fix: prevent sessionHash collision for different users with same messages

Mix SessionContext (ClientIP, UserAgent, APIKeyID) into
GenerateSessionHash 3rd-level fallback to differentiate requests
from different users sending identical content.

Also switch hashContent from SHA256-truncated to XXHash64 for
better performance, and optimize Trie Lua script to match from
longest prefix first.

backend/internal/handler/gateway_handler.go

@@ -203,6 +203,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	}
 
 	// 计算粘性会话hash
+	parsedReq.SessionContext = &service.SessionContext{
+		ClientIP:  ip.GetClientIP(c),
+		UserAgent: c.GetHeader("User-Agent"),
+		APIKeyID:  apiKey.ID,
+	}
 	sessionHash := h.gatewayService.GenerateSessionHash(parsedReq)
 
 	// 获取平台：优先使用强制平台（/antigravity 路由，中间件已设置 request.Context），否则使用分组平台
@@ -962,6 +967,11 @@ func (h *GatewayHandler) CountTokens(c *gin.Context) {
 	}
 
 	// 计算粘性会话 hash
+	parsedReq.SessionContext = &service.SessionContext{
+		ClientIP:  ip.GetClientIP(c),
+		UserAgent: c.GetHeader("User-Agent"),
+		APIKeyID:  apiKey.ID,
+	}
 	sessionHash := h.gatewayService.GenerateSessionHash(parsedReq)
 
 	// 选择支持该模型的账号

backend/internal/handler/gemini_v1beta_handler.go

@@ -233,6 +233,13 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	if sessionHash == "" {
 		// Fallback: 使用通用的会话哈希生成逻辑（适用于其他客户端）
 		parsedReq, _ := service.ParseGatewayRequest(body)
+		if parsedReq != nil {
+			parsedReq.SessionContext = &service.SessionContext{
+				ClientIP:  ip.GetClientIP(c),
+				UserAgent: c.GetHeader("User-Agent"),
+				APIKeyID:  apiKey.ID,
+			}
+		}
 		sessionHash = h.gatewayService.GenerateSessionHash(parsedReq)
 	}
 	sessionKey := sessionHash

backend/internal/repository/gateway_cache.go

@@ -19,25 +19,34 @@ const (
 	// ARGV[2] = TTL seconds (用于刷新)
 	// 返回: 最长匹配的 value (uuid:accountID) 或 nil
 	// 查找成功时自动刷新 TTL，防止活跃会话意外过期
+	// 从最长前缀（完整 chain）开始逐步缩短，第一次命中即返回
 	geminiTrieFindScript = `
 local chain = ARGV[1]
 local ttl = tonumber(ARGV[2])
-local lastMatch = nil
-local path = ""
 
-for part in string.gmatch(chain, "[^-]+") do
-    path = path == "" and part or path .. "-" .. part
-    local val = redis.call('HGET', KEYS[1], path)
+-- 先尝试完整 chain（最常见场景：同一对话的下一轮请求）
+local val = redis.call('HGET', KEYS[1], chain)
+if val and val ~= "" then
+    redis.call('EXPIRE', KEYS[1], ttl)
+    return val
+end
+
+-- 从最长前缀开始逐步缩短（去掉最后一个 "-xxx" 段）
+local path = chain
+while true do
+    local i = string.find(path, "-[^-]*$")
+    if not i or i <= 1 then
+        break
+    end
+    path = string.sub(path, 1, i - 1)
+    val = redis.call('HGET', KEYS[1], path)
     if val and val ~= "" then
-        lastMatch = val
+        redis.call('EXPIRE', KEYS[1], ttl)
+        return val
     end
 end
 
-if lastMatch then
-    redis.call('EXPIRE', KEYS[1], ttl)
-end
-
-return lastMatch
+return nil
 `
 
 	// geminiTrieSaveScript 保存会话到 Trie 的 Lua 脚本

backend/internal/service/gateway_request.go

@@ -9,6 +9,15 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
 )
 
