diff --git a/crawl4ai/async_configs.py b/crawl4ai/async_configs.py
index d3a4d98a..bdc0c9bf 100644
--- a/crawl4ai/async_configs.py
+++ b/crawl4ai/async_configs.py
@@ -1,3 +1,5 @@
+import copy
+import functools
 import importlib
 import os
 import warnings
@@ -35,6 +37,83 @@ from enum import Enum
 UrlMatcher = Union[str, Callable[[str], bool], List[Union[str, Callable[[str], bool]]]]
 
 
+def _with_defaults(cls):
+    """Class decorator: adds set_defaults/get_defaults/reset_defaults classmethods.
+
+    After decorating, every new instance resolves parameters as:
+        explicit arg  >  class-level user defaults  >  hardcoded default
+
+    Usage::
+
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        cfg = BrowserConfig()          # headless=False, viewport_width=1920
+        cfg = BrowserConfig(headless=True)  # explicit wins → headless=True
+    """
+    original_init = cls.__init__
+    sig = inspect.signature(original_init)
+    param_names = [p for p in sig.parameters if p != "self"]
+    valid_params = frozenset(param_names)
+
+    @functools.wraps(original_init)
+    def wrapped_init(self, *args, **kwargs):
+        user_defaults = type(self)._user_defaults
+        if user_defaults:
+            # Determine which params the caller passed explicitly
+            explicit = set(kwargs.keys())
+            for i in range(len(args)):
+                if i < len(param_names):
+                    explicit.add(param_names[i])
+            # Inject user defaults for non-explicit params
+            for key, value in user_defaults.items():
+                if key not in explicit:
+                    kwargs[key] = copy.deepcopy(value)
+        original_init(self, *args, **kwargs)
+
+    cls.__init__ = wrapped_init
+    cls._user_defaults = {}
+
+    @classmethod
+    def set_defaults(klass, **kwargs):
+        """Set class-level default overrides for new instances.
+
+        Args:
+            **kwargs: Parameter names and their default values.
+
+        Raises:
+            ValueError: If any key is not a valid ``__init__`` parameter.
+        """
+        invalid = set(kwargs) - valid_params
+        if invalid:
+            raise ValueError(
+                f"Invalid parameter(s) for {klass.__name__}: {invalid}"
+            )
+        for k, v in kwargs.items():
+            klass._user_defaults[k] = copy.deepcopy(v)
+
+    @classmethod
+    def get_defaults(klass):
+        """Return a deep copy of the current class-level defaults."""
+        return copy.deepcopy(klass._user_defaults)
+
+    @classmethod
+    def reset_defaults(klass, *names):
+        """Clear class-level defaults.
+
+        With no arguments, removes all overrides.
+        With arguments, removes only the named overrides.
+        """
+        if names:
+            for n in names:
+                klass._user_defaults.pop(n, None)
+        else:
+            klass._user_defaults.clear()
+
+    cls.set_defaults = set_defaults
+    cls.get_defaults = get_defaults
+    cls.reset_defaults = reset_defaults
+    return cls
+
+
 class MatchMode(Enum):
     OR = "or"
     AND = "and"
@@ -392,6 +471,7 @@ class ProxyConfig:
         config_dict.update(kwargs)
         return ProxyConfig.from_dict(config_dict)
 
+@_with_defaults
 class BrowserConfig:
     """
     Configuration class for setting up a browser instance and its context in AsyncPlaywrightCrawlerStrategy.
@@ -1032,6 +1112,7 @@ class HTTPCrawlerConfig:
             return config
         return HTTPCrawlerConfig.from_kwargs(config)
 
+@_with_defaults
 class CrawlerRunConfig():
 
     """
diff --git a/tests/test_config_defaults.py b/tests/test_config_defaults.py
new file mode 100644
index 00000000..700886aa
--- /dev/null
+++ b/tests/test_config_defaults.py
@@ -0,0 +1,263 @@
+"""Tests for BrowserConfig.set_defaults / CrawlerRunConfig.set_defaults."""
+
+import pytest
+from crawl4ai.async_configs import BrowserConfig, CrawlerRunConfig
+
+
+@pytest.fixture(autouse=True)
+def _reset_defaults():
+    """Ensure every test starts and ends with a clean slate."""
