Notes on changes to modeling_nemotron_h.py to resolve bugs in orginal code from NVIDIA
Nemotron-Cascade-2-30B-A3B ships with custom model code (modeling_nemotron_h.py) that has two bugs exposed by causal_conv1d 1.6.x. These are bugs in NVIDIA's model code, not in Heretic.
Fix 1: Conv State Cache Dimension Mismatch
File: modeling_nemotron_h.py (in the NemotronHCache.init method)
What was wrong: The cache allocated conv_states tensors with intermediate_size (4096 = mamba_num_heads × mamba_head_dim), but the Mamba mixer's conv1d layer operates on the full projected dimension (6144 = intermediate_size + 2 × n_groups × ssm_state_size), which includes the B and C state channels concatenated with the hidden states. When causal_conv1d_update compared conv_state.shape[1] (4096) against weight.shape[0] (6144), it failed with "weight must have shape (dim, width)".
The fix: Added a conv_dim variable matching the model's own formula from NemotronHMamba2Mixer.init, and used it for the cache allocation:
intermediate_size = config.mamba_num_heads * config.mamba_head_dim
conv_dim = intermediate_size + 2 * config.n_groups * config.ssm_state_size
and
torch.zeros(batch_size, conv_dim, conv_kernel_size, ...)
Fix 2: Prefill Sequence Length Guard
File: modeling_nemotron_h.py (in cuda_kernels_forward)
What was wrong: The single-token decode path was guarded by cache_position[0] > 0, which is true whenever the KV cache has been initialized — but modern transformers can still pass multi-token sequences after cache init (e.g., during the second forward call of generation). The causal_conv1d_update CUDA kernel expects a 2D (batch, dim) input for single-step updates, but received a 3D (batch, seq_len, dim) tensor, causing the shape check to fail.
The fix: Added and hidden_states.shape[1] == 1 to the condition so the single-step CUDA path is only used when there's actually a single token:
if cache_params is not None and cache_position is not None and cache_position[0] > 0 and hidden_states.shape[1] == 1
We're excited to introduce Nemotron-Cascade-2-30B-A3B, an open 30B MoE model with 3B activated parameters that delivers strong reasoning and agentic capabilities. It is post-trained from the Nemotron-3-Nano-30B-A3B-Base. Nemotron-Cascade-2-30B-A3B achieves gold medal performance in both the 2025 International Mathematical Olympiad (IMO) and the International Olympiad in Informatics (IOI). It operates in both thinking and instruct (non-thinking) modes.
Benchmark Results
Benchmark
Nemotron-3-Nano-30B-A3B
Nemotron-3-Super-120B-A12B
Qwen3.5-35B-A3B
Nemotron-Cascade-2-30B-A3B
Math
IMO 2025
-
-
-
🏅 35 pts
IMO AnswerBench
70.4‡
77.2‡
74.8‡
79.3
IMO ProofBench
-
-
-
72.9
AIME 2025
89.1
90.2
91.9‡
92.4 (98.6)†
AIME 2026
89.9‡
89.8‡
91.1‡
90.9 (95.0)†
HMMT Feb25
84.6‡
93.7
89.0
94.6
Code Reasoning
IOI 2025
-
-
348.6‡
🏅 439.3
ICPC World Finals 2025
-
-
-
🏅 10/12
LiveCodeBench v6 (2408-2505)
68.3
78.7
74.6
87.2 (88.4)†
LiveCodeBenchPro 25Q2 (Easy)
54.5‡
81.7‡
81.1‡
87.0 (89.3)†
LiveCodeBenchPro 25Q2 (Med)
3.50‡
23.2‡
17.8‡
27.6 (36.8)†
SciCode
33.3
42.1
38.0
36.4
Knowledge & STEM
MMLU-Redux
-
-
93.3
86.3
MMLU-Pro
78.3
83.7
85.3
79.8
GPQA-Diamond
73.0
79.2
84.2
76.1
HLE (no tool)
10.6
18.3
22.4
17.7
Alignment & Instruction Following
ArenaHard v2 (Avg.)
67.7
-
65.4‡
83.5
– Hard Prompt
72.1
73.9
64.5‡
88.2
– Creative Writing
63.2
-
66.3‡
78.7
IFBench (prompt)
71.5
72.6
70.2
82.9
Scale AI Multi-Challenge
38.5
55.2
60.0
45.3
Long Context & Context Learning
AA-LCR
35.9
58.3
58.5
39.1
LongBench v2
39.6
-
59.0
40.3
NIAH@1M (RULER Subset)
94.8
98.3
94.3‡
99.0
CL-Bench
12.0‡
-
15.5‡
12.2
Agentic
BFCL v4
53.8
-
67.3
52.9
𝜏²-Bench
49.0
61.2
81.2
58.9
Terminal Bench 2.0
8.5
31.0
40.5
21.1
SWE Verified (OpenHands)
38.8
60.5
69.2
50.2
Multilingual
MMLU-ProX
59.5
79.4
81.0
72.5
WMT24++ (en -> xx)
86.2
86.7
87.6‡
84.1
* † Numbers in brackets refers to Tool-Integrated Reasoning (TIR) results.
