diff --git a/examples/docker_repl_example.py b/examples/docker_repl_example.py
index cb00adf..f8c1c5d 100644
--- a/examples/docker_repl_example.py
+++ b/examples/docker_repl_example.py
@@ -18,11 +18,11 @@ class MockLM(BaseLM):
     def __init__(self):
         super().__init__(model_name="mock")
 
-    def completion(self, prompt):
+    def completion(self, prompt, model=None, **kwargs):
         return f"Mock: {str(prompt)[:50]}"
 
-    async def acompletion(self, prompt):
-        return self.completion(prompt)
+    async def acompletion(self, prompt, model=None, **kwargs):
+        return self.completion(prompt, model=model, **kwargs)
 
     def get_usage_summary(self):
         return UsageSummary({"mock": ModelUsageSummary(1, 10, 10)})
diff --git a/examples/modal_repl_example.py b/examples/modal_repl_example.py
index 2b28da1..830a129 100644
--- a/examples/modal_repl_example.py
+++ b/examples/modal_repl_example.py
@@ -16,10 +16,10 @@ class MockLM(BaseLM):
     def __init__(self):
         super().__init__(model_name="mock-model")
 
-    def completion(self, prompt):
+    def completion(self, prompt, model=None, **kwargs):
         return f"Mock response to: {prompt[:50]}"
 
-    async def acompletion(self, prompt):
+    async def acompletion(self, prompt, model=None, **kwargs):
         return self.completion(prompt)
 
     def get_usage_summary(self):
diff --git a/rlm/clients/base_lm.py b/rlm/clients/base_lm.py
index d742650..20c8c2b 100644
--- a/rlm/clients/base_lm.py
+++ b/rlm/clients/base_lm.py
@@ -15,11 +15,11 @@ def __init__(self, model_name: str, **kwargs):
         self.kwargs = kwargs
 
     @abstractmethod
-    def completion(self, prompt: str | dict[str, Any]) -> str:
+    def completion(self, prompt: str | dict[str, Any], model: str | None = None, **kwargs) -> str:
         raise NotImplementedError
 
     @abstractmethod
-    async def acompletion(self, prompt: str | dict[str, Any]) -> str:
+    async def acompletion(self, prompt: str | dict[str, Any], model: str | None = None, **kwargs) -> str:
         raise NotImplementedError
 
     @abstractmethod
diff --git a/rlm/clients/openai.py b/rlm/clients/openai.py
index d5fa999..eb10a0a 100644
--- a/rlm/clients/openai.py
+++ b/rlm/clients/openai.py
@@ -46,7 +46,9 @@ def __init__(
         self.model_output_tokens: dict[str, int] = defaultdict(int)
         self.model_total_tokens: dict[str, int] = defaultdict(int)
 
-    def completion(self, prompt: str | list[dict[str, Any]], model: str | None = None) -> str:
+    def completion(
+        self, prompt: str | list[dict[str, Any]], model: str | None = None, **kwargs
+    ) -> str:
         if isinstance(prompt, str):
             messages = [{"role": "user", "content": prompt}]
         elif isinstance(prompt, list) and all(isinstance(item, dict) for item in prompt):
@@ -58,12 +60,13 @@ def completion(self, prompt: str | list[dict[str, Any]], model: str | None = Non
         if not model:
             raise ValueError("Model name is required for OpenAI client.")
 
-        response = self.client.chat.completions.create(model=model, messages=messages)
+        api_kwargs = {"model": model, "messages": messages, **kwargs}
+        response = self.client.chat.completions.create(**api_kwargs)
         self._track_cost(response, model)
         return response.choices[0].message.content
 
     async def acompletion(
-        self, prompt: str | list[dict[str, Any]], model: str | None = None
+        self, prompt: str | list[dict[str, Any]], model: str | None = None, **kwargs
     ) -> str:
         if isinstance(prompt, str):
             messages = [{"role": "user", "content": prompt}]
@@ -76,19 +79,26 @@ async def acompletion(
         if not model:
             raise ValueError("Model name is required for OpenAI client.")
 