+// SessionContext 粘性会话上下文，用于区分不同来源的请求。
+// 仅在 GenerateSessionHash 第 3 级 fallback（消息内容 hash）时混入，
+// 避免不同用户发送相同消息产生相同 hash 导致账号集中。
+type SessionContext struct {
+	ClientIP  string
+	UserAgent string
+	APIKeyID  int64
+}
+
 // ParsedRequest 保存网关请求的预解析结果
 //
 // 性能优化说明：
@@ -22,15 +31,16 @@ import (
 // 2. 将解析结果 ParsedRequest 传递给 Service 层
 // 3. 避免重复 json.Unmarshal，减少 CPU 和内存开销
 type ParsedRequest struct {
-	Body            []byte // 原始请求体（保留用于转发）
-	Model           string // 请求的模型名称
-	Stream          bool   // 是否为流式请求
-	MetadataUserID  string // metadata.user_id（用于会话亲和）
-	System          any    // system 字段内容
-	Messages        []any  // messages 数组
-	HasSystem       bool   // 是否包含 system 字段（包含 null 也视为显式传入）
-	ThinkingEnabled bool   // 是否开启 thinking（部分平台会影响最终模型名）
-	MaxTokens       int    // max_tokens 值（用于探测请求拦截）
+	Body            []byte          // 原始请求体（保留用于转发）
+	Model           string          // 请求的模型名称
+	Stream          bool            // 是否为流式请求
+	MetadataUserID  string          // metadata.user_id（用于会话亲和）
+	System          any             // system 字段内容
+	Messages        []any           // messages 数组
+	HasSystem       bool            // 是否包含 system 字段（包含 null 也视为显式传入）
+	ThinkingEnabled bool            // 是否开启 thinking（部分平台会影响最终模型名）
+	MaxTokens       int             // max_tokens 值（用于探测请求拦截）
+	SessionContext  *SessionContext // 可选：请求上下文区分因子（nil 时行为不变）
 }
 
 // ParseGatewayRequest 解析网关请求体并返回结构化结果

backend/internal/service/gateway_service.go

@@ -5,7 +5,6 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
-	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -17,6 +16,7 @@ import (
 	"os"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
@@ -26,6 +26,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/Wei-Shaw/sub2api/internal/util/urlvalidator"
+	"github.com/cespare/xxhash/v2"
 	"github.com/google/uuid"
 	"github.com/tidwall/gjson"
 	"github.com/tidwall/sjson"
@@ -490,8 +491,17 @@ func (s *GatewayService) GenerateSessionHash(parsed *ParsedRequest) string {
 		return s.hashContent(cacheableContent)
 	}
 
-	// 3. 最后 fallback: 使用 system + 所有消息的完整摘要串
+	// 3. 最后 fallback: 使用 session上下文 + system + 所有消息的完整摘要串
 	var combined strings.Builder
+	// 混入请求上下文区分因子，避免不同用户相同消息产生相同 hash
+	if parsed.SessionContext != nil {
+		_, _ = combined.WriteString(parsed.SessionContext.ClientIP)
+		_, _ = combined.WriteString(":")
+		_, _ = combined.WriteString(parsed.SessionContext.UserAgent)
+		_, _ = combined.WriteString(":")
+		_, _ = combined.WriteString(strconv.FormatInt(parsed.SessionContext.APIKeyID, 10))
+		_, _ = combined.WriteString("|")
+	}
 	if parsed.System != nil {
 		systemText := s.extractTextFromSystem(parsed.System)
 		if systemText != "" {
@@ -649,8 +659,8 @@ func (s *GatewayService) extractTextFromContent(content any) string {
 }
 
 func (s *GatewayService) hashContent(content string) string {
-	hash := sha256.Sum256([]byte(content))
-	return hex.EncodeToString(hash[:16]) // 32字符
+	h := xxhash.Sum64String(content)
+	return strconv.FormatUint(h, 36)
 }
 