* ‡ For the baseline models, we use official numbers when available, otherwise evaluate them using the recommended settings.
Quick Start
Nemotron-Cascade-2-30B-A3B follows the ChatML template and supports both thinking and instruct (non-thinking) modes. Reasoning content is enclosed within <think> and </think> tags. To activate the instruct (non-thinking) mode, we prepend <think></think> to the beginning of the assistant’s response.
Nemotron-Cascade-2-30B-A3B does not currently support OpenCode; it primarily supports OpenHands for agentic coding and SWE tasks.
To reduce the context length in a multi-turn conversation, when the previous user turn involves thinking mode, only the final summary of the model's output will be added to the conversation history.
Note that we do not define a separate tool role for tool responses; instead, we place them under the user role and warp them with <tool_response> and </tool_response>.
We recommend setting the sampling parameters to temperature = 1.0 and top_p = 0.95.
vLLM setup
Requires vLLM version >= 0.17.1. The following will create API endpoints at http://localhost:8000/v1:
Standard version: Use the following command to create an API endpoint with a maximum context length of 262,144 tokens.
1from transformers import AutoTokenizer
23model_name ='nvidia/Nemotron-Cascade-2-30B-A3B'4tokenizer = AutoTokenizer.from_pretrained(model_name)56'''
7single-turn example
8'''9messages =[10{"role":"system","content":"You are a helpful and harmless assistant.\n\nYou are not allowed to use any tools"},11{"role":"user","content":"calculate 1+1?"}12]1314# thinking mode15prompt_thinking = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=True)16# prompt_thinking = '<|im_start|>system\nYou are a helpful and harmless assistant.\n\nYou are not allowed to use any tools<|im_end|>\n<|im_start|>user\ncalculate 1+1?<|im_end|>\n<|im_start|>assistant\n<think>\n'1718# instruct mode19prompt_instruct = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=False)20# prompt_instruct = '<|im_start|>system\nYou are a helpful and harmless assistant.\n\nYou are not allowed to use any tools<|im_end|>\n<|im_start|>user\ncalculate 1+1?<|im_end|>\n<|im_start|>assistant\n<think></think>'2122'''
23multi-turn example
24'''25messages =[26{"role":"system","content":"You are a helpful and harmless assistant.\n\nYou are not allowed to use any tools"},27{"role":"user","content":"calculate 1+1?"},28{"role":"assistant","content":"<think>THINKING_CONTENT</think>\nTo calculate \\(1 + 1\\):\n\n1. **Identify the operation**: This is a basic addition problem involving two integers.\n2. **Perform the addition**: \n \\(1 + 1 = 2\\).\n\n**Result**: \\(\\boxed{2}\\)",},29{"role":"user","content":"what about 2+2"}30]3132# thinking mode33prompt_thinking = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=True)34# prompt_thinking = '<|im_start|>system\nYou are a helpful and harmless assistant.\n\nYou are not allowed to use any tools<|im_end|>\n<|im_start|>user\ncalculate 1+1?<|im_end|>\n<|im_start|>assistant\n<think></think>\nTo calculate \\(1 + 1\\):\n\n1. **Identify the operation**: This is a basic addition problem involving two integers.\n2. **Perform the addition**: \n \\(1 + 1 = 2\\).\n\n**Result**: \\(\\boxed{2}\\)<|im_end|>\n<|im_start|>user\nwhat about 2+2<|im_end|>\n<|im_start|>assistant\n<think>\n'3536# instruct mode37prompt_instruct = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=False)38# prompt_instruct = '<|im_start|>system\nYou are a helpful and harmless assistant.\n\nYou are not allowed to use any tools<|im_end|>\n<|im_start|>user\ncalculate 1+1?<|im_end|>\n<|im_start|>assistant\n<think></think>\nTo calculate \\(1 + 1\\):\n\n1. **Identify the operation**: This is a basic addition problem involving two integers.\n2. **Perform the addition**: \n \\(1 + 1 = 2\\).\n\n**Result**: \\(\\boxed{2}\\)<|im_end|>\n<|im_start|>user\nwhat about 2+2<|im_end|>\n<|im_start|>assistant\n<think></think>'
Python Tool Use
python
1model_name ='nvidia/Nemotron-Cascade-2-30B-A3B'2tokenizer = AutoTokenizer.from_pretrained(model_name)34SYSTEM_PROMPT ="""# Tools
56You have access to the following functions:
78<tools>
9<function>
10<name>stateful_python_code_exec</name>
11<description>Call this function to execute Python code in a stateful Jupyter notebook environment. Python will respond with the output of the execution or time out after 120.0 seconds.</description>
12<parameters>
13<parameter>
14<name>code</name>
15<type>string</type>
16<description>Code to execute</description>
17</parameter>
18<required>["code"]</required>
19</parameters>
20</function>
21</tools>
2223If you choose to call a function ONLY reply in the following format with NO suffix:
2425<tool_call>
26<function=example_function_name>
27<parameter=example_parameter_1>
28value_1
29</parameter>
30<parameter=example_parameter_2>
31This is the value for the second parameter
32that can span
33multiple lines
34</parameter>
35</function>
36</tool_call>
3738<IMPORTANT>
39Reminder:
40- Function calls MUST follow the specified format: an inner <function=...></function> block must be nested within <tool_call></tool_call> XML tags
41- Required parameters MUST be specified
42- You may provide optional reasoning for your function call in natural language BEFORE the function call, but NOT after
43- If there is no function call available, answer the question like normal with your current knowledge and do not tell the user about function calls
44</IMPORTANT>"""4546messages =[47{"role":"system","content": SYSTEM_PROMPT},48{"role":"user","content":"Solve the following math problem. Put your answer inside \\boxed{}.\n\nIn a school with 2008 students, each student is a member of certain committees. Each committee has at most 1004 members, and every two students are in at least one common committee. Determine the smallest possible number of committees in the school."}49]5051prompt = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=True)52print(prompt)
Agentic Usage
python
1model_name ='nvidia/Nemotron-Cascade-2-30B-A3B'2tokenizer = AutoTokenizer.from_pretrained(model_name)34SYSTEM_PROMPT ="""You are a customer service agent that helps the user. The policy that determines how you should respond to requests from users is described below between the <policy> and </policy> tags.
56In each turn you can either:
7- Send a message to the user.
8- Make a tool call.
9You cannot do both at the same time.
1011<policy>
12_NEED_TO_ADD_POLICY_HERE_
13</policy>
1415Try to be helpful and always follow the policy.
1617# Tools
1819You have access to the following functions:
2021<tools>
22<function>
23<name>_NEED_TO_ADD_FUNCTION_NAME_1_</name>
24<description>_FUNCTION_DESCRIPTION_</description>
25<parameters>
26<parameter>
27<name>_NEED_TO_ADD_PARAMETER_NAME_1_</name>
28<type>_PARAMETER_TYPE_</type>
29<description>_PARAMETER_DESCRIPTION_</description>
30<title>_PARAMETER_TITLE_</title>
31</parameter>
32<parameter>
33<name>_NEED_TO_ADD_PARAMETER_NAME_2_</name>
34<type>_PARAMETER_TYPE_</type>
35<description>_PARAMETER_DESCRIPTION_</description>
36<title>_PARAMETER_TITLE_</title>
37</parameter>
38...... (_MORE_PARAMETERS_TO_ADD_)
39<parameters>
40</function>
41...... (_MORE_FUNCTIONS_TO_ADD_)
42</tools>
43"""4445messages =[46{"role":"system","content": SYSTEM_PROMPT},47{"role":"user","content":"Hello, I'm calling regarding my upcoming stay at your hotel. My guest ID is G90920 and booking ID is B11246 for a Deluxe room on June 5th. I'm traveling with three 6-month-old triplets and need to request three infant cribs for our room. It's currently 30 hours before check-in—could you please confirm if this is feasible and if there are quiet room options available for families with infants?"}48]4950prompt = tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True, enable_thinking=True)51print(prompt)
@article{Nemotron_Cascade_2,
title={Nemotron-Cascade 2: Post-Training LLMs with Cascade RL and Multi-Domain On-Policy Distillation},
author={Yang, Zhuolin and Liu, Zihan and Chen, Yang and Dai, Wenliang and Wang, Boxin and Lin, Sheng-Chieh and Lee, Chankyu and Chen, Yangyi and Jiang, Dongfu and He, Jiafan and Pi, Renjie and Lam, Grace and Lee, Nayeon and Bukharin, Alexander and Shoeybi, Mohammad and Catanzaro, Bryan and Ping, Wei},
year={2026}
}