diff --git a/config.example.yaml b/config.example.yaml
index 22696069f..24e3d99c8 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -104,8 +104,9 @@ quota-exceeded:
 routing:
   strategy: "round-robin" # round-robin (default), fill-first
   # Enable universal session-sticky routing for all clients.
-  # Session IDs are extracted from: X-Session-ID header, Idempotency-Key,
-  # metadata.user_id, conversation_id, or first few messages hash.
+  # Session IDs are extracted from: metadata.user_id (Claude Code session format),
+  # X-Session-ID, Session_id (Codex), X-Amp-Thread-Id (Amp CLI),
+  # X-Client-Request-Id (PI), conversation_id, or first few messages hash.
   # Automatic failover is always enabled when bound auth becomes unavailable.
   session-affinity: false # default: false
   # How long session-to-auth bindings are retained. Default: 1h
diff --git a/internal/config/config.go b/internal/config/config.go
index 9817a8a71..1ee7aed53 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -226,7 +226,9 @@ type RoutingConfig struct {
 
 	// SessionAffinity enables universal session-sticky routing for all clients.
 	// Session IDs are extracted from multiple sources:
-	// X-Session-ID header, Idempotency-Key, metadata.user_id, conversation_id, or message hash.
+	// metadata.user_id (Claude Code session format), X-Session-ID, Session_id (Codex),
+	// X-Amp-Thread-Id (Amp CLI thread), X-Client-Request-Id (PI), metadata.user_id,
+	// conversation_id, or message hash.
 	// Automatic failover is always enabled when bound auth becomes unavailable.
 	SessionAffinity bool `yaml:"session-affinity,omitempty" json:"session-affinity,omitempty"`
 
diff --git a/sdk/cliproxy/auth/selector.go b/sdk/cliproxy/auth/selector.go
index f49979ce4..f0fe237c8 100644
--- a/sdk/cliproxy/auth/selector.go
+++ b/sdk/cliproxy/auth/selector.go
@@ -469,11 +469,14 @@ func NewSessionAffinitySelectorWithConfig(cfg SessionAffinityConfig) *SessionAff
 
 // Pick selects an auth with session affinity when possible.
 // Priority for session ID extraction:
-//  1. metadata.user_id (Claude Code format) - highest priority
+//  1. metadata.user_id (Claude Code format with _session_{uuid}) - highest priority
 //  2. X-Session-ID header
-//  3. metadata.user_id (non-Claude Code format)
-//  4. conversation_id field
-//  5. Hash-based fallback from messages
+//  3. Session_id header (Codex)
+//  4. X-Amp-Thread-Id header (Amp CLI thread ID)
+//  5. X-Client-Request-Id header (PI)
+//  6. metadata.user_id (non-Claude Code format)
+//  7. conversation_id field in request body
+//  8. Stable hash from first few messages content (fallback)
 //
 // Note: The cache key includes provider, session ID, and model to handle cases where
 // a session uses multiple models (e.g., gemini-2.5-pro and gemini-3-flash-preview)
@@ -570,10 +573,12 @@ func (s *SessionAffinitySelector) InvalidateAuth(authID string) {
 // Priority order:
 //  1. metadata.user_id (Claude Code format with _session_{uuid}) - highest priority for Claude Code clients
 //  2. X-Session-ID header
-//  3. X-Amp-Thread-Id header (Amp CLI thread ID)
-//  4. metadata.user_id (non-Claude Code format)
-//  5. conversation_id field in request body
-//  6. Stable hash from first few messages content (fallback)
+//  3. Session_id header (Codex)
+//  4. X-Amp-Thread-Id header (Amp CLI thread ID)
+//  5. X-Client-Request-Id header (PI)
+//  6. metadata.user_id (non-Claude Code format)
+//  7. conversation_id field in request body
+//  8. Stable hash from first few messages content (fallback)
 func ExtractSessionID(headers http.Header, payload []byte, metadata map[string]any) string {
 	primary, _ := extractSessionIDs(headers, payload, metadata)
 	return primary
@@ -609,29 +614,43 @@ func extractSessionIDs(headers http.Header, payload []byte, metadata map[string]
 		}
 	}
 
