diff --git a/backend/internal/service/openai_gateway_chat_completions.go b/backend/internal/service/openai_gateway_chat_completions.go
index a66489da00..ddf9a288a8 100644
--- a/backend/internal/service/openai_gateway_chat_completions.go
+++ b/backend/internal/service/openai_gateway_chat_completions.go
@@ -463,7 +463,7 @@ func (s *OpenAIGatewayService) handleChatBufferedStreamingResponse(
 	// When the terminal event has an empty output array, reconstruct from
 	// accumulated delta events so the client receives the full content.
 	acc.SupplementResponseOutput(finalResponse)
-	if account != nil && account.Platform == PlatformGrok && !hasBillableOpenAIUsage(usage) {
+	if requiresBillableGrokChatUsage(account, originalModel, billingModel, upstreamModel, finalResponse.Model) && !hasBillableOpenAIUsage(usage) {
 		return nil, s.newOpenAIStreamFailoverError(
 			c,
 			account,
diff --git a/backend/internal/service/openai_gateway_chat_completions_raw.go b/backend/internal/service/openai_gateway_chat_completions_raw.go
index 0aee0fa5f5..cdafc30d1b 100644
--- a/backend/internal/service/openai_gateway_chat_completions_raw.go
+++ b/backend/internal/service/openai_gateway_chat_completions_raw.go
@@ -431,7 +431,8 @@ func (s *OpenAIGatewayService) bufferRawChatCompletions(
 	if parsedUsage, ok := extractOpenAIUsageFromJSONBytes(respBody); ok {
 		usage = parsedUsage
 	}
-	if account != nil && account.Platform == PlatformGrok && !hasBillableOpenAIUsage(usage) {
+	responseModel := gjson.GetBytes(respBody, "model").String()
+	if requiresBillableGrokChatUsage(account, originalModel, billingModel, upstreamModel, responseModel) && !hasBillableOpenAIUsage(usage) {
 		return nil, s.newOpenAIStreamFailoverError(
 			c,
 			account,
diff --git a/backend/internal/service/openai_gateway_chat_completions_raw_test.go b/backend/internal/service/openai_gateway_chat_completions_raw_test.go
index c19066f3fc..0a9741889c 100644
--- a/backend/internal/service/openai_gateway_chat_completions_raw_test.go
+++ b/backend/internal/service/openai_gateway_chat_completions_raw_test.go
@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/openai_compat"
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
 	"github.com/tidwall/gjson"
@@ -122,6 +123,41 @@ func TestForwardAsRawChatCompletions_ForcesStreamUsageUpstreamAndPassesUsageDown
 	require.Contains(t, rec.Body.String(), "data: [DONE]")
 }
 
+func TestForwardAsChatCompletions_OpenAICompatibleGrokRawMissingUsageFailsBeforeWrite(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	body := []byte(`{"model":"grok-4.5","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusOK,
+		Header: http.Header{
+			"Content-Type": []string{"application/json"},
+			"X-Request-Id": []string{"rid-openai-compat-grok-no-usage"},
+		},
+		Body: io.NopCloser(strings.NewReader(
+			`{"id":"resp_missing_usage","object":"chat.completion","model":"grok-4.5","choices":[{"index":0,"message":{"role":"assistant","content":"hello"},"finish_reason":"stop"}]}`,
+		)),
+	}}
+	svc := &OpenAIGatewayService{
+		cfg:          rawChatCompletionsTestConfig(),
+		httpUpstream: upstream,
+	}
+	account := rawChatCompletionsTestAccount()
+	account.Name = "openai-compatible-grok"
+	account.Extra = map[string]any{openai_compat.ExtraKeyResponsesSupported: false}
+
+	result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+
+	require.Nil(t, result)
+	var failoverErr *UpstreamFailoverError
+	require.ErrorAs(t, err, &failoverErr)
+	require.False(t, c.Writer.Written(), "unbilled Grok content must not be returned by an OpenAI-compatible account")
+	require.Empty(t, recorder.Body.String())
+}
+
 func TestForwardAsRawChatCompletions_PreservesMappedGPT56MaxEffort(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
diff --git a/backend/internal/service/openai_gateway_usage_integrity.go b/backend/internal/service/openai_gateway_usage_integrity.go
index c1b9e1523b..a70357cc6f 100644
--- a/backend/internal/service/openai_gateway_usage_integrity.go
+++ b/backend/internal/service/openai_gateway_usage_integrity.go
@@ -14,3 +14,19 @@ func hasBillableOpenAIUsage(usage OpenAIUsage) bool {
 		usage.CacheReadInputTokens > 0 ||
 		usage.ImageOutputTokens > 0
 }
+
+// requiresBillableGrokChatUsage identifies Grok traffic by both account
+// platform and model identity. Grok models may be served through generic
+// OpenAI-compatible accounts, so account.Platform alone is not a safe billing
+// boundary.
+func requiresBillableGrokChatUsage(account *Account, models ...string) bool {
+	if account != nil && account.Platform == PlatformGrok {
+		return true
+	}
+	for _, model := range models {
+		if platform, ok := DetectModelPlatform(model); ok && platform == PlatformGrok {
+			return true
+		}
+	}
+	return false
+}
diff --git a/backend/internal/service/openai_gateway_usage_integrity_test.go b/backend/internal/service/openai_gateway_usage_integrity_test.go
new file mode 100644
index 0000000000..3a1dc441f9
--- /dev/null
+++ b/backend/internal/service/openai_gateway_usage_integrity_test.go
@@ -0,0 +1,58 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestRequiresBillableGrokChatUsage(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		account *Account
+		models  []string
+		want    bool
+	}{
+		{
+			name:    "grok platform",
+			account: &Account{Platform: PlatformGrok},
+			models:  []string{"alias"},
+			want:    true,
+		},
+		{
+			name:    "OpenAI-compatible requested Grok model",
+			account: &Account{Platform: PlatformOpenAI},
+			models:  []string{"grok-4.5"},
+			want:    true,
+		},
+		{
+			name:    "OpenAI-compatible mapped Grok model",
+			account: &Account{Platform: PlatformOpenAI},
+			models:  []string{"alias", "grok-4.5"},
+			want:    true,
+		},
+		{
+			name:    "xAI-qualified Grok model",
+			account: &Account{Platform: PlatformOpenAI},
+			models:  []string{"xai/grok-4.5"},
+			want:    true,
+		},
+		{
+			name:    "ordinary OpenAI model",
+			account: &Account{Platform: PlatformOpenAI},
+			models:  []string{"gpt-5.4"},
+			want:    false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			require.Equal(t, tt.want, requiresBillableGrokChatUsage(tt.account, tt.models...))
+		})
+	}
+}