isokl_steering_calibration/README.md

iso-kl-figure

How strongly should we steer a model? How do we compare steering methods when one might be strong and one weak? These are calibration questions.

Treat steering as an intervention: we want maximum behavior change with minimum side effects, like incoherence, format collapse, or random off-target damage.

A useful analogy is a car on the road. A small nudge to the steering wheel gets corrected by the driver. A larger nudge causes a lane change. A very large nudge causes a crash the driver cannot recover from. Some crashes happen immediately. Some take a few seconds to develop, after the driver tries and fails to correct.

We measure the distribution shift caused by steering, especially the worst 5% of per-token KL that would cause a "crash", and find the largest scalar coefficient C that keeps that 5% below a safe threshold (default: 1 nat). At alpha = 1 the steering is delivering exactly that calibrated dose. At alpha = 2 it is doing twice the dose. Iso-KL means: comparing two methods at the same alpha = 1 means they are spending the same per-token KL budget, so any behavioural difference is not just one of them being louder.

Then comes the survival question. If the calibration only looked at the first w tokens of the rollout, do crashes still happen later? Most trajectories either stabilize or go off track within the first 50-100 tokens, but we want to see the long tail too. So at every fork t we ask: can the model still produce one of two valid answer tokens (true or false) when forced into a JSON-schema prefill? If yes, alive at t. If the probability mass on those tokens drops below 0.95, dead, and dead stays dead. Survival S(t) is the fraction still alive at token t, with right-censoring for rollouts that hit EOS before t.

Headline result on Qwen3.5-0.8B (mean_diff, w=512, n=8 held-out prompts):

• alpha in {0, 0.25, 0.5, 0.75, 1.0, 1.5}: 0 of 8 trajectories die at any fork.
• alpha = 2.0: 8 of 8 die. Median death time t = 64.
• alpha = 4.0: 8 of 8 die at t = 0.

A phase transition at roughly 2x the iso-KL coefficient. The calibrated dose has long-horizon headroom; doubling the dose is a slow crash; quadrupling it is an instant one. Figures and table below.

Spaghetti: per-token KL trajectories, coloured by survival

Per-token KL(steered || base) for n=8 held-out long-form prompts on Qwen3.5-0.8B (mean_diff, w=512). One panel per alpha. Each thin line is one trajectory; green = alive at that token (pmass_eval >= 0.95), red = dead. Black line is the median. Dotted horizontal: KL = 1 nat (the calibration target).

How to read this plot, in order:

1. alpha = 0. KL is exactly zero everywhere. This is the base model; nothing has been added. A sanity check that the splicing pipeline does not itself perturb the logits.
2. alpha = 0.25 to 1.0. KL stays well below the 1-nat line for most tokens. The calibration window only required p95 = 1 nat, so most tokens are far below. All trajectories are green: the model is still producing schema-valid answers throughout. The "warm but not hot" zone.
3. alpha = 1.5. KL still mostly below 1 nat but with a fatter upper envelope. All trajectories still alive end-to-end on the held-out set, even though calibration only guaranteed survival up to roughly alpha = 1. Suggests headroom.
4. alpha = 2.0. KL median sits around 1 nat for most of the rollout, with a noisy plateau. The trajectories turn red part-way through. Steering is now strong enough to break format. The phase transition.
5. alpha = 4.0. Every trajectory is red from t = 0 and KL is roughly 4-6 nats throughout. The model is no longer answering the question; it is generating something else.

The diagnostic shape is the alpha = 2 panel: KL is only modestly above the calibration target (less than 2x in nats) but format collapse is total. KL is necessary but not sufficient as a steering budget. Going from "calibrated" to "calibrated x 2" is not a smooth degradation; it crosses a cliff.

Survival: when do trajectories die?

Kaplan-Meier S(t) on the pmass_eval death event (pmass_eval < 0.95). One curve per alpha, n = 8 trajectories each. Dotted vertical: t = 20, the upper end of the calibration window's relevance for the answer span; everything to the right is generalization.

alpha	n	died	censored	S(mid)	S(end)	t at S<=0.5
0.00	8	0	0	1.000	1.000	--
0.25	8	0	0	1.000	1.000	--
0.50	8	0	0	1.000	1.000	--
0.75	8	0	0	1.000	1.000	--
1.00	8	0	0	1.000	1.000	--
1.50	8	0	0	1.000	1.000	--
2.00	8	8	0	0.586	0.321	64
4.00	8	8	0	0.000	0.000	0

