fix(openai): honor Codex session-id header

backend/internal/service/openai_gateway_scheduling.go

@@ -28,6 +28,7 @@ const (
 )
 
 var explicitOpenAIHeaderSessionNames = []string{
+	"session-id",
 	"session_id",
 	"conversation_id",
 	openCodeSessionAffinityHeader,
@@ -145,7 +146,7 @@ func (s *OpenAIGatewayService) GenerateExplicitSessionHash(c *gin.Context, body
 // GenerateSessionHash generates a sticky-session hash for OpenAI requests.
 //
 // Priority:
-//  1. Header: session_id
+//  1. Header: session-id / session_id
 //  2. Header: conversation_id
 //  3. Header: x-session-affinity / x-session-id / x-opencode-session (OpenCode)
 //  4. Header: x-conversation-id (CodeBuddy)

backend/internal/service/openai_gateway_service_test.go

@@ -391,6 +391,7 @@ func TestOpenAIGatewayService_ClientSessionHeaderPriority(t *testing.T) {
 		name  string
 		value string
 	}{
+		{name: "session-id", value: "codex-session"},
 		{name: "session_id", value: "generic-session"},
 		{name: "conversation_id", value: "generic-conversation"},
 		{name: openCodeSessionAffinityHeader, value: "opencode-affinity"},
@@ -416,6 +417,31 @@ func TestOpenAIGatewayService_ClientSessionHeaderPriority(t *testing.T) {
 	require.Equal(t, "body-session", svc.ExtractSessionID(c, body))
 }
 
+func TestOpenAIGatewayService_CodexSessionIDKeepsReconnectHashStable(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodGet, "/v1/responses", nil)
+	c.Request.Header.Set("session-id", "codex-reconnect-session")
+
+	svc := &OpenAIGatewayService{}
+	warmup := []byte(`{
+		"type":"response.create",
+		"model":"gpt-5.6-sol",
+		"generate":false,
+		"tools":[{"type":"custom","name":"exec"}],
+		"input":[{"role":"user","content":"warmup"}]
+	}`)
+	business := []byte(`{
+		"type":"response.create",
+		"model":"gpt-5.6-sol",
+		"input":[{"role":"user","content":"install codex"}]
+	}`)
+
+	require.Equal(t, svc.GenerateSessionHash(c, warmup), svc.GenerateSessionHash(c, business))
+	require.Equal(t, "codex-reconnect-session", svc.ExtractSessionID(c, business))
+}
+
 func TestOpenAIGatewayService_ClientSessionHeadersIgnorePerRequestIDs(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	recorder := httptest.NewRecorder()

backend/internal/service/openai_ws_forwarder_logutil.go

@@ -65,7 +65,10 @@ func resolveOpenAIWSSessionHeaders(c *gin.Context, promptCacheKey string) openAI
 		ConversationSource: "none",
 	}
 	if c != nil && c.Request != nil {
-		if sessionID := strings.TrimSpace(c.Request.Header.Get("session_id")); sessionID != "" {
+		if sessionID := strings.TrimSpace(c.Request.Header.Get("session-id")); sessionID != "" {
+			resolution.SessionID = sessionID
+			resolution.SessionSource = "header_session-id"
+		} else if sessionID := strings.TrimSpace(c.Request.Header.Get("session_id")); sessionID != "" {
 			resolution.SessionID = sessionID
 			resolution.SessionSource = "header_session_id"
 		}

backend/internal/service/openai_ws_forwarder_logutil_test.go

@@ -0,0 +1,37 @@
+package service
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResolveOpenAIWSSessionHeadersPrefersCodexHyphenHeader(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodGet, "/v1/responses", nil)
+	c.Request.Header.Set("session-id", "codex-session")
+	c.Request.Header.Set("session_id", "legacy-session")
+
+	resolution := resolveOpenAIWSSessionHeaders(c, "prompt-cache")
+
+	require.Equal(t, "codex-session", resolution.SessionID)
+	require.Equal(t, "header_session-id", resolution.SessionSource)
+}
+
+func TestResolveOpenAIWSSessionHeadersFallsBackToLegacyHeader(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodGet, "/v1/responses", nil)
+	c.Request.Header.Set("session_id", "legacy-session")
+
+	resolution := resolveOpenAIWSSessionHeaders(c, "prompt-cache")
+
+	require.Equal(t, "legacy-session", resolution.SessionID)
+	require.Equal(t, "header_session_id", resolution.SessionSource)
+}