diff --git a/openfeature/_event_support.py b/openfeature/_event_support.py
index 3928be3e..41fc28f0 100644
--- a/openfeature/_event_support.py
+++ b/openfeature/_event_support.py
@@ -61,6 +61,13 @@ def add_client_handler(
         handlers = _client_handlers[client][event]
         handlers.append(handler)
 
+    # outside the lock intentionally: the immediate-fire status check acquires the registry lock, so calling it
+    # under _client_lock risks lock-order inversion against run_handlers_for_provider (registry lock → _client_lock).
+    # As a consequence, a narrow double-fire is possible: if dispatch_event(client's event) runs concurrently, it
+    # sets the matching provider status (enabling the immediate fire below) and then re-runs every handler for this
+    # client. If _run_immediate_handler lands after that status set but before dispatch snapshots the handler list,
+    # the handler fires twice — once here, once from dispatch. Only happens when the registered event matches the event
+    # being dispatched; otherwise the immediate fire is a no-op.
     _run_immediate_handler(client, event, handler)
 
 
@@ -78,6 +85,7 @@ def add_global_handler(event: ProviderEvent, handler: EventHandler) -> None:
 
     from openfeature.api import get_client  # noqa: PLC0415
 
+    # See comment in add_client_handler for why this runs outside the lock.
     _run_immediate_handler(get_client(), event, handler)
 
 
@@ -134,6 +142,6 @@ def _run_handler(handler: EventHandler, details: EventDetails) -> None:
 
 def clear() -> None:
     with _global_lock:
-        _global_handlers.clear()
-    with _client_lock:
-        _client_handlers.clear()
+        with _client_lock:
+            _global_handlers.clear()
+            _client_handlers.clear()
diff --git a/openfeature/api.py b/openfeature/api.py
index 4585e50e..80fa2434 100644
--- a/openfeature/api.py
+++ b/openfeature/api.py
@@ -62,7 +62,6 @@ def set_provider_and_wait(provider: FeatureProvider, domain: str | None = None)
 
 def clear_providers() -> None:
     provider_registry.clear_providers()
-    _event_support.clear()
 
 
 def get_provider_metadata(domain: str | None = None) -> Metadata:
diff --git a/openfeature/client.py b/openfeature/client.py
index 95dc5b6d..9bf7f513 100644
--- a/openfeature/client.py
+++ b/openfeature/client.py
@@ -1,4 +1,5 @@
 import logging
+import threading
 import typing
 from collections.abc import Awaitable, Mapping, Sequence
 from dataclasses import dataclass
@@ -86,6 +87,7 @@ def __init__(
         self.version = version
         self.context = context or EvaluationContext()
         self.hooks = hooks or []
+        self._hooks_lock = threading.Lock()
 
     @property
     def provider(self) -> FeatureProvider:
@@ -98,7 +100,10 @@ def get_metadata(self) -> ClientMetadata:
         return ClientMetadata(domain=self.domain)
 
     def add_hooks(self, hooks: list[Hook]) -> None:
-        self.hooks = self.hooks + hooks
+        # Guards the read-concat-store against a lost update; this practically never races under the default 5ms GIL
+        # switch interval, but is essential under a no-GIL build.
+        with self._hooks_lock:
+            self.hooks = self.hooks + hooks
 
     def get_boolean_value(
         self,
@@ -468,8 +473,9 @@ def _establish_hooks_and_provider(
 
     def _assert_provider_status(
         self,
+        provider: FeatureProvider,
     ) -> OpenFeatureError | None:
-        status = self.get_provider_status()
+        status = provider_registry.get_provider_status(provider)
         if status == ProviderStatus.NOT_READY:
             return ProviderNotReadyError()
         if status == ProviderStatus.FATAL:
@@ -589,7 +595,7 @@ async def evaluate_flag_details_async(
         )
 
         try:
-            if provider_err := self._assert_provider_status():
+            if provider_err := self._assert_provider_status(provider):
                 error_hooks(
                     flag_type,
                     provider_err,
@@ -765,7 +771,7 @@ def evaluate_flag_details(
         )
 
         try:
-            if provider_err := self._assert_provider_status():
+            if provider_err := self._assert_provider_status(provider):
                 error_hooks(
                     flag_type,
                     provider_err,
diff --git a/openfeature/hook/__init__.py b/openfeature/hook/__init__.py
index 247d316b..b0f142cc 100644
--- a/openfeature/hook/__init__.py
+++ b/openfeature/hook/__init__.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import threading
 import typing
 from collections.abc import Mapping, MutableMapping, Sequence
 from datetime import datetime
@@ -24,6 +25,7 @@
 ]
 
 _hooks: list[Hook] = []
+_hooks_lock = threading.Lock()
 
