diff --git a/backend/internal/service/gateway_anthropic_apikey_passthrough_test.go b/backend/internal/service/gateway_anthropic_apikey_passthrough_test.go
index 3722ec5189..d5976c42b4 100644
--- a/backend/internal/service/gateway_anthropic_apikey_passthrough_test.go
+++ b/backend/internal/service/gateway_anthropic_apikey_passthrough_test.go
@@ -582,7 +582,8 @@ func TestGatewayService_AnthropicAPIKeyPassthrough_CountTokensFiltersGenerationF
 	require.Equal(t, "sys", gjson.GetBytes(sentBody, "system.0.text").String())
 	require.Equal(t, "hello", gjson.GetBytes(sentBody, "messages.0.content").String())
 	require.Equal(t, "tool", gjson.GetBytes(sentBody, "tools.0.name").String())
-	require.Equal(t, int64(1024), gjson.GetBytes(sentBody, "max_tokens").Int())
+	require.False(t, gjson.GetBytes(sentBody, "max_tokens").Exists(),
+		"count_tokens 请求不得携带生成参数 max_tokens")
 	require.Equal(t, "enabled", gjson.GetBytes(sentBody, "thinking.type").String())
 }
 
diff --git a/backend/internal/service/gateway_context_management_test.go b/backend/internal/service/gateway_context_management_test.go
index b0f2e5b820..bdeb2c600b 100644
--- a/backend/internal/service/gateway_context_management_test.go
+++ b/backend/internal/service/gateway_context_management_test.go
@@ -596,6 +596,35 @@ func TestBuildCountTokensRequest_OAuthMimicHaiku_PreservesContextManagementEndTo
 		"count_tokens 路径必须含 token-counting beta")
 }
 
+func TestBuildCountTokensRequest_OAuthMimic_DropsInjectedMaxTokens(t *testing.T) {
+	// OAuth mimicry injects max_tokens=128000 for normal messages requests. It is
+	// invalid for Anthropic's count_tokens endpoint and must be stripped on wire.
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages/count_tokens", nil)
+
+	account := &Account{ID: 413, Platform: PlatformAnthropic, Type: AccountTypeOAuth,
+		Credentials: map[string]any{"access_token": "oauth-tok"},
+		Status:      StatusActive, Schedulable: true,
+	}
+	normalized, _ := normalizeClaudeOAuthRequestBody(
+		[]byte(`{"model":"claude-sonnet-4-5","messages":[]}`),
+		"claude-sonnet-4-5", claudeOAuthNormalizeOptions{},
+	)
+	require.Equal(t, int64(128000), gjson.GetBytes(normalized, "max_tokens").Int(),
+		"precondition: OAuth mimicry injects the Claude Code default")
+
+	svc := &GatewayService{cfg: &config.Config{}}
+	req, _, err := svc.buildCountTokensRequest(
+		context.Background(), c, account, normalized,
+		"oauth-tok", "oauth", "claude-sonnet-4-5", true,
+	)
+	require.NoError(t, err)
+	require.False(t, gjson.GetBytes(readUpstreamBodyForTest(t, req), "max_tokens").Exists(),
+		"count_tokens wire body must not contain max_tokens")
+}
+
 func TestBuildCountTokensRequest_APIKeyHaiku_StripsContextManagementEndToEnd(t *testing.T) {
 	// API-key + haiku + 客户端 header 不带 context-management beta → final beta 不含 → strip
 	gin.SetMode(gin.TestMode)
diff --git a/backend/internal/service/gateway_count_tokens.go b/backend/internal/service/gateway_count_tokens.go
index 500f655660..7b66ac9596 100644
--- a/backend/internal/service/gateway_count_tokens.go
+++ b/backend/internal/service/gateway_count_tokens.go
@@ -584,6 +584,10 @@ func sanitizeCountTokensRequestBody(body []byte) []byte {
 		"stream",
 		"stop_sequences",
 		"stop",
+		// Anthropic's /v1/messages/count_tokens accepts request-input fields only.
+		// max_tokens is a generation parameter; OAuth mimicry may inject it to
+		// resemble Claude Code messages requests, so it must never reach this endpoint.
+		"max_tokens",
 	} {
 		if gjson.GetBytes(out, path).Exists() {
 			if next, ok := deleteJSONPathBytes(out, path); ok {