Merge pull request #6049 from MokoYee/fix/openai-sticky-prefix-system

修复 OpenAI Chat 动态系统消息导致账号亲和漂移

backend/internal/service/openai_content_session_seed.go

@@ -18,8 +18,9 @@ const contentStablePrefixSessionSeedPrefix = "compat_csp_"
 
 // deriveOpenAIContentSessionSeed builds a stable session seed from an
 // OpenAI-format request body. Only fields constant across conversation turns
-// are included: model, tools/functions definitions, system/developer prompts,
-// instructions (Responses API), and the first user message.
+// are included: model, tools/functions definitions, the leading system/developer
+// prompt prefix in Chat messages, instructions (Responses API), and the first
+// user message.
 // Supports both Chat Completions (messages) and Responses API (input).
 func deriveOpenAIContentSessionSeed(body []byte) string {
 	if len(body) == 0 {
@@ -117,15 +118,19 @@ scanRoot:
 
 	msgs := fields[messagesField]
 	if msgs.Exists() && msgs.IsArray() {
+		systemPrefixOpen := true
 		msgs.ForEach(func(_, msg gjson.Result) bool {
 			role := msg.Get("role").String()
 			switch role {
 			case "system", "developer":
-				_, _ = b.WriteString("|system=")
-				if c := msg.Get("content"); c.Exists() {
-					_, _ = b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+				if systemPrefixOpen {
+					_, _ = b.WriteString("|system=")
+					if c := msg.Get("content"); c.Exists() {
+						_, _ = b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+					}
 				}
 			case "user":
+				systemPrefixOpen = false
 				if !firstUserCaptured {
 					_, _ = b.WriteString("|first_user=")
 					if c := msg.Get("content"); c.Exists() {
@@ -133,6 +138,8 @@ scanRoot:
 					}
 					firstUserCaptured = true
 				}
+			default:
+				systemPrefixOpen = false
 			}
 			return true
 		})

backend/internal/service/openai_content_session_seed_test.go

@@ -44,6 +44,65 @@ func TestDeriveOpenAIContentSessionSeed_ChatCompletions_StableAcrossTurns(t *tes
 	require.NotEmpty(t, s1)
 }
 
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_IgnoresLaterSystemMessages(t *testing.T) {
+	turn1 := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"},
+			{"role": "assistant", "content": "Hi there!"},
+			{"role": "user", "content": "How are you?"}
+		]
+	}`)
+	turn2 := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "You are helpful."},
+			{"role": "user", "content": "Hello"},
+			{"role": "assistant", "content": "Hi there!"},
+			{"role": "system", "content": "Return JSON for this turn."},
+			{"role": "user", "content": "How are you?"}
+		]
+	}`)
+
+	require.Equal(t, deriveOpenAIContentSessionSeed(turn1), deriveOpenAIContentSessionSeed(turn2))
+}
+
+func TestDeriveOpenAIContentSessionSeed_ChatCompletions_UsesLeadingSystemDeveloperPrefix(t *testing.T) {
+	firstSystem := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "System A"},
+			{"role": "developer", "content": "Developer B"},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+	changedLaterSystem := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "System A"},
+			{"role": "developer", "content": "Developer C"},
+			{"role": "user", "content": "Hello"}
+		]
+	}`)
+
+	seed := deriveOpenAIContentSessionSeed(firstSystem)
+	require.Contains(t, seed, "System A")
+	require.Contains(t, seed, "Developer B")
+	require.NotEqual(t, seed, deriveOpenAIContentSessionSeed(changedLaterSystem))
+
+	withLaterSystem := []byte(`{
+		"model": "gpt-5.4",
+		"messages": [
+			{"role": "system", "content": "System A"},
+			{"role": "developer", "content": "Developer B"},
+			{"role": "user", "content": "Hello"},
+			{"role": "system", "content": "Dynamic system"}
+		]
+	}`)
+	require.Equal(t, seed, deriveOpenAIContentSessionSeed(withLaterSystem))
+}
+
 func TestDeriveOpenAIContentSessionSeed_ChatCompletions_DifferentFirstUserDiffers(t *testing.T) {
 	req1 := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Question A"}]}`)
 	req2 := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"Question B"}]}`)
@@ -270,7 +329,7 @@ func TestDeriveOpenAIContentSessionSeed_SingleScanMatchesLegacyBytes(t *testing.
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			require.Equal(t, legacyDeriveOpenAIContentSessionSeed(test.body), deriveOpenAIContentSessionSeed(test.body))
+			require.Equal(t, referenceDeriveOpenAIContentSessionSeed(test.body), deriveOpenAIContentSessionSeed(test.body))
 		})
 	}
 }
@@ -283,12 +342,12 @@ func TestDeriveOpenAIContentSessionSeed_AllTruncationOffsetsMatchLegacyBytes(t *
 	for bodyIndex, body := range bodies {
 		for end := 1; end < len(body); end++ {
 			truncated := []byte(body[:end])
-			require.Equalf(t, legacyDeriveOpenAIContentSessionSeed(truncated), deriveOpenAIContentSessionSeed(truncated), "body %d truncated at byte %d", bodyIndex, end)
+			require.Equalf(t, referenceDeriveOpenAIContentSessionSeed(truncated), deriveOpenAIContentSessionSeed(truncated), "body %d truncated at byte %d", bodyIndex, end)
 		}
 	}
 }
 
-func legacyDeriveOpenAIContentSessionSeed(body []byte) string {
+func referenceDeriveOpenAIContentSessionSeed(body []byte) string {
 	if len(body) == 0 {
 		return ""
 	}
@@ -319,15 +378,19 @@ func legacyDeriveOpenAIContentSessionSeed(body []byte) string {
 
 	msgs := gjson.GetBytes(body, "messages")
 	if msgs.Exists() && msgs.IsArray() {
+		systemPrefixOpen := true
 		msgs.ForEach(func(_, msg gjson.Result) bool {
 			role := msg.Get("role").String()
 			switch role {
 			case "system", "developer":
-				_, _ = b.WriteString("|system=")
-				if c := msg.Get("content"); c.Exists() {
-					_, _ = b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+				if systemPrefixOpen {
+					_, _ = b.WriteString("|system=")
+					if c := msg.Get("content"); c.Exists() {
+						_, _ = b.WriteString(normalizeCompatSeedJSON(json.RawMessage(c.Raw)))
+					}
 				}
 			case "user":
+				systemPrefixOpen = false
 				if !firstUserCaptured {
 					_, _ = b.WriteString("|first_user=")
 					if c := msg.Get("content"); c.Exists() {
@@ -335,6 +398,8 @@ func legacyDeriveOpenAIContentSessionSeed(body []byte) string {
 					}
 					firstUserCaptured = true
 				}
+			default:
+				systemPrefixOpen = false
 			}
 			return true
 		})