+    BrowserConfig.reset_defaults()
+    CrawlerRunConfig.reset_defaults()
+    yield
+    BrowserConfig.reset_defaults()
+    CrawlerRunConfig.reset_defaults()
+
+
+# ── Basic API ──────────────────────────────────────────────────────────
+
+
+class TestBasicAPI:
+    def test_set_and_get_defaults(self):
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        d = BrowserConfig.get_defaults()
+        assert d == {"headless": False, "viewport_width": 1920}
+
+    def test_get_defaults_returns_copy(self):
+        BrowserConfig.set_defaults(headers={"X-Foo": "bar"})
+        d = BrowserConfig.get_defaults()
+        d["headers"]["X-Foo"] = "changed"
+        assert BrowserConfig.get_defaults()["headers"]["X-Foo"] == "bar"
+
+    def test_reset_all(self):
+        BrowserConfig.set_defaults(headless=False)
+        BrowserConfig.reset_defaults()
+        assert BrowserConfig.get_defaults() == {}
+
+    def test_reset_selective(self):
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        BrowserConfig.reset_defaults("headless")
+        assert BrowserConfig.get_defaults() == {"viewport_width": 1920}
+
+    def test_invalid_param_raises(self):
+        with pytest.raises(ValueError, match="Invalid parameter"):
+            BrowserConfig.set_defaults(not_a_real_param=42)
+
+    def test_invalid_param_among_valid(self):
+        with pytest.raises(ValueError):
+            BrowserConfig.set_defaults(headless=False, bogus=True)
+        # Nothing should have been stored
+        assert BrowserConfig.get_defaults() == {}
+
+    def test_set_defaults_overwrites(self):
+        BrowserConfig.set_defaults(headless=False)
+        BrowserConfig.set_defaults(headless=True)
+        assert BrowserConfig.get_defaults()["headless"] is True
+
+    def test_crawler_run_config_basic(self):
+        CrawlerRunConfig.set_defaults(verbose=False, scan_full_page=True)
+        d = CrawlerRunConfig.get_defaults()
+        assert d == {"verbose": False, "scan_full_page": True}
+
+
+# ── Default injection ──────────────────────────────────────────────────
+
+
+class TestDefaultInjection:
+    def test_browser_config_defaults_applied(self):
+        BrowserConfig.set_defaults(
+            headless=False,
+            cache_cdp_connection=True,
+            cdp_close_delay=0,
+        )
+        cfg = BrowserConfig()
+        assert cfg.headless is False
+        assert cfg.cache_cdp_connection is True
+        assert cfg.cdp_close_delay == 0
+
+    def test_crawler_run_config_defaults_applied(self):
+        CrawlerRunConfig.set_defaults(verbose=False, scan_full_page=True)
+        cfg = CrawlerRunConfig()
+        assert cfg.verbose is False
+        assert cfg.scan_full_page is True
+
+    def test_partial_defaults(self):
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig()
+        assert cfg.headless is False
+        # Other params keep their hardcoded defaults
+        assert cfg.browser_type == "chromium"
+        assert cfg.viewport_width == 1080
+
+    def test_multiple_instances_get_defaults(self):
+        BrowserConfig.set_defaults(headless=False)
+        c1 = BrowserConfig()
+        c2 = BrowserConfig()
+        assert c1.headless is False
+        assert c2.headless is False
+
+
+# ── Explicit override ──────────────────────────────────────────────────
+
+
+class TestExplicitOverride:
+    def test_explicit_kwarg_wins(self):
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig(headless=True)
+        assert cfg.headless is True
+
+    def test_explicit_same_as_default_still_wins(self):
+        """Even if user passes the same value as user-default, it should be treated as explicit."""