-        response = await self.async_client.chat.completions.create(model=model, messages=messages)
+        api_kwargs = {"model": model, "messages": messages, **kwargs}
+        response = await self.async_client.chat.completions.create(**api_kwargs)
         self._track_cost(response, model)
         return response.choices[0].message.content
 
     def _track_cost(self, response: openai.ChatCompletion, model: str):
         self.model_call_counts[model] += 1
-        self.model_input_tokens[model] += response.usage.prompt_tokens
-        self.model_output_tokens[model] += response.usage.completion_tokens
-        self.model_total_tokens[model] += response.usage.total_tokens
+
+        usage = getattr(response, "usage", None)
+        prompt_tokens = getattr(usage, "prompt_tokens", 0) if usage else 0
+        completion_tokens = getattr(usage, "completion_tokens", 0) if usage else 0
+        total_tokens = getattr(usage, "total_tokens", 0) if usage else 0
+
+        self.model_input_tokens[model] += prompt_tokens
+        self.model_output_tokens[model] += completion_tokens
+        self.model_total_tokens[model] += total_tokens
 
         # Track last call for handler to read
-        self.last_prompt_tokens = response.usage.prompt_tokens
-        self.last_completion_tokens = response.usage.completion_tokens
+        self.last_prompt_tokens = prompt_tokens
+        self.last_completion_tokens = completion_tokens
 
     def get_usage_summary(self) -> UsageSummary:
         model_summaries = {}
diff --git a/rlm/core/comms_utils.py b/rlm/core/comms_utils.py
index 07e92ae..c9cfbc1 100644
--- a/rlm/core/comms_utils.py
+++ b/rlm/core/comms_utils.py
@@ -28,6 +28,7 @@ class LMRequest:
     prompt: str | dict[str, Any] | None = None
     prompts: list[str | dict[str, Any]] | None = None
     model: str | None = None
+    kwargs: dict[str, Any] | None = None
 
     @property
     def is_batched(self) -> bool:
@@ -43,6 +44,8 @@ def to_dict(self) -> dict:
             d["prompts"] = self.prompts
         if self.model is not None:
             d["model"] = self.model
+        if self.kwargs is not None:
+            d["kwargs"] = self.kwargs
         return d
 
     @classmethod
@@ -52,6 +55,7 @@ def from_dict(cls, data: dict) -> "LMRequest":
             prompt=data.get("prompt"),
             prompts=data.get("prompts"),
             model=data.get("model"),
+            kwargs=data.get("kwargs"),
         )
 
 
@@ -220,6 +224,7 @@ def send_lm_request_batched(
     address: tuple[str, int],
     prompts: list[str | dict[str, Any]],
     model: str | None = None,
+    kwargs: dict[str, Any] | None = None,
     timeout: int = 300,
 ) -> list[LMResponse]:
     """Send a batched LM request and return a list of typed responses.
@@ -228,13 +233,14 @@ def send_lm_request_batched(
         address: (host, port) tuple of LM Handler server.
         prompts: List of prompts to send.
         model: Optional model name to use.
+        kwargs: Optional kwargs to pass to completion calls.
         timeout: Socket timeout in seconds.
 
     Returns:
         List of LMResponse objects, one per prompt, in the same order.
     """
     try:
-        request = LMRequest(prompts=prompts, model=model)
+        request = LMRequest(prompts=prompts, model=model, kwargs=kwargs)
         response_data = socket_request(address, request.to_dict(), timeout)
         response = LMResponse.from_dict(response_data)
 
diff --git a/rlm/core/lm_handler.py b/rlm/core/lm_handler.py
index 36d27b3..2dfe68e 100644
--- a/rlm/core/lm_handler.py
+++ b/rlm/core/lm_handler.py
@@ -48,7 +48,8 @@ def _handle_single(self, request: LMRequest, handler: "LMHandler") -> LMResponse
         client = handler.get_client(request.model)
 
