diff --git a/scripts/fit_qwen06b.py b/scripts/fit_qwen06b.py
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+++ b/scripts/fit_qwen06b.py
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+"""Fit the real Qwen3-0.6B Jacobian cache used by the notebooks and README.
+
+(authored by Claude)
+
+64 diverse web-text-ish prompts (hardcoded below so the fit is reproducible),
+mid-layer band, resumable checkpoint. Cost: 1 forward + ceil(1024/8)=128
+backwards per prompt; a few minutes on a 3090.
+
+Run:
+    uv run python scripts/fit_qwen06b.py 2>&1 | tee /tmp/claude-1000/jsteer_fit06b.log
+"""
+from __future__ import annotations
+
+import time
+
+import torch
+from loguru import logger
+from transformers import AutoModelForCausalLM, AutoTokenizer
+
+from jsteer import Jacobian
+
+MODEL = "Qwen/Qwen3-0.6B"
+OUT = "artifacts/qwen3-0.6b.jac"
+CKPT = "artifacts/qwen3-0.6b.ckpt"
+DEVICE = "cuda"
+DTYPE = torch.bfloat16
+
+# Claude: 64 prompts varying topic (news/science/food/sport/tech/nature/history/
+# finance/fiction/how-to/reviews/forum) and register (formal/casual/instructional).
+# Each must tokenize to >17 tokens (jlens skips the first 16 positions and drops
+# the final one); asserted below.
+PROMPTS = [
+    # news / civic
+    "The city council voted last night to repair the old stone bridge downtown, a project residents have been requesting for well over a decade.",
+    "Election officials confirmed on Tuesday that turnout in the rural districts had reached its highest level since records began in the 1970s.",
+    "A water main burst under the high street early this morning, closing three shops and flooding the basement of the public library.",
+    "The transit authority announced fare increases starting next spring, citing rising maintenance costs and a shortfall in state funding for the third consecutive year.",
+    "Firefighters contained the warehouse blaze within two hours, and investigators say the cause appears to be an electrical fault in the loading bay.",
+    # science
+    "Scientists have long argued about whether the early universe expanded smoothly or in sudden bursts that left traces we can still measure today.",
+    "The research team spent four summers tagging migratory birds along the coastline, building one of the largest datasets of its kind in the region.",
+    "A new study suggests that soil bacteria can break down certain plastics far faster than previously thought, though scaling the process remains difficult.",
+    "Astronomers using the survey telescope catalogued thousands of previously unknown asteroids, most of them harmless rocks orbiting far beyond Mars.",
+    "The geology students measured the strata exposed by the landslide, noting how each layer recorded a different chapter of the valley's ancient climate.",
+    "Marine biologists were surprised to find the coral colonies recovering faster in the shaded lagoons than in the brightly lit outer reef.",
+    # food / cooking
+    "Learning to cook well takes patience more than talent, a willingness to taste often, to fail a few times, and to pay attention to detail.",
+    "Start by browning the onions slowly in butter over low heat; rushing this step is the most common reason the soup turns out flat.",
+    "The bakery on Fifth Street still makes its rye bread the old way, with a starter the owner claims has been alive since the fifties.",
+    "If your dough refuses to rise, check the temperature of the water you used, because yeast dies quickly above fifty degrees Celsius.",
+    "We tried the new noodle place near the station last weekend, and honestly the broth alone was worth the forty minute wait in line.",
+    # sport
+    "After months of training, the runners lined up at dawn, breath fogging in the cold air, waiting nervously for the starting gun to fire.",
+    "The home side equalised deep in injury time, sending the away fans silent and the commentators scrambling for superlatives they had already used.",
+    "Her backhand had always been the weaker shot, so she spent the entire off-season rebuilding it with a coach known for his brutal drills.",
+    "The climbing gym gets crowded after work hours, but early on Sunday mornings you can have the whole bouldering wall to yourself.",
+    # tech / software
+    "Software written in a hurry tends to accumulate small mistakes that hide quietly until, one ordinary afternoon, they surface all at once together.",
+    "The outage began when a routine certificate renewal failed silently, and within an hour half the company's internal dashboards were unreachable.",
+    "Before you file a bug report, please check whether the issue reproduces on the latest release, and include the full error message and stack trace.",
+    "Our team migrated the billing service to the new queue system last quarter, and the pager has been remarkably quiet ever since.",
+    "The laptop's battery life is excellent and the keyboard is pleasant, but the webcam remains as grainy as the one on the previous model.",
+    "Version control feels like bureaucracy until the first time it saves you, after which committing early and often becomes a reflex.",
+    # nature / weather
+    "The weather this morning was cold and grey, so I made a large pot of coffee and sat by the window watching the rain fall.",
+    "By late August the meadow behind the house turns gold and dry, and grasshoppers scatter ahead of you with every step through the grass.",
+    "The first frost arrived weeks earlier than usual this year, catching the gardeners with half the tomato crop still green on the vine.",
+    "Wolves returned to the northern valley about a decade ago, and the ripple effects on the deer and the riverbanks are still being studied.",
+    "Fog settled over the harbour before sunrise, muffling the bells of the fishing boats as they felt their way out past the breakwater.",
+    # history
+    "My grandmother used to tell stories about growing up on a small farm, where every season brought a different kind of hard and honest work.",
