Zen Router is a tiny trainable routing model: it reads a prompt and picks the best model to answer it. One forward pass emits the task class, a routing distribution over the model catalog, and a feature embedding — small enough to run in front of every request, on every device.

Model Details

Model Type: Encoder-pooled router (causal LM backbone, routing head)
Base model: Qwen/Qwen3-0.6B (Apache-2.0, Alibaba Qwen), served via the frozen zenlm/zen-nano-0.6b backbone
Architecture: Qwen3 (Qwen3ForCausalLM), 0.6B dense — only the routing heads are trained here; the backbone stays frozen
Parameters: 0.6 billion (frozen backbone) + ~1.2 MB routing heads
License: Apache 2.0
Context Length: 32K tokens
Latency target: <50 ms CPU / <10 ms Metal at Q4
Developed by: Zen AI Team (Hanzo AI)

What it predicts

One forward pass over the prompt (pooled last hidden state) produces three heads:

Task — code, reasoning, math, creative, vision, long-context, cheap-chat, general (the hanzo-router task taxonomy).
Route — a distribution over the model catalog, trained against realized quality/cost/latency, decoded under the caller's SLO (cost ceiling, latency budget, quality floor).
Features — a compact embedding consumed by per-user adaptation (contextual bandit) so routing personalizes online without retraining.

A generative mode is also trained: constrained decode of a single route tag (<route model="..." level="fast|balanced|max"/>) for gateway-side use where only text-in/text-out is available.

Training

Two stages (see training/):

SFT — (prompt → task, best-model) pairs built from eval profiles and gateway telemetry: every routed request settles realized cost, latency, and quality back into the ledger, which becomes labels.
Reward tuning — GRPO on the routing reward quality − λ·cost − μ·latency, the same objective the serving-side SLO uses.

make data     # build dataset from eval JSONL + usage ledger export
make train    # stage-1 SFT
make grpo     # stage-2 reward tuning
make eval     # routing accuracy / regret vs oracle
make quantize # GGUF Q4_K_M + MLX

Measured

Two real SFT runs on this repo's tooling, on real AI-assistant usage. Every number below came from a command that actually ran; nothing is projected. The published weights are from Run B (the frozen-backbone, private-safe run).

Hardware / stack. Apple M4 Max, 128 GB unified memory, macOS 26.5. PyTorch 2.12.1 + Transformers 5.13.0, MPS backend, fp32. Backbone: zen-nano-0.6b (Qwen3, 596M params, hidden 1024, 28 layers).

How the data is built. extract_local_sessions.py and extract_agentic_dataset.py (shared logic in session_common.py) pair each real user prompt with the model that actually served it. quality is a fixed proxy = 1.0 (the counterfactual quality of models that did not run is unobservable), cost = served tokens x the catalog's cost_per_1k, latency_ms = 0 (not in the logs), task = a keyword heuristic ported from hanzo-router's Heuristic::classify. build_dataset.py collapses to one max-reward row per unique prompt. Real model ids are mapped 1:1 to catalog ids (dated Anthropic ids normalized, e.g. claude-opus-4-5-20251101 -> claude-opus-4-5); no collapsing into generic tiers.

Catalog & route head. training/catalog.yaml is the routable universe: 28 models = 3 local zen + zen cloud tiers + Anthropic / OpenAI / Gemini / Grok / DeepSeek / Kimi / MiniMax, priced from the gateway's live config (hanzoai/ai/conf/models.yaml, refreshed from pricing.hanzo.ai; models not yet in it -- e.g. claude-fable-5, gpt-5.5, gemini-3-*, grok-4 -- carry a # verify provider-list price). The route head sizes from the catalog (classes_from: catalog), so the runs printed route head: 24/28 classes (up from 7 in the scaffold) with no code change -- adding a routable model is one catalog row.

Run A -- local logs, full fine-tune

Data: ~/.claude/projects + ~/.codex/sessions. 5,975 raw -> 5,455 unique-prompt rows -> random 80/20 = 4,364 train / 1,091 holdout.
Class balance (the load-bearing caveat): claude-opus-4-8 94.6%, claude-haiku-4-5 4.9%, others <0.3%; 19 catalog models have zero rows. Tasks: code 59.6%, general 24%, cheap_chat 8.5%, math 5.3%, reasoning 2.6%, long_context 0.07%, vision/creative 0%.
Train: full fine-tune (backbone + heads), 1 epoch, batch 16, seq 512, ~17 min; loss 7.23 -> ~0.7. Artifact: 2.4 GB full weights.
Holdout (n=1,091): task_acc 0.824, route_acc 0.961, top3 0.999 -- but route_acc barely clears the 94.2% majority baseline and the head emits only 2 of 24 models. High route_acc here is label memorization, not routing skill.

