @@ func (rcv *Session) SupportedLanguagesLength() int {
 	return 0
 }
 
+func (rcv *Session) ServiceLanguage() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
 func SessionStart(builder *flatbuffers.Builder) {
-	builder.StartObject(5)
+	builder.StartObject(6)
 }
 func SessionAddToken(builder *flatbuffers.Builder, token flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(token), 0)
@@ -115,6 +123,9 @@ func SessionAddSupportedLanguages(builder *flatbuffers.Builder, supportedLanguag
 func SessionStartSupportedLanguagesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
 	return builder.StartVector(4, numElems, 4)
 }
+func SessionAddServiceLanguage(builder *flatbuffers.Builder, serviceLanguage flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(serviceLanguage), 0)
+}
 func SessionEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

ui/src/gen/fbs/scrabblefb/session.ts

@@ -58,8 +58,15 @@ supportedLanguagesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+serviceLanguage():string|null
+serviceLanguage(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+serviceLanguage(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
 static startSession(builder:flatbuffers.Builder) {
-  builder.startObject(5);
+  builder.startObject(6);
 }
 
 static addToken(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset) {
@@ -94,18 +101,23 @@ static startSupportedLanguagesVector(builder:flatbuffers.Builder, numElems:numbe
   builder.startVector(4, numElems, 4);
 }
 
+static addServiceLanguage(builder:flatbuffers.Builder, serviceLanguageOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(5, serviceLanguageOffset, 0);
+}
+
 static endSession(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset, supportedLanguagesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset, supportedLanguagesOffset:flatbuffers.Offset, serviceLanguageOffset:flatbuffers.Offset):flatbuffers.Offset {
   Session.startSession(builder);
   Session.addToken(builder, tokenOffset);
   Session.addUserId(builder, userIdOffset);
   Session.addIsGuest(builder, isGuest);
   Session.addDisplayName(builder, displayNameOffset);
   Session.addSupportedLanguages(builder, supportedLanguagesOffset);
+  Session.addServiceLanguage(builder, serviceLanguageOffset);
   return Session.endSession(builder);
 }
 }

ui/src/lib/codec.test.ts

@@ -171,6 +171,7 @@ describe('codec', () => {
       isGuest: true,
       displayName: 'Me',
       supportedLanguages: ['en', 'ru'],
+      serviceLanguage: '',
     });
   });
 
@@ -311,7 +312,7 @@ describe('codec', () => {
     b.finish(fb.LinkResult.endLinkResult(b));
     const r = decodeLinkResult(b.asUint8Array());
     expect(r.status).toBe('merged');
-    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [] });
+    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [], serviceLanguage: '' });
   });
 
   it('decodes an Invitation with inviter and invitees', () => {

ui/src/lib/codec.ts

@@ -292,6 +292,7 @@ function sessionFromTable(t: fb.Session): Session {
     isGuest: t.isGuest(),
     displayName: s(t.displayName()),
     supportedLanguages,
+    serviceLanguage: s(t.serviceLanguage()),
   };
 }
 

ui/src/lib/mock/data.ts

@@ -25,6 +25,7 @@ export const SESSION: Session = {
   displayName: 'You',
   // Both languages by default, so the mock-driven UI offers every variant.
   supportedLanguages: ['en', 'ru'],
+  serviceLanguage: '',
 };
 
 export const PROFILE: Profile = {

ui/src/lib/model.ts

@@ -234,6 +234,9 @@ export interface Session {
   // variants these languages support (en -> English; ru -> Russian + Эрудит). Empty
   // means ungated (all variants).
   supportedLanguages: string[];
+  // serviceLanguage is the en/ru tag of the Telegram bot this session was minted
+  // through (empty for a non-Telegram login); used to share a link to the same bot.
+  serviceLanguage: string;
 }
 
 // LinkResult is the outcome of an account link/merge step. status is