         start_time = time.perf_counter()
-        content = client.completion(request.prompt)
+        kwargs = request.kwargs or {}
+        content = client.completion(request.prompt, model=request.model, **kwargs)
         end_time = time.perf_counter()
 
         usage_summary = client.get_last_usage()
@@ -67,9 +68,13 @@ def _handle_batched(self, request: LMRequest, handler: "LMHandler") -> LMRespons
         client = handler.get_client(request.model)
 
         start_time = time.perf_counter()
+        kwargs = request.kwargs or {}
 
         async def run_all():
-            tasks = [client.acompletion(prompt) for prompt in request.prompts]
+            tasks = [
+                client.acompletion(prompt, model=request.model, **kwargs)
+                for prompt in request.prompts
+            ]
             return await asyncio.gather(*tasks)
 
         results = asyncio.run(run_all())
@@ -164,9 +169,9 @@ def stop(self):
             self._server = None
             self._thread = None
 
-    def completion(self, prompt: str, model: str | None = None) -> str:
+    def completion(self, prompt: str, model: str | None = None, **kwargs) -> str:
         """Direct completion call (for main process use)."""
-        return self.get_client(model).completion(prompt)
+        return self.get_client(model).completion(prompt, model=model, **kwargs)
 
     def __enter__(self):
         self.start()
diff --git a/rlm/core/rlm.py b/rlm/core/rlm.py
index 80a3792..20690b8 100644
--- a/rlm/core/rlm.py
+++ b/rlm/core/rlm.py
@@ -151,7 +151,7 @@ def _setup_prompt(self, prompt: str | dict[str, Any]) -> list[dict[str, Any]]:
         return message_history
 
     def completion(
-        self, prompt: str | dict[str, Any], root_prompt: str | None = None
+        self, prompt: str | dict[str, Any], root_prompt: str | None = None, **kwargs
     ) -> RLMChatCompletion:
         """
         Recursive Language Model completion call. This is the main entry point for querying an RLM, and
@@ -163,6 +163,7 @@ def completion(
             prompt: A single string or dictionary of messages to pass as context to the model.
             root_prompt: We allow the RLM's root LM to see a (small) prompt that the user specifies. A common example of this
             is if the user is asking the RLM to answer a question, we can pass the question as the root prompt.
+            **kwargs: Optional kwargs to pass to root LM completion calls (e.g., max_tokens, temperature).
         Returns:
             A final answer as a string.
         """
@@ -170,7 +171,7 @@ def completion(
 
         # If we're at max depth, the RLM is an LM, so we fallback to the regular LM.
         if self.depth >= self.max_depth:
-            return self._fallback_answer(prompt)
+            return self._fallback_answer(prompt, **kwargs)
 
         with self._spawn_completion_context(prompt) as (lm_handler, environment):
             message_history = self._setup_prompt(prompt)
@@ -183,6 +184,7 @@ def completion(
                     prompt=current_prompt,
                     lm_handler=lm_handler,
                     environment=environment,
+                    **kwargs,
                 )
 
                 # Check if RLM is done and has a final answer.
@@ -219,7 +221,7 @@ def completion(
 
             # Default behavior: we run out of iterations, provide one final answer
             time_end = time.perf_counter()
-            final_answer = self._default_answer(message_history, lm_handler)
+            final_answer = self._default_answer(message_history, lm_handler, **kwargs)
             usage = lm_handler.get_usage_summary()
             self.verbose.print_final_answer(final_answer)
             self.verbose.print_summary(self.max_iterations, time_end - time_start, usage.to_dict())
@@ -238,13 +240,14 @@ def _completion_turn(
         prompt: str | dict[str, Any],
         lm_handler: LMHandler,
         environment: BaseEnv,
+        **kwargs,
     ) -> RLMIteration:
         """
         Perform a single iteration of the RLM, including prompting the model
         and code execution + tool execution.
         """
         iter_start = time.perf_counter()
-        response = lm_handler.completion(prompt)
+        response = lm_handler.completion(prompt, **kwargs)
         code_block_strs = find_code_blocks(response)
         code_blocks = []
 