Run B -- local + private agentic corpus, FROZEN backbone (published)

Data: Run A's local rows (5,993) plus the private hanzoai/zen-agentic-dataset-private. That corpus is 9.9B tokens / 2.3M samples, but is mostly synthetic identity SFT + git history; the routing signal lives in embedded Claude Code transcripts inside assistant turns. Streaming the 35 train chunks + valid split yielded only 4,420 extractable routing rows (transcripts sit in chunks aa-ad + valid; the rest have no served-model label). Combined and deduped: 9,713 unique rows, stratified 80/20 by route = 7,772 train / 1,941 holdout.
Class balance: still Opus-dominated but more diverse -- claude-opus-4-8 53%, claude-opus-4-5 35%, claude-haiku-4-5 6%, claude-sonnet-4-5 5%, then claude-opus-4-1/claude-fable-5/gpt-5.5/ claude-sonnet-4-6. 8 models carry data (vs 5 in Run A).
Privacy-safe training: --freeze-backbone freezes the backbone (requires_grad=False), precomputes pooled embeddings once (cached), and fits only the three linear heads (12 epochs, seconds; loss 3.04 -> 0.97). The published zen-router.pt is 1.2 MB of head weights only -- the backbone stays the public zenlm/zen-nano-0.6b, so the release cannot memorize or reconstruct any private corpus text.
Holdout (n=1,941): task_acc 0.723, route_acc 0.791, top3 0.990.

metric	Run A (full, local)	Run B (frozen, combined)	reading
route models with data	5	8	corpus adds opus-4-5, sonnet-4-5, opus-4-1
route head size	24	28	auto-sized from catalog
task_acc	0.824	0.723	freezing the backbone costs task expressivity
route_acc	0.961	0.791	Run B beats its 53.3% majority baseline by +25.8 pts and emits 5 models (opus-4-8 x928, opus-4-5 x856, haiku x125, sonnet-4-5 x31, fable x1) -- genuine multi-model discrimination, unlike Run A's memorized single label
route_top3	0.999	0.990
artifact	2.4 GB	1.2 MB	heads only

Run B's route_acc is a real signal: the model learned to tell claude-opus-4-8 from claude-opus-4-5 and route cheap prompts to haiku, scoring 26 points over the majority baseline. Run A's higher route_acc is not.

Honest bottom line. Both corpora are overwhelmingly Claude-Opus-labeled -- this machine and the private corpus both ran Opus for almost everything, and 19-20 of the 28 catalog models never appear. The router head is structurally complete (28 gateway-priced logits) and now does real discrimination among the models it has seen, but true cross-provider routing needs balanced counterfactual labels -- the same prompt served by many models with measured quality/cost/latency. That is exactly what the RouterBench pipeline in benchmarks/ provides, and is the stated path forward.

RouterBench transfer eval (full suite, 36,481 prompts) — a negative result

We ran the published RouterBench cost-quality methodology (Hu et al. 2024, arXiv:2403.12031) on the full suite — all 36,497 examples × 11 models = 401,467 rows, 36,481 prompts with full model coverage — measuring the trained Run-B checkpoint against oracle, random, the non-routing interpolation hull, an untrained prior heuristic, and every individual model (benchmarks/, Apple M4 Max / MPS, backbone fp16, 27.8 min wall-clock to batch-embed + route all prompts). The checkpoint scores AIQ 0.6248 — below the interpolation hull (0.7054), the untrained prior (0.7427), and the oracle upper bound (0.8701). It does not beat statically picking one model here. This is expected and we report it plainly: RouterBench's 11 models are 2023-era with zero overlap with the 2026 catalog, so scores are read through a documented family/tier bridge (benchmarks/checkpoint_map.yaml) — this measures transferred task/tier discrimination, not native in-catalog routing — and the checkpoint saw zero training rows for those targets (its corpus is Claude-Opus-skewed, 8/28 models with data). The head keeps voting frontier tier and cannot trace a cost-penalized frontier. Real RouterBench performance requires training on RouterBench's own per-model labels; the numbers and mapping are fully reproducible (see benchmarks/README.md).