 // replaceModelInBody 替换请求体中的model字段

backend/internal/service/generate_session_hash_test.go

@@ -0,0 +1,843 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// ============ 基础优先级测试 ============
+
+func TestGenerateSessionHash_NilParsedRequest(t *testing.T) {
+	svc := &GatewayService{}
+	require.Empty(t, svc.GenerateSessionHash(nil))
+}
+
+func TestGenerateSessionHash_EmptyRequest(t *testing.T) {
+	svc := &GatewayService{}
+	require.Empty(t, svc.GenerateSessionHash(&ParsedRequest{}))
+}
+
+func TestGenerateSessionHash_MetadataHasHighestPriority(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		MetadataUserID: "session_123e4567-e89b-12d3-a456-426614174000",
+		System:         "You are a helpful assistant.",
+		HasSystem:      true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+
+	hash := svc.GenerateSessionHash(parsed)
+	require.Equal(t, "123e4567-e89b-12d3-a456-426614174000", hash, "metadata session_id should have highest priority")
+}
+
+// ============ System + Messages 基础测试 ============
+
+func TestGenerateSessionHash_SystemPlusMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	withSystem := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+	withoutSystem := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(withSystem)
+	h2 := svc.GenerateSessionHash(withoutSystem)
+	require.NotEmpty(t, h1)
+	require.NotEmpty(t, h2)
+	require.NotEqual(t, h1, h2, "system prompt should be part of digest, producing different hash")
+}
+
+func TestGenerateSessionHash_SystemOnlyProducesHash(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+	}
+	hash := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, hash, "system prompt alone should produce a hash as part of full digest")
+}
+
+func TestGenerateSessionHash_DifferentSystemsSameMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed1 := &ParsedRequest{
+		System:    "You are assistant A.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+	parsed2 := &ParsedRequest{
+		System:    "You are assistant B.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(parsed1)
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEqual(t, h1, h2, "different system prompts with same messages should produce different hashes")
+}
+
+func TestGenerateSessionHash_SameSystemSameMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	mk := func() *ParsedRequest {
+		return &ParsedRequest{
+			System:    "You are a helpful assistant.",
+			HasSystem: true,
+			Messages: []any{
+				map[string]any{"role": "user", "content": "hello"},
+				map[string]any{"role": "assistant", "content": "hi"},
+			},
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(mk())
+	h2 := svc.GenerateSessionHash(mk())
+	require.Equal(t, h1, h2, "same system + same messages should produce identical hash")
+}
+
+func TestGenerateSessionHash_DifferentMessagesProduceDifferentHash(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed1 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "help me with Go"},
+		},
+	}
+	parsed2 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "help me with Python"},
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(parsed1)
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEqual(t, h1, h2, "same system but different messages should produce different hashes")
+}
+
+// ============ SessionContext 核心测试 ============
+
+func TestGenerateSessionHash_DifferentSessionContextProducesDifferentHash(t *testing.T) {
+	svc := &GatewayService{}
+
+	// 相同消息 + 不同 SessionContext → 不同 hash（解决碰撞问题的核心场景）
+	parsed1 := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "192.168.1.1",
+			UserAgent: "Mozilla/5.0",
+			APIKeyID:  100,
+		},
+	}
+	parsed2 := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "10.0.0.1",
+			UserAgent: "curl/7.0",
+			APIKeyID:  200,
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(parsed1)
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEmpty(t, h1)
+	require.NotEmpty(t, h2)
+	require.NotEqual(t, h1, h2, "same messages but different SessionContext should produce different hashes")
+}
+
+func TestGenerateSessionHash_SameSessionContextProducesSameHash(t *testing.T) {
+	svc := &GatewayService{}
+
+	mk := func() *ParsedRequest {
+		return &ParsedRequest{
+			Messages: []any{
+				map[string]any{"role": "user", "content": "hello"},
+			},
+			SessionContext: &SessionContext{
