diff --git a/README.rst b/README.rst
index fc6c48ca..374f6db2 100644
--- a/README.rst
+++ b/README.rst
@@ -94,7 +94,7 @@ You can add a default, pickle-based, persistent cache to your function - meaning
     """Your function now has a persistent cache mapped by argument values!"""
     return {'arg1': arg1, 'arg2': arg2}
 
-Class and object methods can also be cached. Cachier will automatically ignore the `self` parameter when determining the cache key for an object method. **This means that methods will be cached across all instances of an object, which may not be what you want.**
+Class and object methods can also be cached. Cachier will automatically ignore the ``self`` parameter when determining the cache key for an object method. **This means that methods will be cached across all instances of an object, which may not be what you want.** Because this is a common source of bugs, ``@cachier`` raises a ``TypeError`` by default when applied to an instance method (a function whose first parameter is named ``self``). This error is raised when ``@cachier`` is applied (at class definition time), not when the method is called. To opt in to cross-instance cache sharing, pass ``allow_non_static_methods=True``.
 
 .. code-block:: python
 
@@ -107,17 +107,18 @@ Class and object methods can also be cached. Cachier will automatically ignore t
       return arg_1 + arg_2
 
     # Instance method does not depend on object's internal state, so good to cache
-    @cachier()
+    @cachier(allow_non_static_methods=True)
     def good_usage_1(self, arg_1, arg_2):
       return arg_1 + arg_2
 
     # Instance method is calling external service, probably okay to cache
-    @cachier()
+    @cachier(allow_non_static_methods=True)
     def good_usage_2(self, arg_1, arg_2):
       result = self.call_api(arg_1, arg_2)
       return result
 
     # Instance method relies on object attribute, NOT good to cache
+    # @cachier() would raise TypeError here -- this is intentional
     @cachier()
     def bad_usage(self, arg_1, arg_2):
       return arg_1 + arg_2 + self.arg_3
@@ -148,6 +149,7 @@ The following parameters will only be applied to decorators defined after `set_d
 *  `pickle_reload`
 *  `separate_files`
 *  `entry_size_limit`
+*  `allow_non_static_methods`
 
 These parameters can be changed at any time and they will apply to all decorators:
 
diff --git a/src/cachier/config.py b/src/cachier/config.py
index f04d0cff..6b1652e4 100644
--- a/src/cachier/config.py
+++ b/src/cachier/config.py
@@ -66,6 +66,7 @@ class Params:
     cleanup_stale: bool = False
     cleanup_interval: timedelta = timedelta(days=1)
     entry_size_limit: Optional[int] = None
+    allow_non_static_methods: bool = False
 
 
 _global_params = Params()
@@ -130,7 +131,14 @@ def set_global_params(**params: Any) -> None:
       'cleanup_interval', and 'caching_enabled'. In some cores, if the
       decorator was created without concrete value for 'wait_for_calc_timeout',
       calls that check calculation timeouts will fall back to the global
-      'wait_for_calc_timeout' as well.
+      'wait_for_calc_timeout' as well. 'allow_non_static_methods'
+      (decoration-time only) controls whether instance methods are
+      permitted; it is read once when @cachier is applied, not on each call.
+
+    Note that ``allow_non_static_methods`` is a **decoration-time**
+    parameter: it is checked once when the ``@cachier`` decorator is
+    applied and is not re-read on each function call. Changing it via
+    ``set_global_params`` only affects decorators created after the call.
 
     """
     import cachier
diff --git a/src/cachier/core.py b/src/cachier/core.py
index 9031e1fa..d4007b6b 100644
--- a/src/cachier/core.py
+++ b/src/cachier/core.py
@@ -200,6 +200,7 @@ def cachier(
     cleanup_stale: Optional[bool] = None,
     cleanup_interval: Optional[timedelta] = None,
     entry_size_limit: Optional[Union[int, str]] = None,
+    allow_non_static_methods: Optional[bool] = None,
 ):
     """Wrap as a persistent, stale-free memoization decorator.
 
