diff --git a/haystack/components/generators/chat/openai_responses.py b/haystack/components/generators/chat/openai_responses.py
index cf381c056ec..d0a804610bc 100644
--- a/haystack/components/generators/chat/openai_responses.py
+++ b/haystack/components/generators/chat/openai_responses.py
@@ -228,7 +228,7 @@ def _client_kwargs(self) -> dict[str, Any]:
 
     def _warm_up_tools(self) -> None:
         if not self._tools_warmed_up:
-            is_openai_tool = isinstance(self.tools, list) and isinstance(self.tools[0], dict)
+            is_openai_tool = isinstance(self.tools, list) and bool(self.tools) and isinstance(self.tools[0], dict)
             # We only warm up Haystack tools, not OpenAI/MCP tools
             # The type ignore is needed because mypy cannot infer the type correctly
             if not is_openai_tool:
@@ -539,7 +539,10 @@ def _prepare_api_call(  # noqa: PLR0913
                     function_spec = {**t.tool_spec}
                     if not tools_strict:
                         function_spec["strict"] = False
-                    function_spec["parameters"]["additionalProperties"] = False
+                    # Copy the parameters schema before editing it. tool_spec exposes
+                    # Tool.parameters by reference, so mutating it here would permanently alter
+                    # the user's Tool (and any other generator that shares the same Tool instance).
+                    function_spec["parameters"] = {**function_spec["parameters"], "additionalProperties": False}
                     tool_definitions.append({"type": "function", **function_spec})
 
             openai_tools = {"tools": tool_definitions}
diff --git a/releasenotes/notes/fix-openai-responses-tool-schema-mutation-e500ad2c85715f8c.yaml b/releasenotes/notes/fix-openai-responses-tool-schema-mutation-e500ad2c85715f8c.yaml
new file mode 100644
index 00000000000..966c99af279
--- /dev/null
+++ b/releasenotes/notes/fix-openai-responses-tool-schema-mutation-e500ad2c85715f8c.yaml
@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    ``OpenAIResponsesChatGenerator`` no longer mutates the ``parameters`` schema of the ``Tool``
+    objects passed to it. Previously every run wrote ``additionalProperties: False`` into the
+    user's live ``Tool.parameters``, silently altering the tool for any other generator that
+    shared the same ``Tool`` instance and making serialization round trips unstable.
+  - |
+    ``OpenAIResponsesChatGenerator`` no longer raises ``IndexError`` when it is warmed up with an
+    empty ``tools`` list.
diff --git a/test/components/generators/chat/test_openai_responses.py b/test/components/generators/chat/test_openai_responses.py
index 69855986d5c..6b09d719ca4 100644
--- a/test/components/generators/chat/test_openai_responses.py
+++ b/test/components/generators/chat/test_openai_responses.py
@@ -420,6 +420,14 @@ def test_warm_up_with_no_tools_does_not_raise(self, monkeypatch):
         generator.warm_up()
         assert generator._tools_warmed_up
 
+    def test_warm_up_with_empty_tools_list_does_not_raise(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "fake-api-key")
+        # An empty list is a valid ``tools`` value (e.g. when built programmatically); warming up
+        # must not index ``tools[0]`` unguarded.
+        generator = OpenAIResponsesChatGenerator(tools=[])
+        generator.warm_up()
+        assert generator._tools_warmed_up
+
     def test_warm_up_with_openai_tools_does_not_raise(self, monkeypatch):
         monkeypatch.setenv("OPENAI_API_KEY", "fake-api-key")
         generator = OpenAIResponsesChatGenerator(
@@ -493,6 +501,21 @@ def test_run_fail_with_duplicate_tool_names(self, monkeypatch, tools):
             component = OpenAIResponsesChatGenerator(tools=duplicate_tools)
             component.run(chat_messages)
 
+    def test_prepare_api_call_does_not_mutate_tool_parameters(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "test-api-key")
+        parameters = {"type": "object", "properties": {"city": {"type": "string"}}, "required": ["city"]}
+        tool = Tool(name="weather", description="get the weather", parameters=parameters, function=print)
+
+        generator = OpenAIResponsesChatGenerator(tools_strict=False)
+        api_args = generator._prepare_api_call(messages=[ChatMessage.from_user("hi")], tools=[tool])
+
+        # The tool's own schema must not be modified in place, otherwise the same Tool passed to
+        # another generator (or serialized) would carry an additionalProperties flag it never had.
+        assert "additionalProperties" not in tool.parameters
+        assert tool.parameters == {"type": "object", "properties": {"city": {"type": "string"}}, "required": ["city"]}
+        # ...while the payload actually sent to the API still carries additionalProperties=False.
+        assert api_args["tools"][0]["parameters"]["additionalProperties"] is False
+
     def test_run_with_wrong_model(self):
         mock_client = MagicMock()
         mock_client.responses.create.side_effect = OpenAIError("Invalid model name")