+    "The old mill by the river operated for nearly two centuries before the floods of 1953 finally destroyed the wheel and silted the race.",
+    "Medieval copyists made errors just as modern typists do, and scholars can trace manuscript lineages by following those mistakes across the centuries.",
+    "The railway reached the town in 1884, and within a generation the wagon roads over the pass had grassed over almost completely.",
+    "Archaeologists excavating the harbour district found ledgers recording grain shipments, tax disputes, and one memorable complaint about a dishonest rope merchant.",
+    # finance / work
+    "The quarterly report showed modest growth in the services division, offset by continued weakness in hardware sales across the European market.",
+    "Most household budgets fail not because of large purchases but because of small recurring subscriptions nobody remembers signing up for.",
+    "She negotiated the lease renewal down by twelve percent simply by arriving with printouts of comparable rents from the same block.",
+    "The startup burned through its first funding round in fourteen months, mostly on an office it did not need and hires it made too early.",
+    "Remote work suits some teams wonderfully and hollows out others, and the difference usually comes down to how deliberately they communicate in writing.",
+    # fiction-ish / narrative
+    "The lighthouse keeper kept a journal for forty years, and the entries grew shorter each winter until they were single words: wind, rain, waiting.",
+    "She found the letter tucked inside a secondhand atlas, addressed to a street that, as far as she could tell, had never existed.",
+    "The train slowed somewhere between stations and stopped entirely, and for twenty minutes the passengers listened to rain drumming on the roof.",
+    "Every house on the street had a story, but the blue one on the corner had three, and none of them agreed with each other.",
+    "He repaired watches in the back room of the shop, surrounded by tiny drawers whose labels had worn away decades before he arrived.",
+    # how-to / instructional
+    "To bleed a radiator, turn off the heating first, hold a cloth under the valve, and open it slowly until water replaces the hissing air.",
+    "When packing for a long hike, weigh everything, because ounces you ignore in the kitchen become pounds you curse on the switchbacks.",
+    "Sharpen the chisel before each session; a dull edge forces you to push harder, and pushing harder is how fingers end up stitched.",
+    "Back up your photos in two places, one of them offsite, because the drive that fails is always the one holding the originals.",
+    "Plant the garlic in October, mulch it well, and then do absolutely nothing until the scapes curl in early summer.",
+    # reviews / opinion
+    "The novel's middle section drags through two hundred pages of committee meetings, but the final chapters justify nearly all of the patience required.",
+    "This vacuum is louder than our old one and the cord is shorter, but it picks up pet hair the old machine just pushed around.",
+    "The museum's new wing is airy and generous with natural light, though the signage seems designed for people who already know the collection.",
+    "I expected the sequel to coast on the original's charm, and instead it quietly fixed almost every complaint I had scribbled down years ago.",
+    # forum / casual
+    "Does anyone else's cat sprint around the apartment at exactly three in the morning, or have I somehow adopted a very small racehorse?",
+    "Update on the sourdough saga: loaf number nine finally has an open crumb, and I owe it all to a colder proof and a hotter oven.",
+    "We drove the coast road in a rented van last spring, sleeping at trailheads and eating far too many gas station pastries, and I regret nothing.",
+    "My neighbour lends me his ladder every autumn and refuses all payment, so every winter a tray of baklava mysteriously appears on his porch.",
+    "Honest question for the group: how do you keep houseplants alive in a north-facing flat, because I have failed four times running.",
+    # essay / reflective
+    "There is a particular quiet that settles over a library reading room in the late afternoon, part paper, part dust, part concentration.",
+    "Teaching a child to ride a bicycle teaches the adult something too, mostly about when to hold on and when to quietly let go.",
+    "Cities remember their old rivers even after burying them; the streets still dip and curve along channels no map has shown for a century.",
+    "Most advice about writing reduces to the same two acts, performed in alternation: sit down and begin, then cut what you did not need.",
+]
+
+
+def main() -> None:
+    assert len(PROMPTS) == 64, len(PROMPTS)
+    logger.info(f"loading {MODEL} ({DTYPE}) on {DEVICE}")
+    tok = AutoTokenizer.from_pretrained(MODEL)
+    model = AutoModelForCausalLM.from_pretrained(MODEL, dtype=DTYPE).to(DEVICE).eval()
+
+    # SHOULD: every prompt >17 tokens so jlens (skip_first=16, final dropped)
+    # has >=1 valid position. ELSE fit raises on that prompt.
+    n_tok = [len(tok(p, add_special_tokens=True).input_ids) for p in PROMPTS]
+    logger.info(f"prompt token counts: min={min(n_tok)} max={max(n_tok)}")
+    assert min(n_tok) >= 18, f"shortest prompt has {min(n_tok)} tokens"
+
+    t0 = time.monotonic()
+    jac = Jacobian.fit(model, tok, PROMPTS, layers=(0.3, 0.9), dim_batch=8,
+                       max_seq_len=128, checkpoint_path=CKPT)
+    wall = time.monotonic() - t0
+    logger.info(f"fit wall-time: {wall:.1f}s ({wall/60:.1f} min) for {len(PROMPTS)} prompts")
+
+    jac.save(OUT)
+    logger.info(f"saved {OUT}: {jac!r}")
+    # SHOULD: load round-trip returns the same layers. ELSE save/load wiring bug.
+    assert Jacobian.load(OUT).layers == jac.layers
+
+
+if __name__ == "__main__":
+    main()