@@ -260,7 +263,9 @@ def _completion_turn(
             iteration_time=iteration_time,
         )
 
-    def _default_answer(self, message_history: list[dict[str, Any]], lm_handler: LMHandler) -> str:
+    def _default_answer(
+        self, message_history: list[dict[str, Any]], lm_handler: LMHandler, **kwargs
+    ) -> str:
         """
         Default behavior if the RLM runs out of iterations and does not find a final answer.
         It will take the message history, and try to generate a final answer from it.
@@ -271,7 +276,7 @@ def _default_answer(self, message_history: list[dict[str, Any]], lm_handler: LMH
                 "content": "Please provide a final answer to the user's question based on the information provided.",
             }
         ]
-        response = lm_handler.completion(current_prompt)
+        response = lm_handler.completion(current_prompt, **kwargs)
 
         if self.logger:
             self.logger.log(
@@ -285,10 +290,10 @@ def _default_answer(self, message_history: list[dict[str, Any]], lm_handler: LMH
 
         return response
 
-    def _fallback_answer(self, message: str | dict[str, Any]) -> str:
+    def _fallback_answer(self, message: str | dict[str, Any], **kwargs) -> str:
         """
         Fallback behavior if the RLM is actually at max depth, and should be treated as an LM.
         """
         client: BaseLM = get_client(self.backend, self.backend_kwargs)
-        response = client.completion(message)
+        response = client.completion(message, **kwargs)
         return response
diff --git a/rlm/environments/docker_repl.py b/rlm/environments/docker_repl.py
index 6dd8c00..3299bb5 100644
--- a/rlm/environments/docker_repl.py
+++ b/rlm/environments/docker_repl.py
@@ -55,7 +55,9 @@ def _handle_single(self, body: dict) -> dict:
         if not self.lm_handler_address:
             return {"error": "No LM handler configured"}
 
-        request = LMRequest(prompt=body.get("prompt"), model=body.get("model"))
+        request = LMRequest(
+            prompt=body.get("prompt"), model=body.get("model"), kwargs=body.get("kwargs")
+        )
         response = send_lm_request(self.lm_handler_address, request)
 
         if not response.success:
@@ -72,7 +74,10 @@ def _handle_batched(self, body: dict) -> dict:
 
         prompts = body.get("prompts", [])
         responses = send_lm_request_batched(
-            self.lm_handler_address, prompts, model=body.get("model")
+            self.lm_handler_address,
+            prompts,
+            model=body.get("model"),
+            kwargs=body.get("kwargs"),
         )
 
         results = []
@@ -102,17 +107,23 @@ def _build_exec_script(code: str, proxy_port: int) -> str:
 PROXY = "http://host.docker.internal:{proxy_port}"
 STATE = "/workspace/state.dill"
 
-def llm_query(prompt, model=None):
+def llm_query(prompt, model=None, **kwargs):
     try:
-        r = requests.post(f"{{PROXY}}/llm_query", json={{"prompt": prompt, "model": model}}, timeout=300)
+        payload = {{"prompt": prompt, "model": model}}
+        if kwargs:
+            payload["kwargs"] = kwargs
+        r = requests.post(f"{{PROXY}}/llm_query", json=payload, timeout=300)
         d = r.json()
         return d.get("response") or f"Error: {{d.get('error')}}"
     except Exception as e:
         return f"Error: {{e}}"
 
-def llm_query_batched(prompts, model=None):
+def llm_query_batched(prompts, model=None, **kwargs):
     try:
-        r = requests.post(f"{{PROXY}}/llm_query_batched", json={{"prompts": prompts, "model": model}}, timeout=300)
+        payload = {{"prompts": prompts, "model": model}}
+        if kwargs:
+            payload["kwargs"] = kwargs
+        r = requests.post(f"{{PROXY}}/llm_query_batched", json=payload, timeout=300)
         d = r.json()
         return d.get("responses") or [f"Error: {{d.get('error')}}"] * len(prompts)
     except Exception as e:
diff --git a/rlm/environments/local_repl.py b/rlm/environments/local_repl.py
index b818380..60f3bd3 100644
--- a/rlm/environments/local_repl.py
+++ b/rlm/environments/local_repl.py
@@ -166,18 +166,19 @@ def _final_var(self, variable_name: str) -> str:
             return str(self.locals[variable_name])
         return f"Error: Variable '{variable_name}' not found"
 
