| # Agentic post-training, measured: Nex-N2-mini vs its base — on one RTX 5090 |
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| **Rig:** single RTX 5090 32GB · llama.cpp b9562 · native OpenAI tool-calling (`--jinja`) · temp 0 |
| **Subjects:** `Nex-N2-mini` (35B-A3B, agentic post-train) vs its base `Qwen3.5-35B-A3B`, both Q4_K_M |
| **Harness:** the rig's new **Agentic Score** — a model-agnostic native tool-calling loop over 15 |
| deterministic, programmatically-checked tasks (tool-use chains, multi-step dependencies, sandboxed |
| coding). Calibration-grade, not SWE-bench; a sub-5% gap is a tie. |
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| --- |
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| ## The finding |
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| Nex-N2's headline claim is that "Adaptive Thinking" cuts ~20% of tokens at zero performance loss. On |
| this rig, **the token claim is not just true — it's an understatement.** Nex-N2-mini does the same |
| agentic work in **93 tokens/task vs the base's 264 — a 65% cut, ~2.8x leaner.** But "zero performance |
| loss" does **not** hold: the post-train trades **13 points of task success** for that efficiency. |
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| | axis | base Qwen3.5-35B-A3B | Nex-N2-mini | read | |
| |---|---|---|---| |
| | **Agentic Score** | **98.0** | 92.0 | base wins overall | |
| | Task success | **100%** (15/15) | 86.7% (13/15) | base completes more | |
| | Tool efficiency | 0.90 | **0.933** | Nex slightly tighter | |
| | Loop stability | 100% | 100% | tie — neither stalls | |
| | **Tokens / task** | 264.3 | **93.3** | **Nex −65%** | |
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| Per axis, Nex's misses are concentrated: chain 4/5, multistep **5/5**, coding 4/5. The base is clean 5/5/5. |
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| ## The mechanism (why the post-train loses success) |
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| Both failures trace to the *same* cause — terse Adaptive Thinking skips the sanity-check a longer pass |
| would have caught: |
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| 1. **`chain_vram_sq`** — Nex searched (got 32GB), then called `calc("32^2")`. The calculator is Python |
| `eval`, where `^` is **XOR**, so it returned **34**. Nex trusted the surprising number and answered |
| 34. The base used a power/multiply expression and got 1024. *A model that paused on "34 ≠ 32²" would |
| have caught it; Nex didn't pause.* |
| 2. **`coding_sum_evens`** — Nex's code `print()`ed the answer instead of assigning `result` (the tool's |
| documented contract), got an error, then **guessed 90** (correct: 110). The base followed the |
| contract and verified in the sandbox. |
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| Same tools, same prompts, temp 0, both reproduced on a re-run. This is the agentic cost of aggressive |
| brevity: fewer tokens, fewer self-checks. |
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| ## Methodology note (the harness, suspected first) |
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| The first Nex run scored 73% — and was **wrong**. Traces showed the mock `web_search` only matched an |
| exact key, so the model's reasonable paraphrases ("RTX 5090 specs VRAM GB") returned *"no match"* and it |
| looped to the step cap; a second task tripped a safety refusal. Those were **harness artifacts, not |
| model weakness.** Fixed (search now tolerates phrasing like a real engine; tasks ground via named tools; |
| the safety-trap task reframed), re-ran both models on the identical corrected set. The numbers above are |
| from the corrected harness. Suspect the harness before the model — every time. |
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| ## Worth it? |
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| - **Reach for Nex-N2-mini** if tokens, latency, or serving cost dominate and your tasks are |
| well-specified — it does the same tool-driving for a third of the tokens, with tighter tool use and |
| zero stalls. |
| - **Reach for the base** if maximum task completion matters more than token budget — it self-checks |
| surprising tool outputs and honors tool contracts that the terse post-train skips. |
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| Adaptive Thinking is a real, large efficiency win. Just not a free one. |
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| --- |
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| *Harness: `lib/agentic/native/` in notwitcheer/llm-bench-rig (17 unit tests). Charts: |
| `reports/agentic-nex-n2-mini.png`, `reports/agentic-tokens.png`.* |
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