Single-forward routing latency (Run B, n=1,941). One prompt, tokenize + forward, MPS fp32: mean 95.7 ms, p50 57.0 ms, p99 303.6 ms. Unquantized eager Transformers; the <50 ms CPU / <10 ms Metal target is at Q4 (see make quantize, not run here). For reference, llama.cpp prompt-processing of zen-nano-0.6b at Q4_K_M on this box runs 9,092 tok/s (a 1k-token forward ~= 112 ms).

Mechanism vs. model. The Rust routing mechanism this model plugs into (hanzo-router + enso) decides in 1.6 us (rules) to 16 us (learned featurize + bilinear select) -- 4-5 orders of magnitude below one 0.6B forward, so the encoder-mode router forward dominates end-to-end cost, as designed.

Reproduce Run B: python training/extract_local_sessions.py --out data/evals-local.jsonl

python training/extract_agentic_dataset.py -> combine -> build_dataset.py -> stratified split -> python training/sft.py --config training/config.scaled.yaml --freeze-backbone -> python -m eval.eval_routing --model out/zen-router --data data/routing-eval.jsonl.

Serving & integration

Served by hanzo-engine (OpenAI-compatible, local or cloud) — dense 0.6B quantized runs on laptops, phones (Metal/Vulkan), and CPUs.
Rust: implements the hanzo-router Classifier seam (task head) and the enso Featurizer seam (feature head); enso's per-user LinUCB consumes the features, so cold-start falls back to rules and improves online.
Gateway: cloud-api resolves the route tag to a provider route (primary + fallbacks) and meters it through the billing gate; rate-limit and spend snapshots from the usage plane bias decoding away from providers near their window limits.

See docs/integration.md.

Quantized deployment

The published weights are heads only (1.2 MB), so serving splits in two: the public zen-nano-0.6b backbone runs quantized in llama.cpp embedding mode and the three linear heads run in the caller in numpy (a 1024×28 matmul is microseconds). No torch at serve time. Export + recipe live in export/ (export_heads.py, route_gguf.py, verify_parity.py, QUANTIZED.md).

prompt ─▶ llama.cpp (zen-nano Q4_K_M, --pooling last --embd-normalize -1)
            └─ raw last-token hidden state x (1024-d, NOT normalized)
                 └─ numpy: route/task = argmax(W·x + b);  feat = W_feat·x + b_feat

Correctness. llama.cpp --pooling last --embd-normalize -1 emits the same raw post-final-norm hidden state HF returns as last_hidden_state (torch f16 norm 101.4 / llama 102.7, element-wise match). Normalization must be off — the head bias is calibrated to the ≈100 norm; an L2-normalized (norm 1) vector lets the bias swamp the signal and collapses routing to one label.

Parity (torch fp32/MPS pipeline vs GGUF+heads, same heads, n=100 holdout, Apple M4 Max, llama.cpp build 9430 Metal):

quant	size	task argmax	route argmax	route top-3	pooled cosine
Q4_K_M	397 MB	89%	95%	100%	0.969
Q8_0	639 MB	95%	97%	100%	0.988

The quantized backbone reproduces the torch routing pick 95% of the time at Q4 (97% at Q8) and never drops it out of the top-3. Q4_K_M is the default; Q8_0 for fidelity-sensitive use.

Latency (Q4_K_M, resident llama-server, prompt → model id, n=200 holdout mix; token lengths min 3 / p50 159 / mean 405 / max 1373):

mean 132 ms p50 97 ms p99 351 ms min 12.5 ms

Stratified p50 by prompt length: 1–32 tok 50 ms · 129–512 tok 107 ms · 513+ tok 251 ms — a ~40–50 ms Metal-dispatch + HTTP floor plus linear prompt-processing (p50 tracks the earlier llama-bench pp1024 ≈ 112 ms). For reference the fp32 torch/MPS path measured p50 57 ms on the same holdout: for a 0.6B model, in-process MPS matmuls beat a Q4 llama.cpp HTTP round-trip, so the quantized path's win here is footprint (397 MB vs 2.4 GB) and portability (CPU / edge / Vulkan, no torch), not raw M4-Max latency. See export/QUANTIZED.md for the full recipe and reproduction.

Formats

PyTorch (safetensors), GGUF (Q2_K–F16), MLX.

zenlm/zen-router

Zen Router 0.6B