@@ -287,6 +288,13 @@ def cachier(
         Maximum serialized size of a cached value. Values exceeding the limit
         are returned but not cached. Human readable strings like ``"10MB"`` are
         allowed.
+    allow_non_static_methods : bool, optional
+        If True, allows ``@cachier`` to decorate instance methods (functions
+        whose first parameter is named ``self``). By default, decorating an
+        instance method raises ``TypeError`` because the ``self`` argument is
+        ignored for cache-key computation, meaning all instances share the
+        same cache -- which is rarely the intended behaviour. Set this to
+        ``True`` only when cross-instance cache sharing is intentional.
 
     """
     # Check for deprecated parameters
@@ -356,6 +364,23 @@ def cachier(
 
     def _cachier_decorator(func):
         core.set_func(func)
+
+        # Guard: raise TypeError when decorating an instance method unless
+        # explicitly opted in.  The 'self' parameter is ignored for cache-key
+        # computation, so all instances share the same cache.
+        if core.func_is_method:
+            _allow_methods = _update_with_defaults(allow_non_static_methods, "allow_non_static_methods")
+            if not _allow_methods:
+                raise TypeError(
+                    f"@cachier cannot decorate instance method "
+                    f"'{func.__qualname__}' because the 'self' parameter is "
+                    "excluded from cache-key computation and all instances "
+                    "would share a single cache. Pass allow_non_static_methods=True "
+                    "to the decorator or call "
+                    "set_global_params(allow_non_static_methods=True) if "
+                    "cross-instance cache sharing is intentional."
+                )
+
         is_coroutine = inspect.iscoroutinefunction(func)
 
         if backend == "mongo":
@@ -468,7 +493,6 @@ def _call(*args, max_age: Optional[timedelta] = None, **kwds):
                         _print("max_age is negative. Cached result considered stale.")
                         nonneg_max_age = False
                     else:
-                        assert max_age is not None  # noqa: S101
                         max_allowed_age = min(_stale_after, max_age)
                 # note: if max_age < 0, we always consider a value stale
                 if nonneg_max_age and (now - entry.time <= max_allowed_age):
@@ -557,7 +581,6 @@ async def _call_async(*args, max_age: Optional[timedelta] = None, **kwds):
                         _print("max_age is negative. Cached result considered stale.")
                         nonneg_max_age = False
                     else:
-                        assert max_age is not None  # noqa: S101
                         max_allowed_age = min(_stale_after, max_age)
                 # note: if max_age < 0, we always consider a value stale
                 if nonneg_max_age and (now - entry.time <= max_allowed_age):
diff --git a/src/cachier/cores/base.py b/src/cachier/cores/base.py
index 547382da..1e099458 100644
--- a/src/cachier/cores/base.py
+++ b/src/cachier/cores/base.py
@@ -48,11 +48,22 @@ def __init__(
         self.wait_for_calc_timeout = wait_for_calc_timeout
         self.lock = threading.RLock()
         self.entry_size_limit = entry_size_limit
+        self.func_is_method: bool = False
 
     def set_func(self, func):
         """Set the function this core will use.
 
-        This has to be set before any method is called. Also determine if the function is an object method.
+        This must be called before any other method is invoked.  In addition
+        to storing ``func`` on the instance, this method inspects the
+        function's signature and sets ``self.func_is_method`` to ``True``
+        when the first parameter is named ``"self"``.
+
+        Notes
+        -----
+        Detection is name-based: only ``func_params[0] == "self"`` is
+        checked.  ``@classmethod`` functions whose first parameter is
+        conventionally named ``cls`` are not detected as methods --
+        this is a known gap.
 
         """
         # unwrap if the function is functools.partial
diff --git a/tests/mongo_tests/test_async_mongo_core.py b/tests/mongo_tests/test_async_mongo_core.py
index bcb9a233..d92226bd 100644
--- a/tests/mongo_tests/test_async_mongo_core.py
+++ b/tests/mongo_tests/test_async_mongo_core.py
@@ -76,7 +76,7 @@ async def async_mongetter():
     call_count = 0
 
     class _MongoMethods:
-        @cachier(mongetter=async_mongetter)
+        @cachier(mongetter=async_mongetter, allow_non_static_methods=True)
         async def async_cached_mongo_method_args_kwargs(self, x: int, y: int) -> int:
             nonlocal call_count
             call_count += 1
diff --git a/tests/redis_tests/test_async_redis_core.py b/tests/redis_tests/test_async_redis_core.py
index 4d730d6b..e5dec8f7 100644
--- a/tests/redis_tests/test_async_redis_core.py
+++ b/tests/redis_tests/test_async_redis_core.py
@@ -84,7 +84,7 @@ async def get_redis_client():
     call_count = 0
 