+				ClientIP:  "192.168.1.1",
+				UserAgent: "Mozilla/5.0",
+				APIKeyID:  100,
+			},
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(mk())
+	h2 := svc.GenerateSessionHash(mk())
+	require.Equal(t, h1, h2, "same messages + same SessionContext should produce identical hash")
+}
+
+func TestGenerateSessionHash_MetadataOverridesSessionContext(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		MetadataUserID: "session_123e4567-e89b-12d3-a456-426614174000",
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "192.168.1.1",
+			UserAgent: "Mozilla/5.0",
+			APIKeyID:  100,
+		},
+	}
+
+	hash := svc.GenerateSessionHash(parsed)
+	require.Equal(t, "123e4567-e89b-12d3-a456-426614174000", hash,
+		"metadata session_id should take priority over SessionContext")
+}
+
+func TestGenerateSessionHash_NilSessionContextBackwardCompatible(t *testing.T) {
+	svc := &GatewayService{}
+
+	withCtx := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: nil,
+	}
+	withoutCtx := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(withCtx)
+	h2 := svc.GenerateSessionHash(withoutCtx)
+	require.Equal(t, h1, h2, "nil SessionContext should produce same hash as no SessionContext")
+}
+
+// ============ 多轮连续会话测试 ============
+
+func TestGenerateSessionHash_ContinuousConversation_HashChangesWithMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 模拟连续会话：每增加一轮对话，hash 应该不同（内容累积变化）
+	round1 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: ctx,
+	}
+
+	round2 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+			map[string]any{"role": "assistant", "content": "Hi there!"},
+			map[string]any{"role": "user", "content": "How are you?"},
+		},
+		SessionContext: ctx,
+	}
+
+	round3 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+			map[string]any{"role": "assistant", "content": "Hi there!"},
+			map[string]any{"role": "user", "content": "How are you?"},
+			map[string]any{"role": "assistant", "content": "I'm doing well!"},
+			map[string]any{"role": "user", "content": "Tell me a joke"},
+		},
+		SessionContext: ctx,
+	}
+
+	h1 := svc.GenerateSessionHash(round1)
+	h2 := svc.GenerateSessionHash(round2)
+	h3 := svc.GenerateSessionHash(round3)
+
+	require.NotEmpty(t, h1)
+	require.NotEmpty(t, h2)
+	require.NotEmpty(t, h3)
+	require.NotEqual(t, h1, h2, "different conversation rounds should produce different hashes")
+	require.NotEqual(t, h2, h3, "each new round should produce a different hash")
+	require.NotEqual(t, h1, h3, "round 1 and round 3 should differ")
+}
+
+func TestGenerateSessionHash_ContinuousConversation_SameRoundSameHash(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 同一轮对话重复请求（如重试）应产生相同 hash
+	mk := func() *ParsedRequest {
+		return &ParsedRequest{
+			System:    "You are a helpful assistant.",
+			HasSystem: true,
+			Messages: []any{
+				map[string]any{"role": "user", "content": "hello"},
+				map[string]any{"role": "assistant", "content": "Hi there!"},
+				map[string]any{"role": "user", "content": "How are you?"},
+			},
+			SessionContext: ctx,
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(mk())
+	h2 := svc.GenerateSessionHash(mk())
+	require.Equal(t, h1, h2, "same conversation state should produce identical hash on retry")
+}
+
+// ============ 消息回退测试 ============
+
+func TestGenerateSessionHash_MessageRollback(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 模拟消息回退：用户删掉最后一轮再重发
+	original := &ParsedRequest{
+		System:    "System prompt",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "msg1"},
+			map[string]any{"role": "assistant", "content": "reply1"},
+			map[string]any{"role": "user", "content": "msg2"},
+			map[string]any{"role": "assistant", "content": "reply2"},
+			map[string]any{"role": "user", "content": "msg3"},
+		},
+		SessionContext: ctx,
+	}
+
+	// 回退到 msg2 后，用新的 msg3 替代
+	rollback := &ParsedRequest{
+		System:    "System prompt",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "msg1"},
+			map[string]any{"role": "assistant", "content": "reply1"},
+			map[string]any{"role": "user", "content": "msg2"},
+			map[string]any{"role": "assistant", "content": "reply2"},
+			map[string]any{"role": "user", "content": "different msg3"},
+		},
+		SessionContext: ctx,
+	}
+
+	hOrig := svc.GenerateSessionHash(original)
+	hRollback := svc.GenerateSessionHash(rollback)