-	// 3. X-Amp-Thread-Id header (Amp CLI thread ID)
+	// 3. Session_id header (Codex)
+	if headers != nil {
+		if sid := headers.Get("Session_id"); sid != "" {
+			return "codex:" + sid, ""
+		}
+	}
+
+	// 4. X-Amp-Thread-Id header (Amp CLI thread ID)
 	if headers != nil {
 		if tid := headers.Get("X-Amp-Thread-Id"); tid != "" {
 			return "amp:" + tid, ""
 		}
 	}
 
+	// 5. X-Client-Request-Id header (PI)
+	if headers != nil {
+		if rid := headers.Get("X-Client-Request-Id"); rid != "" {
+			return "clientreq:" + rid, ""
+		}
+	}
+
 	if len(payload) == 0 {
 		return "", ""
 	}
 
-	// 4. metadata.user_id (non-Claude Code format)
+	// 6. metadata.user_id (non-Claude Code format)
 	userID := gjson.GetBytes(payload, "metadata.user_id").String()
 	if userID != "" {
 		return "user:" + userID, ""
 	}
 
-	// 5. conversation_id field
+	// 7. conversation_id field
 	if convID := gjson.GetBytes(payload, "conversation_id").String(); convID != "" {
 		return "conv:" + convID, ""
 	}
 
-	// 6. Hash-based fallback from message content
+	// 8. Hash-based fallback from message content
 	return extractMessageHashIDs(payload)
 }
 
diff --git a/sdk/cliproxy/auth/selector_test.go b/sdk/cliproxy/auth/selector_test.go
index c3041b5ba..f6682c6fc 100644
--- a/sdk/cliproxy/auth/selector_test.go
+++ b/sdk/cliproxy/auth/selector_test.go
@@ -776,6 +776,46 @@ func TestExtractSessionID_Headers(t *testing.T) {
 	}
 }
 
+func TestExtractSessionID_CodexSessionIDHeader(t *testing.T) {
+	t.Parallel()
+
+	headers := make(http.Header)
+	headers.Set("Session_id", "codex-session-123")
+
+	got := ExtractSessionID(headers, nil, nil)
+	want := "codex:codex-session-123"
+	if got != want {
+		t.Errorf("ExtractSessionID() with Session_id = %q, want %q", got, want)
+	}
+}
+
+func TestExtractSessionID_ClientRequestIDHeader(t *testing.T) {
+	t.Parallel()
+
+	headers := make(http.Header)
+	headers.Set("X-Client-Request-Id", "pi-session-123")
+
+	got := ExtractSessionID(headers, nil, nil)
+	want := "clientreq:pi-session-123"
+	if got != want {
+		t.Errorf("ExtractSessionID() with X-Client-Request-Id = %q, want %q", got, want)
+	}
+}
+
+func TestExtractSessionID_CodexSessionIDPriorityOverClientRequestID(t *testing.T) {
+	t.Parallel()
+
+	headers := make(http.Header)
+	headers.Set("X-Client-Request-Id", "pi-session-123")
+	headers.Set("Session_id", "codex-session-456")
+
+	got := ExtractSessionID(headers, nil, nil)
+	want := "codex:codex-session-456"
+	if got != want {
+		t.Errorf("ExtractSessionID() = %q, want %q (Session_id should take priority over X-Client-Request-Id)", got, want)
+	}
+}
+
 func TestExtractSessionID_AmpThreadId(t *testing.T) {
 	t.Parallel()
 
@@ -789,6 +829,20 @@ func TestExtractSessionID_AmpThreadId(t *testing.T) {
 	}
 }
 
+func TestExtractSessionID_AmpThreadIdPriorityOverClientRequestID(t *testing.T) {
+	t.Parallel()
+
+	headers := make(http.Header)
+	headers.Set("X-Amp-Thread-Id", "T-priority-test")
+	headers.Set("X-Client-Request-Id", "pi-session-123")
+
+	got := ExtractSessionID(headers, nil, nil)
+	want := "amp:T-priority-test"
+	if got != want {
+		t.Errorf("ExtractSessionID() = %q, want %q (X-Amp-Thread-Id should take priority over X-Client-Request-Id)", got, want)
+	}
+}
+
 // TestExtractSessionID_AmpThreadIdLowerPriority verifies X-Amp-Thread-Id is lower
 // priority than Claude Code metadata.user_id but higher than conversation_id.
 func TestExtractSessionID_AmpThreadIdPriority(t *testing.T) {