     class _RedisMethods:
-        @cachier(backend="redis", redis_client=get_redis_client)
+        @cachier(backend="redis", redis_client=get_redis_client, allow_non_static_methods=True)
         async def async_cached_redis_method_args_kwargs(self, x: int, y: int) -> int:
             nonlocal call_count
             call_count += 1
diff --git a/tests/test_async_core.py b/tests/test_async_core.py
index 5adeedb9..770c0ee4 100644
--- a/tests/test_async_core.py
+++ b/tests/test_async_core.py
@@ -261,7 +261,9 @@ class MyClass:
             def __init__(self, value):
                 self.value = value
 
-            @cachier(backend="memory")
+            # allow_non_static_methods=True: cross-instance cache sharing
+            # is intentional in this test
+            @cachier(backend="memory", allow_non_static_methods=True)
             async def async_method(self, x):
                 await asyncio.sleep(0.1)
                 return x * self.value
@@ -290,7 +292,9 @@ class MyClass:
             def __init__(self, value):
                 self.value = value
 
-            @cachier(backend="memory")
+            # allow_non_static_methods=True: cross-instance cache sharing
+            # is intentional in this test
+            @cachier(backend="memory", allow_non_static_methods=True)
             async def async_method(self, x):
                 await asyncio.sleep(0.1)
                 return x * self.value
@@ -311,6 +315,15 @@ async def async_method(self, x):
 
         obj1.async_method.clear_cache()
 
+    async def test_guard_raises_without_opt_in(self):
+        """Test that @cachier raises TypeError for async instance methods without opt-in."""
+        with pytest.raises(TypeError, match="allow_non_static_methods"):
+
+            class MyClass:
+                @cachier(backend="memory")
+                async def async_method(self, x):
+                    return x
+
 
 # =============================================================================
 # Sync Function Compatibility Tests
diff --git a/tests/test_caching_regression.py b/tests/test_caching_regression.py
index 82f857cb..8b018885 100644
--- a/tests/test_caching_regression.py
+++ b/tests/test_caching_regression.py
@@ -105,7 +105,7 @@ def __init__(self, cache_ttl=None):
                 cachier.enable_caching()
 
         # Use memory backend to avoid file cache persistence issues
-        @cachier.cachier(backend="memory")
+        @cachier.cachier(backend="memory", allow_non_static_methods=True)
         def test(self, param):
             self.counter += 1
             return param
diff --git a/tests/test_config.py b/tests/test_config.py
index 19e7f528..ccd0f2a0 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -2,14 +2,17 @@
 
 import pytest
 
-from cachier.config import get_default_params, set_default_params
+from cachier.config import get_default_params, get_global_params, set_default_params, set_global_params
 
 
 def test_set_default_params_deprecated():
     """Test that set_default_params shows deprecation warning."""
-    # Test lines 103-111: deprecation warning
-    with pytest.warns(DeprecationWarning, match="set_default_params.*deprecated.*set_global_params"):
-        set_default_params(stale_after=60)
+    original = get_global_params().stale_after
+    try:
+        with pytest.warns(DeprecationWarning, match="set_default_params.*deprecated.*set_global_params"):
+            set_default_params(stale_after=60)
+    finally:
+        set_global_params(stale_after=original)
 
 
 def test_get_default_params_deprecated():
diff --git a/tests/test_core_lookup.py b/tests/test_core_lookup.py
index 10675b6c..599aa194 100644
--- a/tests/test_core_lookup.py
+++ b/tests/test_core_lookup.py
@@ -8,6 +8,7 @@
 def test_get_default_params():
     params = get_global_params()
     assert sorted(vars(params).keys()) == [
+        "allow_non_static_methods",
         "allow_none",
         "backend",
         "cache_dir",
diff --git a/tests/test_general.py b/tests/test_general.py
index 280709e4..1592c9fb 100644
--- a/tests/test_general.py
+++ b/tests/test_general.py
@@ -185,7 +185,7 @@ def dummy_func(arg_1, arg_2):
 )
 def test_ignore_self_in_methods(mongetter, backend):
     class DummyClass:
-        @cachier.cachier(backend=backend, mongetter=mongetter)
+        @cachier.cachier(backend=backend, mongetter=mongetter, allow_non_static_methods=True)
         def takes_2_seconds(self, arg_1, arg_2):
             """Some function."""
             sleep(2)
@@ -257,7 +257,7 @@ def test():
 
 def test_global_disable_method():
     class Test:
-        @cachier.cachier()
+        @cachier.cachier(allow_non_static_methods=True)
         def test(self):
             return True
 