How to read this table, in order:

1. Censored is always 0 here, so survival numbers are not biased by short rollouts. Every trajectory reaches every fork t in {0, 5, ..., w}.
2. alpha 0 to 1.5: died = 0, S = 1.0 everywhere. Format holds for the full rollout. The held-out set generalizes past the calibration window (t = 20 dotted line) all the way to t = 512. The 1-nat KL budget set on calibration prompts continues to be a survivable budget on a different prompt set, more than an order of magnitude past where calibration looked.
3. alpha = 2.0: died = 8 of 8 within the rollout; median death at t = 64. All trajectories eventually break, but they survive the first 64 tokens, about 3x the calibration window. Format collapse is gradual at this dose.
4. alpha = 4.0: median death at t = 0. The first fork already shows pmass_eval < 0.95. Format collapse is immediate.
5. Read t at S<=0.5 as a half-life. It scales nonlinearly with alpha: 64 tokens at 2x, 0 tokens at 4x. Doubling the dose past the calibration point does not double the half-life; it eliminates it.

The calibrated alpha (alpha = 1) earns a high-confidence survival claim within this experiment: 8 of 8 trajectories survived all 512 forks on a held-out prompt set. Doubling the dose still leaves a usable window. Quadrupling it does not.

Reproduce

uv sync --extra all
just smoke         # tiny-random model, ~1 min CPU
just calibrate     # one (model, method, seed, window) cell
just trajectory
just plot

The headline run for the figures here:

uv run --extra all python scripts/run_cell.py \
  --model Qwen/Qwen3.5-0.8B --method mean_diff --seed 0 --window 512 \
  --out-root outputs/qwen35_w512_dense \
  --run-id Qwen3.5-0.8B_mean_diff_s0_w512_dense \
  --compute-pmass --skip-pmass-qa --fork-log \
  --alphas 0.0 0.25 0.5 0.75 1.0 1.5 2.0 4.0 \
  --render-figs --render-threshold 0.95

Outputs land under outputs/qwen35_w512_dense/<run-id>/figs_auto/{survival,spaghetti,aggregate}/.

What this repo is and isn't

In scope:

• One figure family (spaghetti, survival, aggregate) and one calibration script.
• 3 methods: mean_diff, directional_ablation, pca.
• A branch_pmass metric: fork-and-teacher-force probability mass on a forced-choice answer token after schema prefill.

Out of scope:

• A method zoo beyond those three.
• A norm-matching baseline. Iso-KL says nothing about whether two methods of equal KL move the same distance in activation space.
• A threshold sweep. The pmass < 0.95 cutoff is a single point; neighbours might disagree.

Per-trajectory KL normalization (OLMo-2 1B, w=4096)

The figures above use raw per-token KL in nats. That works when every panel shares an alpha-scale, but cross-alpha comparison gets harder once trajectories vary in their own baseline KL noise (some prompts are just spikier than others, even at alpha = 1).

So we re-plot KL divided by each trajectory's own p95 over the first 20 tokens. The dashed reference at y = 1 is then the calibration scale by construction. Values above 1 are excursions above that trajectory's own early-rollout budget. The y-axis is shared across panels so the alpha sweep reads as one picture.

Per-token KL(steered || base) / p95(KL[:20]) for OLMo-2-0425-1B (mean_diff, w=4096, n=24 held-out prompts). One panel per alpha. Blue = alive at that token (pmass_eval >= 0.75 at the nearest fork s <= t); orange = dead (irreversible). Black tick = EOS (right-censored). Dashed = calibration scale = 1. Black line = median across alive+dead trajectories. Threshold here is 0.75, looser than the 0.95 used for Qwen, because OLMo's early pmass is noisier.

How to read it, in order:

1. alpha = 0. No steering, KL = 0, denom = 0, all trajectories skipped. Empty panel is correct.
2. alpha = 0.25 to 0.75. Median sits well below 1. Most trajectories never approach the calibration scale even past 4000 tokens. The 1-nat budget set on a w = 4096 calibration window has long-horizon headroom for OLMo-2 1B.
3. alpha = 1.0. Median hovers a bit under 1 with one outlier traveller above 2. Iso-KL is doing what it claims at the trajectory-median level on a different prompt set than calibration saw.
4. alpha = 1.5 to 2.0. Median crosses 1 and stays there. Most trajectories die. KL is now sustainably over budget.
5. alpha = 4.0. Every trajectory dies. Median rides above 1 throughout. Same instant-collapse story as the Qwen panel.