-    def _llm_query(self, prompt: str, model: str | None = None) -> str:
+    def _llm_query(self, prompt: str, model: str | None = None, **kwargs) -> str:
         """Query the LM via socket connection to the handler.
 
         Args:
             prompt: The prompt to send to the LM.
             model: Optional model name to use (if handler has multiple clients).
+            **kwargs: Optional kwargs to pass to the completion call (e.g., max_tokens, temperature).
         """
         if not self.lm_handler_address:
             return "Error: No LM handler configured"
 
         try:
-            request = LMRequest(prompt=prompt, model=model)
+            request = LMRequest(prompt=prompt, model=model, kwargs=kwargs if kwargs else None)
             response = send_lm_request(self.lm_handler_address, request)
 
             if not response.success:
@@ -192,12 +193,13 @@ def _llm_query(self, prompt: str, model: str | None = None) -> str:
         except Exception as e:
             return f"Error: LM query failed - {e}"
 
-    def _llm_query_batched(self, prompts: list[str], model: str | None = None) -> list[str]:
+    def _llm_query_batched(self, prompts: list[str], model: str | None = None, **kwargs) -> list[str]:
         """Query the LM with multiple prompts concurrently.
 
         Args:
             prompts: List of prompts to send to the LM.
             model: Optional model name to use (if handler has multiple clients).
+            **kwargs: Optional kwargs to pass to the completion calls (e.g., max_tokens, temperature).
 
         Returns:
             List of responses in the same order as input prompts.
@@ -206,7 +208,7 @@ def _llm_query_batched(self, prompts: list[str], model: str | None = None) -> li
             return ["Error: No LM handler configured"] * len(prompts)
 
         try:
-            responses = send_lm_request_batched(self.lm_handler_address, prompts, model=model)
+            responses = send_lm_request_batched(self.lm_handler_address, prompts, model=model, kwargs=kwargs if kwargs else None)
 
             results = []
             for response in responses:
diff --git a/rlm/environments/modal_repl.py b/rlm/environments/modal_repl.py
index 2acfed1..e4d02f4 100644
--- a/rlm/environments/modal_repl.py
+++ b/rlm/environments/modal_repl.py
@@ -166,12 +166,15 @@ def _build_exec_script(code: str, broker_port: int = 8080) -> str:
 
 BROKER_URL = "http://127.0.0.1:{broker_port}"
 
-def llm_query(prompt, model=None):
+def llm_query(prompt, model=None, **kwargs):
     """Query the LM via the broker."""
     try:
+        payload = {{"type": "single", "prompt": prompt, "model": model}}
+        if kwargs:
+            payload["kwargs"] = kwargs
         response = requests.post(
             f"{{BROKER_URL}}/enqueue",
-            json={{"type": "single", "prompt": prompt, "model": model}},
+            json=payload,
             timeout=300,
         )
         data = response.json()
@@ -182,12 +185,15 @@ def llm_query(prompt, model=None):
         return f"Error: LM query failed - {{e}}"
 