+	require.NotEqual(t, hOrig, hRollback, "rollback with different last message should produce different hash")
+}
+
+func TestGenerateSessionHash_MessageRollbackSameContent(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 回退后重新发送相同内容 → 相同 hash（合理的粘性恢复）
+	mk := func() *ParsedRequest {
+		return &ParsedRequest{
+			System:    "System prompt",
+			HasSystem: true,
+			Messages: []any{
+				map[string]any{"role": "user", "content": "msg1"},
+				map[string]any{"role": "assistant", "content": "reply1"},
+				map[string]any{"role": "user", "content": "msg2"},
+			},
+			SessionContext: ctx,
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(mk())
+	h2 := svc.GenerateSessionHash(mk())
+	require.Equal(t, h1, h2, "rollback and resend same content should produce same hash")
+}
+
+// ============ 相同 System、不同用户消息 ============
+
+func TestGenerateSessionHash_SameSystemDifferentUsers(t *testing.T) {
+	svc := &GatewayService{}
+
+	// 两个不同用户使用相同 system prompt 但发送不同消息
+	user1 := &ParsedRequest{
+		System:    "You are a code reviewer.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "Review this Go code"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "1.1.1.1",
+			UserAgent: "vscode",
+			APIKeyID:  1,
+		},
+	}
+	user2 := &ParsedRequest{
+		System:    "You are a code reviewer.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "Review this Python code"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "2.2.2.2",
+			UserAgent: "vscode",
+			APIKeyID:  2,
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(user1)
+	h2 := svc.GenerateSessionHash(user2)
+	require.NotEqual(t, h1, h2, "different users with different messages should get different hashes")
+}
+
+func TestGenerateSessionHash_SameSystemSameMessageDifferentContext(t *testing.T) {
+	svc := &GatewayService{}
+
+	// 这是修复的核心场景：两个不同用户发送完全相同的 system + messages（如 "hello"）
+	// 有了 SessionContext 后应该产生不同 hash
+	user1 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "1.1.1.1",
+			UserAgent: "Mozilla/5.0",
+			APIKeyID:  10,
+		},
+	}
+	user2 := &ParsedRequest{
+		System:    "You are a helpful assistant.",
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "2.2.2.2",
+			UserAgent: "Mozilla/5.0",
+			APIKeyID:  20,
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(user1)
+	h2 := svc.GenerateSessionHash(user2)
+	require.NotEqual(t, h1, h2, "CRITICAL: same system+messages but different users should get different hashes")
+}
+
+// ============ SessionContext 各字段独立影响测试 ============
+
+func TestGenerateSessionHash_SessionContext_IPDifference(t *testing.T) {
+	svc := &GatewayService{}
+
+	base := func(ip string) *ParsedRequest {
+		return &ParsedRequest{
+			Messages: []any{
+				map[string]any{"role": "user", "content": "test"},
+			},
+			SessionContext: &SessionContext{
+				ClientIP:  ip,
+				UserAgent: "same-ua",
+				APIKeyID:  1,
+			},
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(base("1.1.1.1"))
+	h2 := svc.GenerateSessionHash(base("2.2.2.2"))
+	require.NotEqual(t, h1, h2, "different IP should produce different hash")
+}
+
+func TestGenerateSessionHash_SessionContext_UADifference(t *testing.T) {
+	svc := &GatewayService{}
+
+	base := func(ua string) *ParsedRequest {
+		return &ParsedRequest{
+			Messages: []any{
+				map[string]any{"role": "user", "content": "test"},
+			},
+			SessionContext: &SessionContext{
+				ClientIP:  "1.1.1.1",
+				UserAgent: ua,
+				APIKeyID:  1,
+			},
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(base("Mozilla/5.0"))
+	h2 := svc.GenerateSessionHash(base("curl/7.0"))
+	require.NotEqual(t, h1, h2, "different User-Agent should produce different hash")
+}
+
+func TestGenerateSessionHash_SessionContext_APIKeyIDDifference(t *testing.T) {
+	svc := &GatewayService{}
+
+	base := func(keyID int64) *ParsedRequest {
+		return &ParsedRequest{
+			Messages: []any{
+				map[string]any{"role": "user", "content": "test"},
+			},
+			SessionContext: &SessionContext{
+				ClientIP:  "1.1.1.1",
+				UserAgent: "same-ua",
+				APIKeyID:  keyID,
+			},
+		}
+	}
+
+	h1 := svc.GenerateSessionHash(base(1))
+	h2 := svc.GenerateSessionHash(base(2))
+	require.NotEqual(t, h1, h2, "different APIKeyID should produce different hash")
+}
+
+// ============ 多用户并发相同消息场景 ============
+
+func TestGenerateSessionHash_MultipleUsersSameFirstMessage(t *testing.T) {
+	svc := &GatewayService{}
+
+	// 模拟 5 个不同用户同时发送 "hello" → 应该产生 5 个不同的 hash