What surprised me, and what doesn't yet have an explanation:

• Random dead traces at low alpha. At alpha = 0.25 to 0.75, KL stays comfortably under 1 for almost every token, and yet 7 to 12 of 24 trajectories register a death event at some fork. They are not dying because the budget was exceeded; they are dying for some other reason that p95 KL does not see. Possibilities: a single high-KL spike at one token that p95 averages away; a low-KL but format-fragile region where small logit shifts knock pmass below 0.75; calibration noise on the pmass side (threshold 0.75 is loose, and OLMo's base pmass at some forks is already close to it). I don't know which yet.
• alpha = 1.0 has fewer deaths than alpha = 0.75 (11 vs 12). Within noise at n = 24, but the monotonicity of "more steering = more death" is not crisp at this scale.
• Censoring drops to ~0 above alpha = 0.75. Steering past the calibrated dose seems to suppress EOS, so trajectories run to the window length even when off-format. That makes died counts more comparable across high-alpha panels but means low-alpha "censored" is partly the model's own brevity, not the steering's.

So: per-trajectory normalization makes the cross-alpha comparison cleaner, and confirms iso-KL is roughly delivering its claimed dose on held-out prompts. It also makes the "KL is necessary but not sufficient" point sharper: the low-alpha panels show death events at well under the calibration scale.

Reproduce:

uv run --extra all python scripts/spaghetti_kl_alive.py \
  --runs-root outputs/olmo1b_w4096_dense/_OLMo-2-0425-1B_mean_diff_s0_w4096_dense_n24_single \
  --out outputs/olmo1b_w4096_dense/OLMo-2-0425-1B_mean_diff_s0_w4096_dense_n24/figs_thr075_quantile_shaded/spaghetti_norm \
  --window 4096 --threshold 0.75 --model-contains OLMo-2-0425-1B \
  --roll 301 --line-lw 0.4

Toggle off normalization with --no-normalize-kl; change the calibration window with --calib-tokens N.

Honest caveats

• Iso-KL is one fairness criterion, not the criterion. Matching p95 per-token KL means matching distributional disagreement under greedy decoding in the calibration window. It does not match intervention norm, behavioural effect size, or human-perceived quality.
• Pmass is a proxy. It scores whether the model still produces one of two specific tokens after we splice in a schema. A model that has gone off-format but is otherwise coherent gets scored as dead.
• n = 8. Held-out prompts, not held-out seeds. The phase-transition shape is consistent at this scale; the exact half-life at alpha = 2 is not.
• One model so far. Gemma 4B, Gemma 12B (4-bit), OLMo-2 1B, and OLMo-3 7B at w = 4096 are queued. The phase-transition story may or may not survive scaling.

Glossary

Operational definitions used in this repo, not textbook ones. Skim and come back as needed.

• steering coefficient (alpha): scalar multiplier on the steering vector added to residual-stream activations at one layer. alpha = 1.0 is the iso-KL-calibrated coefficient C. alpha = 0 is the unsteered base model.
• iso-KL calibration: bisection on alpha such that the 95th-percentile per-token KL(steered || base) over a w-token calibration window equals 1 nat. Output is c_star, the coefficient at alpha = 1.
• p95 KL: 95th percentile of per-token KL(steered || base) over the calibration window. The "worst 5%" measure used by calibration.
• window (w): length in tokens of the calibration rollout. w = 512 for the figures here.
• pmass: probability the model puts on the set {true, false} after we splice in a JSON schema prefill ('\nI should answer now.</think>{"value": ') at fork token t.
• pmass_eval: pmass on a held-out prompt set, at every fork t.
• fork point t: the token in the rollout where we splice the schema and read pmass.
• death: irreversible. Alive at t means pmass_eval(t) >= 0.95. First time it drops below, dead, and stays dead. Below 0.95 the JSON schema stops being a stable attractor.
• right-censoring: the rollout hit EOS at length L < t, so we never see fork t. Drops out of the at-risk denominator from L onward; not death.
• survival S(t): Kaplan-Meier estimator of the fraction alive at fork t.
• mean_diff: steering vector built as the difference of mean activations on a contrastive prompt pair.
• spaghetti plot: every individual KL trajectory drawn as one thin line, alive segments green, dead red, black line is the median.