 
-def llm_query_batched(prompts, model=None):
+def llm_query_batched(prompts, model=None, **kwargs):
     """Query the LM with multiple prompts."""
     try:
+        payload = {{"type": "batched", "prompts": prompts, "model": model}}
+        if kwargs:
+            payload["kwargs"] = kwargs
         response = requests.post(
             f"{{BROKER_URL}}/enqueue",
-            json={{"type": "batched", "prompts": prompts, "model": model}},
+            json=payload,
             timeout=300,
         )
         data = response.json()
@@ -405,10 +411,11 @@ def _handle_llm_request(self, req_data: dict) -> dict:
         """Handle an LLM request from the sandbox."""
         req_type = req_data.get("type")
         model = req_data.get("model")
+        kwargs = req_data.get("kwargs")
 
         if req_type == "single":
             prompt = req_data.get("prompt")
-            request = LMRequest(prompt=prompt, model=model)
+            request = LMRequest(prompt=prompt, model=model, kwargs=kwargs)
             response = send_lm_request(self.lm_handler_address, request)
 
             if not response.success:
@@ -422,7 +429,9 @@ def _handle_llm_request(self, req_data: dict) -> dict:
 
         elif req_type == "batched":
             prompts = req_data.get("prompts", [])
-            responses = send_lm_request_batched(self.lm_handler_address, prompts, model=model)
+            responses = send_lm_request_batched(
+                self.lm_handler_address, prompts, model=model, kwargs=kwargs
+            )
 
             results = []
             for resp in responses:
diff --git a/tests/clients/_openai.py b/tests/clients/_openai.py
new file mode 100644
index 0000000..f5e1ba6
--- /dev/null
+++ b/tests/clients/_openai.py
@@ -0,0 +1,63 @@
+"""Basic tests for OpenAIClient."""
+
+import os
+
+from dotenv import load_dotenv
+
+from rlm.clients.openai import OpenAIClient
+
+load_dotenv()
+
+
+def test_openai_completion_with_kwargs():
+    """Test that kwargs are passed through to OpenAI API."""
+    api_key = os.environ.get("OPENAI_API_KEY")
+    base_url = os.environ.get("OPENAI_BASE_URL")
+    model_name = "openai/gpt-5.2"
+
+    if not api_key:
+        print("Skipping test: OPENAI_API_KEY not set")
+        return
+
+    client = OpenAIClient(api_key=api_key, model_name=model_name, base_url=base_url)
+    prompt = "What is the capital of France?"
+
+    try:
+        # Test with kwargs
+        sample_kwargs = {"reasoning_effort": "high", "extra_body": {"usage": {"include": True}}}
+        result = client.completion(prompt, **sample_kwargs)
+        print(f"OpenAI response with {sample_kwargs=}:\n{result}")
+        assert result is not None
+        assert len(result) > 0
+    except Exception as e:
+        print(f"OpenAIClient error: {e}")
+        raise
+
+
+def test_openai_completion_without_kwargs():
+    """Test that completion works without kwargs."""
+    api_key = os.environ.get("OPENAI_API_KEY")
+    base_url = os.environ.get("OPENAI_BASE_URL")
+    model_name = "openai/gpt-4.1-mini"
+
+    if not api_key:
+        print("Skipping test: OPENAI_API_KEY not set")
+        return
+
+    client = OpenAIClient(api_key=api_key, model_name=model_name, base_url=base_url)
+    prompt = "What is the capital of France?"
+
+    try:
+        # Test without kwargs
+        result = client.completion(prompt)
+        print(f"OpenAI response without kwargs:\n{result}")
+        assert result is not None
+        assert len(result) > 0
+    except Exception as e:
+        print(f"OpenAIClient error: {e}")
+        raise
+
+
+if __name__ == "__main__":
+    test_openai_completion_with_kwargs()
+    test_openai_completion_without_kwargs()
diff --git a/tests/mock_lm.py b/tests/mock_lm.py
index e4793b1..3719a9c 100644
--- a/tests/mock_lm.py
+++ b/tests/mock_lm.py
@@ -8,10 +8,10 @@ class MockLM(BaseLM):
     def __init__(self):
         super().__init__(model_name="mock-model")
 
-    def completion(self, prompt):
+    def completion(self, prompt, model=None, **kwargs):
         return f"Mock response to: {prompt[:50]}"
 
-    async def acompletion(self, prompt):
+    async def acompletion(self, prompt, model=None, **kwargs):
         return self.completion(prompt)
 
     def get_usage_summary(self):