Reuse Codex route validation for same-session business STATE updates

.downstream/native-domains.md

@@ -112,7 +112,12 @@ cookie ordering are unchanged.
 It has a 90-second budget. Each target/model is validated with two completed
 HTTP 200 streams, at most 45 seconds each: the first must return STATE, and
 the second may omit STATE or return the same value. Qualification includes
-the target identity, actual model, exit, client headers, STATE and TLS profile.
+the target identity, actual model, exit, client headers and TLS profile.
+The probe clears the business STATE and validates its own two-shot STATE, so
+echoing the next business STATE does not invalidate an otherwise identical live
+proof. The sender keeps that exact business STATE and its existing continuation
+isolation. Native session/window identity, model, tier, exit and the original
+candidate expiry still constrain reuse; no credential gains a longer lease.
 Target probes preserve the template's native session/thread/window headers and
 generate only a fresh legacy `session_id` for each shot, matching ranxi's full
 probe contract. The effective service tier in the gateway-owned routing hint is

backend/internal/service/codex_gateway_borrow.go

@@ -805,8 +805,12 @@ func (s *CodexGatewayBorrowService) prepareSource(ctx context.Context, rev uint6
 
 func borrowRequestFingerprint(req *http.Request, model, proxy string, profile *tlsfingerprint.Profile, cookie string) string {
 	tlsJSON, _ := json.Marshal(profile)
-	values := []string{cookie, model, proxy, req.URL.EscapedPath(), req.Header.Get("Authorization"), req.Header.Get("ChatGPT-Account-ID"), req.Header.Get("User-Agent"), req.Header.Get("Originator"), req.Header.Get("Version"), req.Header.Get("X-Codex-Turn-State"), string(tlsJSON)}
+	values := []string{cookie, model, proxy, req.URL.EscapedPath(), req.Header.Get("Authorization"), req.Header.Get("ChatGPT-Account-ID"), req.Header.Get("User-Agent"), req.Header.Get("Originator"), req.Header.Get("Version"), string(tlsJSON)}
 	values = append(values, codexBorrowRequestServiceTier(req))
+	// Probes clear business STATE and mint their own independent two-shot STATE.
+	// A client's next turn must not discard that live route proof merely because
+	// it echoes a new business STATE. Apply still preserves the client's exact
+	// STATE; the candidate lease and native session identity remain part of reuse.
 	// Include the native identity and routing policy preserved by the probe.
 	// Per-request tracing IDs and the probe's random legacy session_id are not
 	// qualification identities and must not defeat reuse or failure cooldowns.
@@ -951,7 +955,7 @@ func (s *CodexGatewayBorrowService) apply(req *http.Request, account *Account, m
 			s.qualifications = make(map[string]codexGatewayBorrowTargetCheck)
 		}
 		qualificationKey := fmt.Sprintf("%d:%s:%s", account.ID, model, key)
-		// Bound request-specific STATE/header variants; expired proofs are discarded.
+		// Bound request-identity variants; expired proofs are discarded.
 		if len(s.qualifications) >= 1024 {
 			oldestKey := ""
 			var oldest time.Time

backend/internal/service/codex_gateway_borrow_core_test.go

@@ -259,7 +259,15 @@ func TestCodexGatewayBorrowTargetCacheModelAndFingerprintIsolation(t *testing.T)
 	_, app, err := s.Apply(template, a, "gpt-6.1-sol", "target-exit", nil, false)
 	require.NoError(t, err)
 	require.True(t, app.Applied, "hot cache is read before global target lock")
-	for _, header := range []string{"Authorization", "ChatGPT-Account-ID", "User-Agent", "Originator", "Version", "X-Codex-Turn-State"} {
+	continuation := template.Clone(template.Context())
+	continuation.Header.Set("X-Codex-Turn-State", "next-business-state")
+	continuation.Header.Set("X-Client-Request-ID", "next-trace")
+	wire, reused, err := s.Apply(continuation, a, "gpt-6.1-sol", "target-exit", nil, true)
+	require.NoError(t, err, "business STATE is cleared from the independent probe, so it cannot invalidate its proof")
+	require.Equal(t, app.ExpiresAt, reused.ExpiresAt, "reuse never extends the candidate lease")
+	require.Equal(t, "next-business-state", wire.Header.Get("X-Codex-Turn-State"))
+	require.Equal(t, "business-state", template.Header.Get("X-Codex-Turn-State"), "business histories remain isolated")
+	for _, header := range []string{"Authorization", "ChatGPT-Account-ID", "User-Agent", "Originator", "Version", "session-id", "thread-id", "x-codex-window-id", "x-codex-installation-id"} {
 		req := template.Clone(template.Context())
 		req.Header.Set(header, req.Header.Get(header)+"-changed")
 		_, _, err := s.Apply(req, a, "gpt-6.1-sol", "target-exit", nil, true)

backend/internal/service/codex_gateway_borrow_diagnose_test.go

@@ -240,6 +240,7 @@ func testCodexBorrowDiagnoseHTTPToolContinuation(t *testing.T, tier string) {
 	}
 	require.True(t, results[1].ToolRoundTrip)
 	require.Equal(t, 2, business)
+	require.Equal(t, 2, probes, "the returned business STATE must not revalidate the same live route")
 	require.EqualValues(t, business+probes, count)
 	require.LessOrEqual(t, count, int32(8))
 	usage := s.Status().RecentUsage[0]