diff --git a/crawl4ai/deep_crawling/bff_strategy.py b/crawl4ai/deep_crawling/bff_strategy.py
index 511fde692..a6e8768ef 100644
--- a/crawl4ai/deep_crawling/bff_strategy.py
+++ b/crawl4ai/deep_crawling/bff_strategy.py
@@ -36,7 +36,7 @@ class BestFirstCrawlingStrategy(DeepCrawlStrategy):
     def __init__(
         self,
         max_depth: int,
-        filter_chain: FilterChain = FilterChain(),
+        filter_chain: Optional[FilterChain] = None,
         url_scorer: Optional[URLScorer] = None,
         include_external: bool = False,
         score_threshold: float = -infinity,
@@ -49,7 +49,7 @@ def __init__(
         should_cancel: Optional[Callable[[], Union[bool, Awaitable[bool]]]] = None,
     ):
         self.max_depth = max_depth
-        self.filter_chain = filter_chain
+        self.filter_chain = filter_chain if filter_chain is not None else FilterChain()
         self.url_scorer = url_scorer
         self.include_external = include_external
         self.score_threshold = score_threshold
diff --git a/crawl4ai/deep_crawling/bfs_strategy.py b/crawl4ai/deep_crawling/bfs_strategy.py
index dfb759272..d7d3fa532 100644
--- a/crawl4ai/deep_crawling/bfs_strategy.py
+++ b/crawl4ai/deep_crawling/bfs_strategy.py
@@ -25,7 +25,7 @@ class BFSDeepCrawlStrategy(DeepCrawlStrategy):
     def __init__(
         self,
         max_depth: int,
-        filter_chain: FilterChain = FilterChain(),
+        filter_chain: Optional[FilterChain] = None,
         url_scorer: Optional[URLScorer] = None,
         include_external: bool = False,
         score_threshold: float = -infinity,
@@ -38,7 +38,7 @@ def __init__(
         should_cancel: Optional[Callable[[], Union[bool, Awaitable[bool]]]] = None,
     ):
         self.max_depth = max_depth
-        self.filter_chain = filter_chain
+        self.filter_chain = filter_chain if filter_chain is not None else FilterChain()
         self.url_scorer = url_scorer
         self.include_external = include_external
         self.score_threshold = score_threshold
diff --git a/tests/deep_crawling/test_default_filter_chain_isolation.py b/tests/deep_crawling/test_default_filter_chain_isolation.py
new file mode 100644
index 000000000..512a907f7
--- /dev/null
+++ b/tests/deep_crawling/test_default_filter_chain_isolation.py
@@ -0,0 +1,42 @@
+"""Regression tests for the mutable-default-argument bug in deep-crawl strategy
+constructors: filter_chain defaulted to a single FilterChain() instance shared
+across every strategy created without an explicit filter_chain=, leaking
+FilterChain.stats counters and add_filter() mutations across unrelated
+strategy instances.
+"""
+
+from crawl4ai.deep_crawling.bff_strategy import BestFirstCrawlingStrategy
+from crawl4ai.deep_crawling.bfs_strategy import BFSDeepCrawlStrategy
+from crawl4ai.deep_crawling.dfs_strategy import DFSDeepCrawlStrategy
+
+
+def test_bfs_strategy_instances_get_independent_default_filter_chains():
+    a = BFSDeepCrawlStrategy(max_depth=2)
+    b = BFSDeepCrawlStrategy(max_depth=3)
+    assert a.filter_chain is not b.filter_chain
+
+
+def test_dfs_strategy_instances_get_independent_default_filter_chains():
+    a = DFSDeepCrawlStrategy(max_depth=2)
+    b = DFSDeepCrawlStrategy(max_depth=3)
+    assert a.filter_chain is not b.filter_chain
+
+
+def test_best_first_strategy_instances_get_independent_default_filter_chains():
+    a = BestFirstCrawlingStrategy(max_depth=2)
+    b = BestFirstCrawlingStrategy(max_depth=3)
+    assert a.filter_chain is not b.filter_chain
+
+
+def test_adding_a_filter_to_one_default_chain_does_not_leak_to_another_instance():
+    a = BFSDeepCrawlStrategy(max_depth=2)
+    b = BFSDeepCrawlStrategy(max_depth=3)
+
+    class _DummyFilter:
+        def apply(self, url):
+            return True
+
+    a.filter_chain.add_filter(_DummyFilter())
+
+    assert len(a.filter_chain.filters) == 1
+    assert len(b.filter_chain.filters) == 0