fix(openai): avoid implicit image sticky sessions

backend/internal/handler/openai_images.go

@@ -117,12 +117,7 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 		return
 	}
 
-	sessionHash := ""
-	if parsed.Multipart {
-		sessionHash = h.gatewayService.GenerateSessionHashWithFallback(c, nil, parsed.StickySessionSeed())
-	} else {
-		sessionHash = h.gatewayService.GenerateSessionHash(c, body)
-	}
+	sessionHash := h.gatewayService.GenerateExplicitSessionHash(c, body)
 
 	maxAccountSwitches := h.maxAccountSwitches
 	switchCount := 0

backend/internal/service/openai_gateway_service.go

@@ -1125,6 +1125,35 @@ func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) str
 	return sessionID
 }
 
+func explicitOpenAISessionID(c *gin.Context, body []byte) string {
+	if c == nil {
+		return ""
+	}
+
+	sessionID := strings.TrimSpace(c.GetHeader("session_id"))
+	if sessionID == "" {
+		sessionID = strings.TrimSpace(c.GetHeader("conversation_id"))
+	}
+	if sessionID == "" && len(body) > 0 {
+		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
+	}
+	return sessionID
+}
+
+// GenerateExplicitSessionHash generates a sticky-session hash only from explicit
+// client session signals. It intentionally skips content-derived fallback and is
+// used by stateless endpoints such as /v1/images.
+func (s *OpenAIGatewayService) GenerateExplicitSessionHash(c *gin.Context, body []byte) string {
+	sessionID := explicitOpenAISessionID(c, body)
+	if sessionID == "" {
+		return ""
+	}
+
+	currentHash, legacyHash := deriveOpenAISessionHashes(sessionID)
+	attachOpenAILegacySessionHashToGin(c, legacyHash)
+	return currentHash
+}
+
 // GenerateSessionHash generates a sticky-session hash for OpenAI requests.
 //
 // Priority:
@@ -1137,13 +1166,7 @@ func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte)
 		return ""
 	}
 
-	sessionID := strings.TrimSpace(c.GetHeader("session_id"))
-	if sessionID == "" {
-		sessionID = strings.TrimSpace(c.GetHeader("conversation_id"))
-	}
-	if sessionID == "" && len(body) > 0 {
-		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
-	}
+	sessionID := explicitOpenAISessionID(c, body)
 	if sessionID == "" && len(body) > 0 {
 		sessionID = deriveOpenAIContentSessionSeed(body)
 	}

backend/internal/service/openai_gateway_service_test.go

@@ -227,6 +227,41 @@ func TestOpenAIGatewayService_GenerateSessionHash_AttachesLegacyHashToContext(t
 	require.NotEmpty(t, openAILegacySessionHashFromContext(c.Request.Context()))
 }
 
+func TestOpenAIGatewayService_GenerateExplicitSessionHash_SkipsContentFallback(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	svc := &OpenAIGatewayService{}
+	body := []byte(`{"model":"gpt-image-2","prompt":"draw a cat"}`)
+
+	t.Run("stateless image body stays unstuck", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(rec)
+		c.Request = httptest.NewRequest(http.MethodPost, "/v1/images/generations", nil)
+
+		require.Empty(t, svc.GenerateExplicitSessionHash(c, body))
+		require.Empty(t, openAILegacySessionHashFromContext(c.Request.Context()))
+	})
+
+	t.Run("prompt_cache_key is explicit", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(rec)
+		c.Request = httptest.NewRequest(http.MethodPost, "/v1/images/generations", nil)
+
+		got := svc.GenerateExplicitSessionHash(c, []byte(`{"model":"gpt-image-2","prompt_cache_key":"image-session"}`))
+		require.Equal(t, fmt.Sprintf("%016x", xxhash.Sum64String("image-session")), got)
+		require.NotEmpty(t, openAILegacySessionHashFromContext(c.Request.Context()))
+	})
+
+	t.Run("header overrides body", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(rec)
+		c.Request = httptest.NewRequest(http.MethodPost, "/v1/images/generations", nil)
+		c.Request.Header.Set("session_id", "header-session")
+
+		got := svc.GenerateExplicitSessionHash(c, []byte(`{"prompt_cache_key":"body-session"}`))
+		require.Equal(t, fmt.Sprintf("%016x", xxhash.Sum64String("header-session")), got)
+	})
+}
+
 func TestOpenAIGatewayService_GenerateSessionHashWithFallback(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()