@@ -270,7 +270,7 @@ def test(self):
 
 def test_global_disable_method_with_args():
     class Test:
-        @cachier.cachier()
+        @cachier.cachier(allow_non_static_methods=True)
         def test(self, test):
             return test
 
@@ -286,7 +286,7 @@ class Test:
         def __init__(self, val):
             self.val = val
 
-        @cachier.cachier()
+        @cachier.cachier(allow_non_static_methods=True)
         def test(self, test=0):
             return self.val + test
 
@@ -302,7 +302,7 @@ class Test:
         def __init__(self, val):
             self.val = val
 
-        @cachier.cachier()
+        @cachier.cachier(allow_non_static_methods=True)
         def test(self, test1, test2=0):
             return self.val + test1 + test2
 
diff --git a/tests/test_smoke.py b/tests/test_smoke.py
index 31f2bd2f..1f38afd3 100644
--- a/tests/test_smoke.py
+++ b/tests/test_smoke.py
@@ -1,6 +1,8 @@
 """Smoke tests for cachier - fast, no external service dependencies."""
 
 import datetime
+import hashlib
+import pickle
 
 import pytest
 
@@ -168,3 +170,70 @@ def func():
         return 1
 
     assert func.cache_dpath() is None
+
+
+@pytest.mark.smoke
+def test_classmethod_not_guarded():
+    """Test that @classmethod (cls) does not trigger the instance method guard.
+
+    Note: Place ``@classmethod`` above ``@cachier`` so that ``@cachier``
+    decorates the underlying function before it is wrapped as a classmethod.
+    This way cachier sees ``cls`` (not ``self``) as the first parameter and
+    the instance-method guard is not triggered.
+
+    """
+
+    # A custom hash_func is needed because the default pickle-based
+    # hash function cannot serialise the local class object passed as
+    # ``cls``.
+    def _hash_ignore_cls(args, kwds):
+        filtered = {k: v for k, v in kwds.items() if k != "cls"}
+        return hashlib.sha256(pickle.dumps((args, sorted(filtered.items())))).hexdigest()
+
+    class Foo:
+        @classmethod
+        @cachier_decorator(backend="memory", hash_func=_hash_ignore_cls)
+        def method(cls, x):
+            return x + 1
+
+    Foo.method.clear_cache()
+    assert Foo.method(2) == 3
+    Foo.method.clear_cache()
+
+
+@pytest.mark.smoke
+def test_instance_method_global_opt_out_reset():
+    """Test that resetting allow_non_static_methods=False re-enables the guard."""
+    original = get_global_params().allow_non_static_methods
+    try:
+        set_global_params(allow_non_static_methods=True)
+        set_global_params(allow_non_static_methods=False)
+        with pytest.raises(TypeError, match="instance method"):
+
+            class Foo:
+                @cachier_decorator(backend="memory")
+                def method(self, x):
+                    return x
+    finally:
+        set_global_params(allow_non_static_methods=original)
+
+
+@pytest.mark.smoke
+def test_instance_method_skip_cache():
+    """Test that cachier__skip_cache=True works for methods with allow_non_static_methods."""
+    call_count = 0
+
+    class Foo:
+        @cachier_decorator(backend="memory", allow_non_static_methods=True)
+        def method(self, x):
+            nonlocal call_count
+            call_count += 1
+            return x * 2
+
+    obj = Foo()
+    obj.method.clear_cache()
+    assert obj.method(5) == 10
+    assert call_count == 1
+    assert obj.method(5, cachier__skip_cache=True) == 10
+    assert call_count == 2  # recalculated, not from cache
+    obj.method.clear_cache()