+	hashes := make(map[string]bool)
+	for i := 0; i < 5; i++ {
+		parsed := &ParsedRequest{
+			Messages: []any{
+				map[string]any{"role": "user", "content": "hello"},
+			},
+			SessionContext: &SessionContext{
+				ClientIP:  "192.168.1." + string(rune('1'+i)),
+				UserAgent: "client-" + string(rune('A'+i)),
+				APIKeyID:  int64(i + 1),
+			},
+		}
+		h := svc.GenerateSessionHash(parsed)
+		require.NotEmpty(t, h)
+		require.False(t, hashes[h], "hash collision detected for user %d", i)
+		hashes[h] = true
+	}
+	require.Len(t, hashes, 5, "5 different users should produce 5 unique hashes")
+}
+
+// ============ 连续会话粘性：多轮对话同一用户 ============
+
+func TestGenerateSessionHash_SameUserGrowingConversation(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "browser", APIKeyID: 42}
+
+	// 模拟同一用户的连续会话，每轮 hash 不同但同用户重试保持一致
+	messages := []map[string]any{
+		{"role": "user", "content": "msg1"},
+		{"role": "assistant", "content": "reply1"},
+		{"role": "user", "content": "msg2"},
+		{"role": "assistant", "content": "reply2"},
+		{"role": "user", "content": "msg3"},
+		{"role": "assistant", "content": "reply3"},
+		{"role": "user", "content": "msg4"},
+	}
+
+	prevHash := ""
+	for round := 1; round <= len(messages); round += 2 {
+		// 构建前 round 条消息
+		msgs := make([]any, round)
+		for j := 0; j < round; j++ {
+			msgs[j] = messages[j]
+		}
+		parsed := &ParsedRequest{
+			System:         "System",
+			HasSystem:      true,
+			Messages:       msgs,
+			SessionContext: ctx,
+		}
+		h := svc.GenerateSessionHash(parsed)
+		require.NotEmpty(t, h, "round %d hash should not be empty", round)
+
+		if prevHash != "" {
+			require.NotEqual(t, prevHash, h, "round %d hash should differ from previous round", round)
+		}
+		prevHash = h
+
+		// 同一轮重试应该相同
+		h2 := svc.GenerateSessionHash(parsed)
+		require.Equal(t, h, h2, "retry of round %d should produce same hash", round)
+	}
+}
+
+// ============ 多轮消息内容结构化测试 ============
+
+func TestGenerateSessionHash_MultipleUserMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 5 条用户消息（无 assistant 回复）
+	parsed := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "first"},
+			map[string]any{"role": "user", "content": "second"},
+			map[string]any{"role": "user", "content": "third"},
+			map[string]any{"role": "user", "content": "fourth"},
+			map[string]any{"role": "user", "content": "fifth"},
+		},
+		SessionContext: ctx,
+	}
+
+	h := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, h)
+
+	// 修改中间一条消息应该改变 hash
+	parsed2 := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "first"},
+			map[string]any{"role": "user", "content": "CHANGED"},
+			map[string]any{"role": "user", "content": "third"},
+			map[string]any{"role": "user", "content": "fourth"},
+			map[string]any{"role": "user", "content": "fifth"},
+		},
+		SessionContext: ctx,
+	}
+
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEqual(t, h, h2, "changing any message should change the hash")
+}
+
+func TestGenerateSessionHash_MessageOrderMatters(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	parsed1 := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "alpha"},
+			map[string]any{"role": "user", "content": "beta"},
+		},
+		SessionContext: ctx,
+	}
+	parsed2 := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "beta"},
+			map[string]any{"role": "user", "content": "alpha"},
+		},
+		SessionContext: ctx,
+	}
+
+	h1 := svc.GenerateSessionHash(parsed1)
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEqual(t, h1, h2, "message order should affect the hash")
+}
+
+// ============ 复杂内容格式测试 ============
+
+func TestGenerateSessionHash_StructuredContent(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 结构化 content（数组形式）
+	parsed := &ParsedRequest{
+		Messages: []any{
+			map[string]any{
+				"role": "user",
+				"content": []any{
+					map[string]any{"type": "text", "text": "Look at this"},
+					map[string]any{"type": "text", "text": "And this too"},
+				},
+			},
+		},
+		SessionContext: ctx,
+	}
+
+	h := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, h, "structured content should produce a hash")
+}
+
+func TestGenerateSessionHash_ArraySystemPrompt(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 数组格式的 system prompt
+	parsed := &ParsedRequest{
+		System: []any{
+			map[string]any{"type": "text", "text": "You are a helpful assistant."},
