diff --git a/litellm/llms/zai/chat/transformation.py b/litellm/llms/zai/chat/transformation.py
index a3a85934275..d8571fa7132 100644
--- a/litellm/llms/zai/chat/transformation.py
+++ b/litellm/llms/zai/chat/transformation.py
@@ -68,14 +68,21 @@ class ZAIChatConfig(OpenAIGPTConfig):
         reasoning_params: Final = MappingProxyType(
             {k: v for k, v in non_default_params.items() if k in ZAI_REASONING_PARAMS and k in supported_openai_params}
         )
-        optional_params.update(
-            (k, v)
-            for k, v in non_default_params.items()
-            if k in supported_openai_params and k not in ZAI_REASONING_PARAMS
-        )
-        if reasoning_params:
-            optional_params["extra_body"] = {  # mutable-ok: the OpenAI SDK json-encodes extra_body from a plain dict
-                **(optional_params.get("extra_body") or MappingProxyType({})),
-                **reasoning_params,
+        passthrough_params: Final = MappingProxyType(
+            {
+                k: v
+                for k, v in non_default_params.items()
+                if k in supported_openai_params and k not in ZAI_REASONING_PARAMS
             }
-        return optional_params
+        )
+        if not reasoning_params:
+            return {**optional_params, **passthrough_params}  # mutable-ok: base class returns a dict
+        extra_body: Final = {  # mutable-ok: the OpenAI SDK json-encodes extra_body from a plain dict
+            **(optional_params.get("extra_body") or MappingProxyType({})),
+            **reasoning_params,
+        }
+        return {  # mutable-ok: base class returns a dict
+            **optional_params,
+            **passthrough_params,
+            "extra_body": extra_body,
+        }
diff --git a/tests/test_litellm/llms/zai/test_zai_provider.py b/tests/test_litellm/llms/zai/test_zai_provider.py
index e513aa5a450..2429e7c05c3 100644
--- a/tests/test_litellm/llms/zai/test_zai_provider.py
+++ b/tests/test_litellm/llms/zai/test_zai_provider.py
@@ -175,17 +175,25 @@ def test_thinking_and_reasoning_effort_move_into_extra_body(local_model_cost_map
     assert result["extra_body"] == {"thinking": {"type": "disabled"}, "reasoning_effort": "low"}
 
 
-def test_existing_extra_body_is_kept_and_not_mutated(local_model_cost_map):
+def test_caller_optional_params_and_extra_body_are_not_mutated(local_model_cost_map):
     from litellm.llms.zai.chat.transformation import ZAIChatConfig
 
     caller_extra_body = {"already_here": True}
+    caller_optional_params = {"stream": False, "extra_body": caller_extra_body}
     result = ZAIChatConfig()._map_openai_params(
-        non_default_params={"thinking": {"type": "disabled"}},
-        optional_params={"extra_body": caller_extra_body},
+        non_default_params={"max_tokens": 100, "thinking": {"type": "disabled"}},
+        optional_params=caller_optional_params,
         model="glm-4.7",
         drop_params=False,
     )
-    assert result["extra_body"] == {"already_here": True, "thinking": {"type": "disabled"}}
+    assert result == {
+        "stream": False,
+        "max_tokens": 100,
+        "extra_body": {"already_here": True, "thinking": {"type": "disabled"}},
+    }
+    assert result is not caller_optional_params
+    assert caller_optional_params == {"stream": False, "extra_body": {"already_here": True}}
+    assert caller_optional_params["extra_body"] is caller_extra_body
     assert caller_extra_body == {"already_here": True}