 
 # https://openfeature.dev/specification/sections/hooks/#requirement-461
@@ -150,15 +152,20 @@ def supports_flag_value_type(self, flag_type: FlagType) -> bool:
         """
         return True
 
+# while the lock guarantees safety, even without it there was never a loss within 50.000 runs (with the default GIL
+# switch interval of 5ms). only when the switch interval was significantly shortened to 0.1 microseconds, losses were
+# observed without locks every now and then. with a no-GIL python, the lock would be essential
 
 def add_hooks(hooks: list[Hook]) -> None:
-    global _hooks
-    _hooks = _hooks + hooks
+    with  _hooks_lock:
+        global _hooks
+        _hooks = _hooks + hooks
 
 
 def clear_hooks() -> None:
-    global _hooks
-    _hooks = []
+    with _hooks_lock:
+        global _hooks
+        _hooks = []
 
 
 def get_hooks() -> list[Hook]:
diff --git a/openfeature/provider/__init__.py b/openfeature/provider/__init__.py
index 1b2b5206..e02aab78 100644
--- a/openfeature/provider/__init__.py
+++ b/openfeature/provider/__init__.py
@@ -261,5 +261,6 @@ def emit_provider_stale(self, details: ProviderEventDetails) -> None:
         self.emit(ProviderEvent.PROVIDER_STALE, details)
 
     def emit(self, event: ProviderEvent, details: ProviderEventDetails) -> None:
-        if hasattr(self, "_on_emit"):
-            self._on_emit(self, event, details)
+        on_emit = getattr(self, "_on_emit", None)
+        if on_emit is not None:
+            on_emit(self, event, details)
diff --git a/openfeature/provider/_registry.py b/openfeature/provider/_registry.py
index e46caadd..63fa6ce3 100644
--- a/openfeature/provider/_registry.py
+++ b/openfeature/provider/_registry.py
@@ -1,6 +1,6 @@
 import threading
 
-from openfeature._event_support import run_handlers_for_provider
+from openfeature._event_support import run_handlers_for_provider, clear as clear_event_handlers
 from openfeature.evaluation_context import EvaluationContext, get_evaluation_context
 from openfeature.event import (
     ProviderEvent,
@@ -54,9 +54,16 @@ def set_provider(
             self._shutdown_if_unused(old_provider)
 
     def get_provider(self, domain: str | None) -> FeatureProvider:
-        if domain is None:
-            return self._default_provider
-        return self._providers.get(domain, self._default_provider)
+        # defensive lock under the GIL as the op is basically atomic
+        # but we might want to keep it so a provider that's about
+        # to be shut down isn't returned
+        # however it contributes to a potential deadlock that is currently
+        # still in place (clear_providers: registry's lock -> _event_support's lock;
+        # run_handlers_for_provider: _event_support's lock -> registry's lock)
+        with self._lock:
+            if domain is None:
+                return self._default_provider
+            return self._providers.get(domain, self._default_provider)
 
     def set_default_provider(
         self, provider: FeatureProvider, wait_for_init: bool = False
@@ -83,7 +90,8 @@ def set_default_provider(
             self._shutdown_if_unused(old_provider)
 
     def get_default_provider(self) -> FeatureProvider:
-        return self._default_provider
+        with self._lock:
+            return self._default_provider
 
     def clear_providers(self) -> None:
         self.shutdown()
@@ -93,11 +101,13 @@ def clear_providers(self) -> None:
             self._provider_status = {
                 self._default_provider: ProviderStatus.READY,
             }
+            clear_event_handlers()
 
     def shutdown(self) -> None:
         with self._lock:
             providers = {self._default_provider, *self._providers.values()}
 
+        # do we want to move this inside the lock? it allows a narrow double-shutdown window
         for provider in providers:
             self._shutdown_provider(provider)
 
@@ -214,7 +224,12 @@ def _shutdown_provider(
         provider.detach()
 
     def get_provider_status(self, provider: FeatureProvider) -> ProviderStatus:
-        return self._provider_status.get(provider, ProviderStatus.NOT_READY)
+        # defensive lock under the GIL as the op is basically atomic
+        # but we might want to keep it so a provider that's about
+        # to be shut down isn't returned
+        # however, removing it would enable moving _run_immediate_handler into the lock i think
+        with self._lock:
+            return self._provider_status.get(provider, ProviderStatus.NOT_READY)
 
     def dispatch_event(
         self,
diff --git a/openfeature/transaction_context/__init__.py b/openfeature/transaction_context/__init__.py
index 15ac7e01..e44f6135 100644
--- a/openfeature/transaction_context/__init__.py
+++ b/openfeature/transaction_context/__init__.py
@@ -1,3 +1,5 @@
+import threading
+
 from openfeature.evaluation_context import EvaluationContext
 from openfeature.transaction_context.context_var_transaction_context_propagator import (
     ContextVarsTransactionContextPropagator,
@@ -21,13 +23,15 @@
 _evaluation_transaction_context_propagator: TransactionContextPropagator = (
     NoOpTransactionContextPropagator()
 )
+_propagator_lock = threading.Lock()
 
