diff --git a/CHANGELOG.md b/CHANGELOG.md
index 41be77c..3bc8488 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,7 +22,7 @@ These breaking changes apply to Python JSONPath in its default configuration. We
 - Added the [Keys filter selector](https://jg-rp.github.io/python-jsonpath/syntax/#keys-filter-selector).
 - Added the [Singular query selector](https://jg-rp.github.io/python-jsonpath/syntax/#singular-query-selector).
 - We now use the [regex] package, if available, instead of `re` for match and search function extensions. See [optional dependencies](https://jg-rp.github.io/python-jsonpath/#optional-dependencies).
-- Added the `strict` argument to all [convenience functions](https://jg-rp.github.io/python-jsonpath/convenience/), the CLI and the `JSONPathEnvironment` constructor. When `strict=True`, all extensions to RFC 9535 and any lax parsing rules will be disabled.
+- Added the `strict` argument to all [convenience functions](https://jg-rp.github.io/python-jsonpath/convenience/), the CLI and the `JSONPathEnvironment` constructor. When `strict=True`, all extensions to RFC 9535, any non-standard function extensions and any lax parsing rules will be disabled.
 - Added class variable `JSONPathEnvironment.max_recursion_depth` to control the maximum recursion depth of descendant segments.
 - Added pretty exception messages.
 
diff --git a/docs/functions.md b/docs/functions.md
index 4ddf99f..72eb5f9 100644
--- a/docs/functions.md
+++ b/docs/functions.md
@@ -2,6 +2,10 @@
 
 A filter function is a named function that can be called as part of a [filter selector](syntax.md#filter-selector). Here we describe built in filters. You can [define your own function extensions](advanced.md#function-extensions) too.
 
+!!! note
+
+    If you pass `strict=True` when calling [`findall()`](convenience.md#jsonpath.findall), [`finditer()`](convenience.md#jsonpath.finditer), etc., Only standard functions - those defined by RFC 9535 - will be enabled. The standard functions are `count`, `length`, `match`, `search` and `value`.
+
 ## `count()`
 
 ```text
diff --git a/jsonpath/env.py b/jsonpath/env.py
index ee674ed..048b050 100644
--- a/jsonpath/env.py
+++ b/jsonpath/env.py
@@ -450,11 +450,13 @@ def setup_function_extensions(self) -> None:
         self.function_extensions["match"] = function_extensions.Match()
         self.function_extensions["search"] = function_extensions.Search()
         self.function_extensions["value"] = function_extensions.Value()
-        self.function_extensions["isinstance"] = function_extensions.IsInstance()
-        self.function_extensions["is"] = self.function_extensions["isinstance"]
-        self.function_extensions["typeof"] = function_extensions.TypeOf()
-        self.function_extensions["type"] = self.function_extensions["typeof"]
-        self.function_extensions["startswith"] = function_extensions.StartsWith()
+
+        if not self.strict:
+            self.function_extensions["isinstance"] = function_extensions.IsInstance()
+            self.function_extensions["is"] = self.function_extensions["isinstance"]
+            self.function_extensions["typeof"] = function_extensions.TypeOf()
+            self.function_extensions["type"] = self.function_extensions["typeof"]
+            self.function_extensions["startswith"] = function_extensions.StartsWith()
 
     def validate_function_extension_signature(
         self, token: Token, args: List[Any]
diff --git a/tests/test_strictness.py b/tests/test_strictness.py
index d4fe39a..5344b1e 100644
--- a/tests/test_strictness.py
+++ b/tests/test_strictness.py
@@ -1,6 +1,7 @@
 import pytest
 
 from jsonpath import JSONPathEnvironment
+from jsonpath import JSONPathNameError
 
 
 @pytest.fixture()
@@ -73,3 +74,34 @@ def test_singular_path_selector_without_root_identifier(
     }
 
     assert env.findall(query, data) == [{"q": [4, 5, 6]}]
+
+
+def test_isinstance_is_disabled_in_strict_mode() -> None:
+    env = JSONPathEnvironment(strict=True)
+
+    query = "$.some[?is(@.thing, 'string')]"
+    with pytest.raises(JSONPathNameError):
+        env.compile(query)
+
+    query = "$.some[?isinstance(@.thing, 'string')]"
+    with pytest.raises(JSONPathNameError):
+        env.compile(query)
+
+
+def test_typeof_is_disabled_in_strict_mode() -> None:
+    env = JSONPathEnvironment(strict=True)
+
+    query = "$.some[?type(@.thing) == 'string']"
+    with pytest.raises(JSONPathNameError):
+        env.compile(query)
+
+    query = "$.some[?typeof(@.thing) == 'string']"
+    with pytest.raises(JSONPathNameError):
+        env.compile(query)
+
+
+def test_startswith_is_disabled_in_strict_mode() -> None:
+    env = JSONPathEnvironment(strict=True)
+    query = "$[?startswith(@, 'ab')]"
+    with pytest.raises(JSONPathNameError):
+        env.compile(query)