diff --git a/app/Services/Crawl/Adapters/ArxivApiAdapter.php b/app/Services/Crawl/Adapters/ArxivApiAdapter.php
index 3a10f4c..a8081ba 100644
--- a/app/Services/Crawl/Adapters/ArxivApiAdapter.php
+++ b/app/Services/Crawl/Adapters/ArxivApiAdapter.php
@@ -3,6 +3,7 @@
 namespace App\Services\Crawl\Adapters;
 
 use App\Models\CrawlSource;
+use App\Models\Paper;
 use App\Services\Crawl\ArxivAbsEnricher;
 use App\Services\Crawl\ArxivMetadataParser;
 use App\Services\Crawl\ArxivRequestGate;
@@ -41,6 +42,9 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
         $maxResults = min(200, max(1, (int) ($params['max_results'] ?? 50)));
         $maxPages = min(20, max(1, (int) ($params['max_pages'] ?? 1)));
         $pageSize = 50;
+        $skipImported = ($params['skip_imported'] ?? true) !== false;
+        $importedIds = $skipImported ? $this->loadImportedExternalIds() : [];
+        $maxScanPages = $this->resolveMaxScanPages($maxPages, $maxResults, $skipImported);
 
         if ((bool) config('crawl.arxiv.prefer_html_search', false)) {
             return $this->finalizeItems($this->requireHtmlSearchItems($keywordRaw, $maxResults));
@@ -49,10 +53,9 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
         $items = [];
         $seen = [];
 
-        for ($page = 0; $page < $maxPages && count($items) < $maxResults; $page++) {
+        for ($page = 0; $page < $maxScanPages && count($items) < $maxResults; $page++) {
             $start = $page * $pageSize;
-            $batchSize = min($pageSize, $maxResults - count($items));
-            $batch = $this->fetchApiPage($keywordRaw, $start, $batchSize);
+            $batch = $this->fetchApiPage($keywordRaw, $start, $pageSize);
             if ($batch === []) {
                 break;
             }
@@ -62,10 +65,18 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
                     continue;
                 }
                 $seen[$item->externalId] = true;
+
+                if ($skipImported && isset($importedIds[$item->externalId])) {
+                    continue;
+                }
+
                 $items[] = $item;
+                if (count($items) >= $maxResults) {
+                    break 2;
+                }
             }
 
-            if (count($batch) < $batchSize) {
+            if (count($batch) < $pageSize) {
                 break;
             }
         }
@@ -74,6 +85,10 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
             return $this->finalizeItems($items);
         }
 
+        if ($keywordRaw === '') {
+            throw new \RuntimeException('arXiv API 未返回结果，请稍后重试');
+        }
+
         return $this->finalizeItems($this->requireHtmlSearchItems($keywordRaw, min($maxResults, $pageSize)));
     }
 
@@ -100,7 +115,12 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
             return [];
         }
 
-        return $this->parseAtomFeed($response->body(), $keywordRaw);
+        $body = $response->body();
+        if (str_contains($body, 'arxiv.org/api/errors') || str_contains($body, '<title>Error</title>')) {
+            return [];
+        }
+
+        return $this->parseAtomFeed($body, $keywordRaw);
     }
 
     /**
@@ -340,4 +360,30 @@ class ArxivApiAdapter implements CrawlerAdapterInterface
 
         return $enrichAbs ? $this->absEnricher->enrichMany($items) : $items;
     }
+
+    /**
+     * @return array<string, true>
+     */
+    protected function loadImportedExternalIds(): array
+    {
+        $ids = Paper::query()
+            ->where('source', 'crawl')
+            ->whereNotNull('external_id')
+            ->pluck('external_id')
+            ->all();
+
+        return array_fill_keys($ids, true);
+    }
+
+    protected function resolveMaxScanPages(int $maxPages, int $maxResults, bool $skipImported): int
+    {
+        $maxPages = min(20, max(1, $maxPages));
+        if (! $skipImported) {
+            return $maxPages;
+        }
+
+        $minForTarget = (int) ceil($maxResults / 50);
+
+        return min(200, max($maxPages, $minForTarget * 10));
+    }
 }
diff --git a/app/Services/Crawl/CrawlKeywordParser.php b/app/Services/Crawl/CrawlKeywordParser.php
index 1a416fc..442b87a 100644
--- a/app/Services/Crawl/CrawlKeywordParser.php
+++ b/app/Services/Crawl/CrawlKeywordParser.php
@@ -86,7 +86,7 @@ class CrawlKeywordParser
     {
         $phrases = self::parsePhrases($raw);
         if ($phrases === []) {
-            return 'all:*';
+            return 'cat:*';
         }
 
         $clauseParts = [];
@@ -106,7 +106,7 @@ class CrawlKeywordParser
         }
 
         if ($clauseParts === []) {
-            return 'all:*';
+            return 'cat:*';
         }
 
         return count($clauseParts) === 1
diff --git a/tests/Unit/CrawlKeywordParserTest.php b/tests/Unit/CrawlKeywordParserTest.php
new file mode 100644
index 0000000..85a5d26
--- /dev/null
+++ b/tests/Unit/CrawlKeywordParserTest.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Tests\Unit;
+
+use App\Services\Crawl\CrawlKeywordParser;
+use PHPUnit\Framework\TestCase;
+
+class CrawlKeywordParserTest extends TestCase
+{
+    public function test_build_arxiv_search_query_without_keyword_uses_category_wildcard(): void
+    {
+        $this->assertSame('cat:*', CrawlKeywordParser::buildArxivSearchQuery(''));
+        $this->assertSame('cat:*', CrawlKeywordParser::buildArxivSearchQuery(null));
+    }
+
+    public function test_build_arxiv_search_query_with_phrase(): void
+    {
+        $this->assertSame('(all:machine AND all:learning)', CrawlKeywordParser::buildArxivSearchQuery('machine learning'));
+    }
+}