diff --git a/backend/internal/service/openai_gateway_grok_cache.go b/backend/internal/service/openai_gateway_grok_cache.go
index bbdd5be44a..dac9e975e1 100644
--- a/backend/internal/service/openai_gateway_grok_cache.go
+++ b/backend/internal/service/openai_gateway_grok_cache.go
@@ -15,6 +15,7 @@ const (
 	grokConversationIDHeader        = "X-Grok-Conv-Id"
 	grokFreeCacheNativeToolsJSON    = `[{"type":"web_search"},{"type":"x_search"}]`
 	grokFreeCacheDisabledToolChoice = "none"
+	grokFreeRolling24hTokenLimit    = int64(2_000_000)
 )
 
 // resolveGrokCacheIdentity derives one stable, tenant-isolated routing identity
@@ -152,22 +153,69 @@ func isKnownGrokFreeAccount(account *Account) bool {
 	if account == nil || !account.IsGrokOAuth() {
 		return false
 	}
+	freeSignal := false
+	paidSignal := false
+	inferredFreeSignal := false
 	if billing, err := grokBillingSnapshotFromExtra(account.Extra); err == nil && billing != nil {
 		if tier := strings.TrimSpace(billing.Plan); tier != "" {
-			return isGrokFreeSubscriptionTier(tier)
+			if isGrokFreeSubscriptionTier(tier) {
+				freeSignal = true
+			} else if !isGrokUnknownSubscriptionTier(tier) {
+				paidSignal = true
+			}
+		}
+		if billing.UsagePercent != nil || billing.UsedPercent != nil ||
+			(billing.MonthlyLimitCents != nil && *billing.MonthlyLimitCents > 0) {
+			paidSignal = true
+		}
+		// xAI deliberately reports an empty plan for Free accounts; only paid
+		// subscriptions receive a SuperGrok plan/monthly limit. A successful
+		// monthly billing observation with no paid signal is therefore positive
+		// Free evidence, not an unknown tier. Keep partial probes fail-closed.
+		if strings.TrimSpace(billing.MonthlyUpdatedAt) != "" ||
+			(billing.StatusCode >= http.StatusOK && billing.StatusCode < http.StatusMultipleChoices &&
+				!billing.Partial && len(billing.FailedWindows) == 0) {
+			inferredFreeSignal = true
 		}
 	}
 	if snapshot, err := grokQuotaSnapshotFromExtra(account.Extra); err == nil && snapshot != nil {
 		if tier := strings.TrimSpace(snapshot.SubscriptionTier); tier != "" {
-			return isGrokFreeSubscriptionTier(tier)
+			if isGrokFreeSubscriptionTier(tier) {
+				freeSignal = true
+			} else if !isGrokUnknownSubscriptionTier(tier) {
+				paidSignal = true
+			}
+		}
+		if snapshot.Tokens != nil && snapshot.Tokens.Limit != nil &&
+			*snapshot.Tokens.Limit == grokFreeRolling24hTokenLimit {
+			inferredFreeSignal = true
 		}
 	}
-	return isGrokFreeSubscriptionTier(account.GetCredential("subscription_tier"))
+	if tier := strings.TrimSpace(account.GetCredential("subscription_tier")); tier != "" {
+		if isGrokFreeSubscriptionTier(tier) {
+			freeSignal = true
+		} else if !isGrokUnknownSubscriptionTier(tier) {
+			paidSignal = true
+		}
+	}
+	// Explicit paid evidence always wins over an inferred Free signal. This
+	// protects upgraded/stale accounts whose previous quota snapshot still
+	// carries the historical 2M Free token limit.
+	return !paidSignal && (freeSignal || inferredFreeSignal)
 }
 