 
 def set_transaction_context_propagator(
     transaction_context_propagator: TransactionContextPropagator,
 ) -> None:
     global _evaluation_transaction_context_propagator
-    _evaluation_transaction_context_propagator = transaction_context_propagator
+    with _propagator_lock:
+        _evaluation_transaction_context_propagator = transaction_context_propagator
 
 
 def clear_transaction_context_propagator() -> None:
@@ -35,11 +39,12 @@ def clear_transaction_context_propagator() -> None:
 
 
 def get_transaction_context() -> EvaluationContext:
-    return _evaluation_transaction_context_propagator.get_transaction_context()
+    with _propagator_lock:
+        propagator = _evaluation_transaction_context_propagator
+        return propagator.get_transaction_context()
 
 
 def set_transaction_context(evaluation_context: EvaluationContext) -> None:
-    global _evaluation_transaction_context_propagator
-    _evaluation_transaction_context_propagator.set_transaction_context(
-        evaluation_context
-    )
+    with _propagator_lock:
+        propagator = _evaluation_transaction_context_propagator
+        propagator.set_transaction_context(evaluation_context)
diff --git a/tests/test_client.py b/tests/test_client.py
index 44b49e5f..64598936 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -4,11 +4,12 @@
 import types
 import uuid
 from concurrent.futures import ThreadPoolExecutor
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, Mock
 
 import pytest
 
 from openfeature import _event_support, api
+from openfeature import client as client_module
 from openfeature.api import (
     add_hooks,
     clear_hooks,
@@ -20,7 +21,7 @@
 from openfeature.client import OpenFeatureClient, _typecheck_flag_value
 from openfeature.evaluation_context import EvaluationContext
 from openfeature.event import EventDetails, ProviderEvent, ProviderEventDetails
-from openfeature.exception import ErrorCode, OpenFeatureError
+from openfeature.exception import ErrorCode, OpenFeatureError, ProviderFatalError
 from openfeature.flag_evaluation import FlagResolutionDetails, FlagType, Reason
 from openfeature.hook import Hook
 from openfeature.provider import FeatureProvider, ProviderStatus
@@ -291,9 +292,10 @@ def test_provider_should_return_error_status_if_failed():
 async def test_should_shortcircuit_if_provider_is_not_ready(
     no_op_provider_client, monkeypatch
 ):
-    # Given
     monkeypatch.setattr(
-        no_op_provider_client, "get_provider_status", lambda: ProviderStatus.NOT_READY
+        provider_registry,
+        "get_provider_status",
+        lambda provider: ProviderStatus.NOT_READY,
     )
     spy_hook = MagicMock(spec=Hook)
     no_op_provider_client.add_hooks([spy_hook])
@@ -321,9 +323,10 @@ async def test_should_shortcircuit_if_provider_is_not_ready(
 async def test_should_shortcircuit_if_provider_is_in_irrecoverable_error_state(
     no_op_provider_client, monkeypatch
 ):
-    # Given
     monkeypatch.setattr(
-        no_op_provider_client, "get_provider_status", lambda: ProviderStatus.FATAL
+        provider_registry,
+        "get_provider_status",
+        lambda provider: ProviderStatus.FATAL,
     )
     spy_hook = MagicMock(spec=Hook)
     no_op_provider_client.add_hooks([spy_hook])
@@ -768,3 +771,32 @@ def test_should_noop_if_provider_does_not_support_tracking(monkeypatch):
     set_provider(provider)
     client = get_client()
     client.track(tracking_event_name="test")
+
+
+def test_assert_provider_status_uses_passed_provider_not_current_registry_state():
+    fatal_provider = NoOpProvider()
+    ready_provider = NoOpProvider()
+
+    registry_mock = Mock()
+    registry_mock.get_provider_status.side_effect = lambda p: (
+        ProviderStatus.FATAL if p is fatal_provider else ProviderStatus.READY
+    )
+    registry_mock.get_provider.return_value = ready_provider
+
+    original = client_module.provider_registry
+    client_module.provider_registry = registry_mock
+    try:
+        c = OpenFeatureClient(domain=None, version=None)
+        assert c.provider is ready_provider, (
+            "test setup: self.provider should resolve via the patched registry"
+        )
+
+        err = c._assert_provider_status(fatal_provider)
+        assert isinstance(err, ProviderFatalError), (
+            "status check used self.provider (READY) instead of the captured "
+            "fatal_provider — TOCTOU regression"
+        )
+        registry_mock.get_provider_status.assert_any_call(fatal_provider)
+        assert c._assert_provider_status(ready_provider) is None
+    finally:
+        client_module.provider_registry = original