diff --git a/src/fastmcp/utilities/json_schema.py b/src/fastmcp/utilities/json_schema.py
index b33c4f945..75320e1a3 100644
--- a/src/fastmcp/utilities/json_schema.py
+++ b/src/fastmcp/utilities/json_schema.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import copy
 from collections import defaultdict
 from typing import Any
 
@@ -287,6 +288,7 @@ def _prune_param(schema: dict[str, Any], param: str) -> dict[str, Any]:
     """Return a new schema with *param* removed from `properties`, `required`,
     and (if no longer referenced) `$defs`.
     """
+    schema = copy.deepcopy(schema)
 
     # ── 1. drop from properties/required ──────────────────────────────
     props = schema.get("properties", {})
diff --git a/src/fastmcp/utilities/openapi/json_schema_converter.py b/src/fastmcp/utilities/openapi/json_schema_converter.py
index 5d0e02eed..e044d5bf9 100644
--- a/src/fastmcp/utilities/openapi/json_schema_converter.py
+++ b/src/fastmcp/utilities/openapi/json_schema_converter.py
@@ -173,7 +173,7 @@ def _convert_nullable_field(schema: dict[str, Any]) -> dict[str, Any]:
     elif "anyOf" in result:
         # Add null to anyOf if not present
         if not any(item.get("type") == "null" for item in result["anyOf"]):
-            result["anyOf"].append({"type": "null"})
+            result["anyOf"] = [*result["anyOf"], {"type": "null"}]
     elif "allOf" in result:
         # Wrap allOf in anyOf with null option
         result["anyOf"] = [{"allOf": result.pop("allOf")}, {"type": "null"}]
diff --git a/tests/utilities/openapi/test_nullable_fields.py b/tests/utilities/openapi/test_nullable_fields.py
index 2331fc3ed..ff1881e46 100644
--- a/tests/utilities/openapi/test_nullable_fields.py
+++ b/tests/utilities/openapi/test_nullable_fields.py
@@ -8,6 +8,7 @@ from fastmcp import FastMCP
 from fastmcp.client import Client
 from fastmcp.server.providers.openapi import OpenAPIProvider
 from fastmcp.utilities.openapi.json_schema_converter import (
+    _convert_nullable_field,
     convert_openapi_schema_to_json_schema,
 )
 
@@ -556,3 +557,14 @@ class TestNullableInputSchemaIntegration:
 
                 assert "nullable" not in bio_prop
                 assert bio_prop["type"] == ["string", "null"]
+
+
+class TestConvertNullableFieldMutation:
+    """Test that _convert_nullable_field does not mutate its input."""
+
+    def test_does_not_mutate_anyof_list(self):
+        """_convert_nullable_field should not append to the original anyOf list."""
+        schema = {"anyOf": [{"type": "string"}], "nullable": True}
+        original_len = len(schema["anyOf"])
+        _convert_nullable_field(schema)
+        assert len(schema["anyOf"]) == original_len
diff --git a/tests/utilities/test_json_schema.py b/tests/utilities/test_json_schema.py
index 644366057..5c5f4c860 100644
--- a/tests/utilities/test_json_schema.py
+++ b/tests/utilities/test_json_schema.py
@@ -1,3 +1,4 @@
+import copy
 from unittest.mock import patch
 
 from jsonref import replace_refs
@@ -51,6 +52,17 @@ class TestPruneParam:
         result = _prune_param(schema, "foo")
         assert "required" not in result
 
+    def test_does_not_mutate_input(self):
+        """Test that _prune_param does not mutate the original schema."""
+        schema = {
+            "type": "object",
+            "properties": {"a": {"type": "string"}, "b": {"type": "integer"}},
+            "required": ["a", "b"],
+        }
+        original = copy.deepcopy(schema)
+        _prune_param(schema, "a")
+        assert schema == original
+
 
 class TestDereferenceRefs:
     """Tests for the dereference_refs function."""