 func isGrokFreeSubscriptionTier(tier string) bool {
 	switch strings.ToLower(strings.TrimSpace(tier)) {
-	case "free", "grok-free", "grok_free", "free-tier", "free_tier":
+	case "free", "grok-free", "grok_free", "free-tier", "free_tier", "basic", "grok-basic", "grok_basic":
+		return true
+	default:
+		return false
+	}
+}
+
+func isGrokUnknownSubscriptionTier(tier string) bool {
+	switch strings.ToLower(strings.TrimSpace(tier)) {
+	case "", "unknown", "n/a", "none":
 		return true
 	default:
 		return false
diff --git a/backend/internal/service/openai_gateway_grok_cache_test.go b/backend/internal/service/openai_gateway_grok_cache_test.go
index 4f811a4530..c31ee7bc37 100644
--- a/backend/internal/service/openai_gateway_grok_cache_test.go
+++ b/backend/internal/service/openai_gateway_grok_cache_test.go
@@ -296,6 +296,31 @@ func TestGrokFreeMessagesFunctionToolCacheRouteRequiresKnownFreeTier(t *testing.
 			}(),
 			wantMix: true,
 		},
+		{
+			name: "free successful billing has blank plan",
+			account: func() *Account {
+				a := healthyGrokOAuthGatewayTestAccount(9111, "access-token")
+				a.Extra = map[string]any{grokBillingExtraKey: map[string]any{
+					"status_code":        http.StatusOK,
+					"source":             "billing_probe",
+					"monthly_updated_at": "2026-07-15T05:00:00Z",
+				}}
+				return a
+			}(),
+			wantMix: true,
+		},
+		{
+			name: "free rolling token quota",
+			account: func() *Account {
+				a := healthyGrokOAuthGatewayTestAccount(9112, "access-token")
+				a.Extra = map[string]any{grokQuotaSnapshotExtraKey: map[string]any{
+					"headers_observed": true,
+					"tokens":           map[string]any{"limit": grokFreeRolling24hTokenLimit},
+				}}
+				return a
+			}(),
+			wantMix: true,
+		},
 		{
 			name: "supergrok remains unchanged",
 			account: func() *Account {
@@ -304,6 +329,33 @@ func TestGrokFreeMessagesFunctionToolCacheRouteRequiresKnownFreeTier(t *testing.
 				return a
 			}(),
 		},
+		{
+			name: "paid billing overrides stale free quota",
+			account: func() *Account {
+				a := healthyGrokOAuthGatewayTestAccount(9121, "access-token")
+				a.Extra = map[string]any{
+					grokBillingExtraKey: map[string]any{"plan": "SuperGrok", "status_code": http.StatusOK},
+					grokQuotaSnapshotExtraKey: map[string]any{
+						"headers_observed": true,
+						"tokens":           map[string]any{"limit": grokFreeRolling24hTokenLimit},
+					},
+				}
+				return a
+			}(),
+		},
+		{
+			name: "partial billing without monthly evidence remains unknown",
+			account: func() *Account {
+				a := healthyGrokOAuthGatewayTestAccount(9122, "access-token")
+				a.Extra = map[string]any{grokBillingExtraKey: map[string]any{
+					"status_code":    http.StatusOK,
+					"source":         "billing_probe",
+					"partial":        true,
+					"failed_windows": []string{"monthly"},
+				}}
+				return a
+			}(),
+		},
 		{
 			name:    "unknown tier remains unchanged",
 			account: healthyGrokOAuthGatewayTestAccount(913, "access-token"),
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index 60997b396f..71ca5cc010 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -1473,7 +1473,11 @@ func TestForwardAsAnthropicForGrokFunctionToolUsesCacheCapableMixedRoute(t *test
 	c.Set("api_key", &APIKey{ID: 5403})
 
 	account := healthyGrokOAuthGatewayTestAccount(58, "access-token")
-	account.Credentials["subscription_tier"] = "free"
+	account.Extra = map[string]any{grokBillingExtraKey: map[string]any{
+		"status_code":        http.StatusOK,
+		"source":             "billing_probe",
+		"monthly_updated_at": "2026-07-15T05:00:00Z",
+	}}
 	repo := &grokQuotaAccountRepo{
 		mockAccountRepoForPlatform: &mockAccountRepoForPlatform{
 			accountsByID: map[int64]*Account{58: account},