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig(headless=False)
+        assert cfg.headless is False
+
+    def test_explicit_none_wins(self):
+        BrowserConfig.set_defaults(cdp_url="ws://localhost:9222")
+        cfg = BrowserConfig(cdp_url=None)
+        assert cfg.cdp_url is None
+
+    def test_mixed_explicit_and_default(self):
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        cfg = BrowserConfig(headless=True)
+        assert cfg.headless is True  # explicit
+        assert cfg.viewport_width == 1920  # from user default
+
+
+# ── Mutable isolation ──────────────────────────────────────────────────
+
+
+class TestMutableIsolation:
+    def test_list_default_not_shared(self):
+        BrowserConfig.set_defaults(cookies=[{"name": "a", "value": "1"}])
+        c1 = BrowserConfig()
+        c2 = BrowserConfig()
+        c1.cookies.append({"name": "b", "value": "2"})
+        assert len(c2.cookies) == 1  # c2 should be unaffected
+
+    def test_dict_default_not_shared(self):
+        BrowserConfig.set_defaults(headers={"X-Foo": "bar"})
+        c1 = BrowserConfig()
+        c2 = BrowserConfig()
+        c1.headers["X-New"] = "val"
+        assert "X-New" not in c2.headers
+
+    def test_set_defaults_input_not_mutated(self):
+        original = {"X-Foo": "bar"}
+        BrowserConfig.set_defaults(headers=original)
+        cfg = BrowserConfig()
+        cfg.headers["X-Added"] = "val"
+        assert "X-Added" not in original
+        assert "X-Added" not in BrowserConfig.get_defaults()["headers"]
+
+
+# ── Special processing ─────────────────────────────────────────────────
+
+
+class TestSpecialProcessing:
+    def test_browser_mode_builtin_sets_managed(self):
+        BrowserConfig.set_defaults(browser_mode="builtin")
+        cfg = BrowserConfig()
+        assert cfg.use_managed_browser is True
+
+    def test_viewport_dict_overrides_dimensions(self):
+        BrowserConfig.set_defaults(viewport={"width": 1920, "height": 1080})
+        cfg = BrowserConfig()
+        assert cfg.viewport_width == 1920
+        assert cfg.viewport_height == 1080
+
+    def test_proxy_string_converted_to_proxy_config(self):
+        BrowserConfig.set_defaults(proxy="http://user:pass@proxy:8080")
+        cfg = BrowserConfig()
+        assert cfg.proxy_config is not None
+        assert cfg.proxy_config.server == "http://proxy:8080"
+
+    def test_crawler_run_config_proxy_dict_converted(self):
+        CrawlerRunConfig.set_defaults(
+            proxy_config={"server": "http://proxy:8080"}
+        )
+        cfg = CrawlerRunConfig()
+        from crawl4ai.async_configs import ProxyConfig
+        assert isinstance(cfg.proxy_config, ProxyConfig)
+
+
+# ── Clone / from_kwargs ────────────────────────────────────────────────
+
+
+class TestCloneAndFromKwargs:
+    def test_clone_preserves_user_default_values(self):
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        cfg = BrowserConfig()
+        cloned = cfg.clone()
+        assert cloned.headless is False
+        assert cloned.viewport_width == 1920
+
+    def test_clone_with_override(self):
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig()
+        cloned = cfg.clone(headless=True)
+        assert cloned.headless is True
+
+    def test_from_kwargs_explicit_values(self):
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig.from_kwargs({"headless": True})
+        assert cfg.headless is True
+
+
+# ── Dump / Load round-trip ─────────────────────────────────────────────
+
+
+class TestDumpLoad:
+    def test_dump_load_preserves_user_defaults(self):
+        BrowserConfig.set_defaults(headless=False, viewport_width=1920)
+        cfg = BrowserConfig()
+        data = cfg.dump()
+        loaded = BrowserConfig.load(data)
+        assert loaded.headless is False
+        assert loaded.viewport_width == 1920
+
+    def test_dump_load_survives_reset(self):
+        """Values should be baked into serialized data, independent of class defaults."""
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig()
+        data = cfg.dump()
+        BrowserConfig.reset_defaults()
+        loaded = BrowserConfig.load(data)
+        assert loaded.headless is False
+
+    def test_crawler_run_config_dump_load(self):
+        CrawlerRunConfig.set_defaults(verbose=False, scan_full_page=True)
+        cfg = CrawlerRunConfig()
+        data = cfg.dump()
+        CrawlerRunConfig.reset_defaults()
+        loaded = CrawlerRunConfig.load(data)
+        assert loaded.verbose is False
+        assert loaded.scan_full_page is True
+
+    def test_to_dict_includes_user_default_values(self):
+        BrowserConfig.set_defaults(headless=False)
+        cfg = BrowserConfig()
+        d = cfg.to_dict()
+        assert d["headless"] is False
+
+
+# ── Class isolation ────────────────────────────────────────────────────
+
+
+class TestClassIsolation:
+    def test_browser_defaults_dont_leak_to_crawler(self):
+        BrowserConfig.set_defaults(verbose=False)
+        cfg = CrawlerRunConfig()
+        assert cfg.verbose is True  # CrawlerRunConfig hardcoded default
+
+    def test_crawler_defaults_dont_leak_to_browser(self):
+        CrawlerRunConfig.set_defaults(verbose=False)
+        cfg = BrowserConfig()
+        assert cfg.verbose is True  # BrowserConfig hardcoded default
+
+    def test_independent_reset(self):
+        BrowserConfig.set_defaults(headless=False)
+        CrawlerRunConfig.set_defaults(verbose=False)
+        BrowserConfig.reset_defaults()
+        assert BrowserConfig.get_defaults() == {}
+        assert CrawlerRunConfig.get_defaults() == {"verbose": False}