+			map[string]any{"type": "text", "text": "Be concise."},
+		},
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: ctx,
+	}
+
+	h := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, h, "array system prompt should produce a hash")
+}
+
+// ============ SessionContext 与 cache_control 优先级 ============
+
+func TestGenerateSessionHash_CacheControlOverridesSessionContext(t *testing.T) {
+	svc := &GatewayService{}
+
+	// 当有 cache_control: ephemeral 时，使用第 2 级优先级
+	// SessionContext 不应影响结果
+	parsed1 := &ParsedRequest{
+		System: []any{
+			map[string]any{
+				"type":          "text",
+				"text":          "You are a tool-specific assistant.",
+				"cache_control": map[string]any{"type": "ephemeral"},
+			},
+		},
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "1.1.1.1",
+			UserAgent: "ua1",
+			APIKeyID:  100,
+		},
+	}
+	parsed2 := &ParsedRequest{
+		System: []any{
+			map[string]any{
+				"type":          "text",
+				"text":          "You are a tool-specific assistant.",
+				"cache_control": map[string]any{"type": "ephemeral"},
+			},
+		},
+		HasSystem: true,
+		Messages: []any{
+			map[string]any{"role": "user", "content": "hello"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "2.2.2.2",
+			UserAgent: "ua2",
+			APIKeyID:  200,
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(parsed1)
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.Equal(t, h1, h2, "cache_control ephemeral has higher priority, SessionContext should not affect result")
+}
+
+// ============ 边界情况 ============
+
+func TestGenerateSessionHash_EmptyMessages(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		Messages: []any{},
+		SessionContext: &SessionContext{
+			ClientIP:  "1.1.1.1",
+			UserAgent: "test",
+			APIKeyID:  1,
+		},
+	}
+
+	// 空 messages + 只有 SessionContext 时，combined.Len() > 0 因为有 context 写入
+	h := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, h, "empty messages with SessionContext should still produce a hash from context")
+}
+
+func TestGenerateSessionHash_EmptyMessagesNoContext(t *testing.T) {
+	svc := &GatewayService{}
+
+	parsed := &ParsedRequest{
+		Messages: []any{},
+	}
+
+	h := svc.GenerateSessionHash(parsed)
+	require.Empty(t, h, "empty messages without SessionContext should produce empty hash")
+}
+
+func TestGenerateSessionHash_SessionContextWithEmptyFields(t *testing.T) {
+	svc := &GatewayService{}
+
+	// SessionContext 字段为空字符串和零值时仍应影响 hash
+	withEmptyCtx := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "test"},
+		},
+		SessionContext: &SessionContext{
+			ClientIP:  "",
+			UserAgent: "",
+			APIKeyID:  0,
+		},
+	}
+	withoutCtx := &ParsedRequest{
+		Messages: []any{
+			map[string]any{"role": "user", "content": "test"},
+		},
+	}
+
+	h1 := svc.GenerateSessionHash(withEmptyCtx)
+	h2 := svc.GenerateSessionHash(withoutCtx)
+	// 有 SessionContext（即使字段为空）仍然会写入分隔符 "::" 等
+	require.NotEqual(t, h1, h2, "empty-field SessionContext should still differ from nil SessionContext")
+}
+
+// ============ 长对话历史测试 ============
+
+func TestGenerateSessionHash_LongConversation(t *testing.T) {
+	svc := &GatewayService{}
+
+	ctx := &SessionContext{ClientIP: "1.2.3.4", UserAgent: "test", APIKeyID: 1}
+
+	// 构建 20 轮对话
+	messages := make([]any, 0, 40)
+	for i := 0; i < 20; i++ {
+		messages = append(messages, map[string]any{
+			"role":    "user",
+			"content": "user message " + string(rune('A'+i)),
+		})
+		messages = append(messages, map[string]any{
+			"role":    "assistant",
+			"content": "assistant reply " + string(rune('A'+i)),
+		})
+	}
+
+	parsed := &ParsedRequest{
+		System:         "System prompt",
+		HasSystem:      true,
+		Messages:       messages,
+		SessionContext: ctx,
+	}
+
+	h := svc.GenerateSessionHash(parsed)
+	require.NotEmpty(t, h)
+
+	// 再加一轮应该不同
+	moreMessages := make([]any, len(messages)+2)
+	copy(moreMessages, messages)
+	moreMessages[len(messages)] = map[string]any{"role": "user", "content": "one more"}
+	moreMessages[len(messages)+1] = map[string]any{"role": "assistant", "content": "ok"}
+
+	parsed2 := &ParsedRequest{
+		System:         "System prompt",
+		HasSystem:      true,
+		Messages:       moreMessages,
+		SessionContext: ctx,
+	}
+
+	h2 := svc.GenerateSessionHash(parsed2)
+	require.NotEqual(t, h, h2, "adding more messages to long conversation should change hash")
+}