Which layers matter?
Residual, MLP, head, module, and position-specific ablations measure how much output distributions move when a component is removed.
Mechanistic interpretability plus training perturbation
A causal atlas for small language models.
I tested where behavior moves inside small models, then used that map to target LoRA, steering, ablation, patching, and negative controls. The result is not a universal theory of language models. It is a practical rule: map the model you plan to edit before you edit it.
5models or model families mapped
12default task families in the main suite
39LFM2.5-230M SFT runs in the sweep
13.8xfewer LoRA params on the JSON result
What got mapped
The atlas links behavior to components, training changes, activations, and interventions. The core measurements are causal: remove, patch, steer, train, compare.
Can clean activations recover behavior?
Activation patching tests whether a component carries behavior by transferring clean or trained activations into a different run.
Where does LoRA write?
Adapters are trained on controlled task families, then compared against the base model by layer, module, norm, and downstream behavior.
Can directions move behavior?
Activation additions test whether a direction can increase, suppress, or damage a behavior. Steering claims remain control-sensitive.
What data shape trains cleanly?
The data-format ablation holds content fixed across 6 formats. Training loss is real. Behavioral judging is still pending real eval runs.
What failed?
Full SFT OOMed on 8GB VRAM, naive layer skipping broke outputs, and some early patching and scoring setups were not useful.
The claims I would share
These are the claims that survive the current audit. Confidence is intentionally conservative.
| Claim | Evidence | Confidence | Details |
|---|---|---|---|
| Qwen2.5-0.5B has a stable L2 causal hub under the main suite. | L2 is top under ablation across 12 families. Phase 3 replicated hub=L2 across seeds 42, 137, and 2026 with std=0.0. Position-specific ablation shows first-token and last-token effects, not a uniform layer. | High | Open 0.5B atlas |
| Qwen hub location changes with scale and task suite breadth. | Full-suite hubs: Qwen2.5-0.5B L2, Qwen2.5-1.5B L14, Qwen2.5-3B L34. Each replicated across 3 seeds with std=0.0. The 1.5B hub was revised from L26 to L14 after widening the suite. | High | Open Phase 3 |
| Atlas-guided LoRA is parameter-efficient on JSON schema following. | On Qwen2.5-0.5B JSON, atlas-guided LoRA used 319K trainable params and reached exact_match=1.000. All-linear also reached 1.000 but used 4.4M params. That is 13.8x more parameters. | High for JSON | Open LoRA evidence |
| Generic conclusions do not transfer cleanly across tasks. | Factual and code tasks did not match the clean JSON story. For code semantics, all-linear won exact match; atlas-guided had lower loss than random-matched but did not reach exact-match parity. | Medium | Open task split |
| Adapter effects are late-layer effects, not upstream propagation. | Adapter-only ablation on the JSON adapter gives norm-effect correlation around 0.85, with effects concentrated around L19-L23. The older "norm-effect separation" story is refuted. | Medium | Open adapter notes |
| Naive layer skipping is not a free speedup. | Layer skipping produced 0% top-5 token overlap across tested skip configs in the audited claim. Recovery fine-tuning is still open work. | High negative result | Open skip result |
| 4-bit NF4 is the practical inference choice in the tested 1.5B setup. | Qwen2.5-1.5B bf16 ran at 18.8 tok/s, 4-bit NF4 at 17.1 tok/s, and 8-bit at 9.0 tok/s. Quality stayed close in the qualitative checks. Causal-surface drift is still under test. | Medium-high | Open quant notes |
How the story changed
The useful part of the project is not that every early guess held up. Several did not. That is the point of the atlas.
Phase 1
Initial Qwen2.5-0.5B atlas. Found L2, LoRA rewiring, late-layer trained activations, and a suspiciously strong factual knockout result.
Phase 2
Scale and controls. Some claims held; some shifted. SmolLM2 did not support a simple universal hub rule.
Phase 3
Multi-seed replication and practical tests. The strongest result is atlas-guided LoRA for JSON with 13.8x fewer parameters.
LFM2 SFT sweep
This is the applied training branch of the atlas: 39 supervised fine-tuning runs on LFM2.5-230M across datasets, optimizers, LoRA rank, target modules, learning rate, steps, and data volume.
Dataset choice moved the result most.
In the Phase 8 sweep, smol-magpie-ultra reached loss 1.25 while Alpaca sat around 6.07. The dataset gap was much larger than optimizer or learning-rate gaps.
Open SFT sweepAdafactor was the practical default.
On smol-magpie-ultra, Adafactor gave loss 1.267 and KL 0.109 while using less memory than AdamW. Lion drifted more in this setup.
Open optimizer tableRank 8 was the efficiency point.
Higher rank helped less with each step up. Rank 8 used 213K trainable params in the sweep; rank 32 used 852K for a smaller additional loss gain.
Open rank sweepHub-targeted LoRA kept drift low.
The hub plus o_proj setup used about 65K params and KL 0.039. All-linear used far more params and moved the model more, while reaching lower loss.
Open target sweepVerbose won loss after content control.
When the same 300 examples were rendered into 6 formats, multi-turn verbose had the lowest loss at 1.372. Bad-format control was second, which is why loss cannot be treated as quality.
Open format ablationBehavioral ranking is not settled.
Phase 9R fixed the eval stack and marked mock-judge numbers as pipeline validation only. The page treats loss rankings as real and behavioral claims as pending real eval.
Open evidence caveatSide results worth keeping
These are useful, but I would not sell them as final laws.
LFM2.5-230M behaves differently from Qwen.
In the LFM2.5-230M pass, L0 had the largest measured effect and early-layer steering moved outputs more. This used a corrected operator/MLP hook because full-layer zeroing caused cascade-zero artifacts.
Open LFM2 atlasFormat affects training loss, but behavior is not settled.
The 6-format ablation holds content fixed. Multi-turn verbose gave the lowest training loss in the current run. The mock-judge data is pipeline validation only, not behavioral evidence.
Open format detailsThe useful LFM2 recipe is narrow.
For memory-constrained LFM2 SFT, the best current default is good multi-turn data, Adafactor, rank 8, and hub-targeted modules when low drift matters.
Open recipe evidenceSteering can work, but controls matter.
Steering looked stronger when tested at the right layers in larger Qwen models. Random-vector and shuffled-label controls are still required before treating this as a clean task-specific intervention.
Open steering notesShort prompts are not the whole story.
Long-task checks suggest some hub locations and steering effects change with prompt length. This limits how far short synthetic prompts can be stretched.
Open robustness notesWhat I do not claim
This is the part that keeps the page honest.
No universal law
The hub locations are model-specific. Even within Qwen, scale and training change the answer. Across architectures, the story changes again.
No probe-only claims
The page does not rely on attention maps, probes, or SAE labels alone. Claims need an intervention, a metric, and a control path.
No hidden nulls
Full SFT did not fit on 8GB VRAM. Full-residual patching was trivial. Early clean/corrupt pairs had tokenization problems. JSON knockout had limited room to suppress targets.
No publication claim yet
This is a reproducible research project, not a finished paper. The strongest results are ready to build on; several controls are still open.
Practical rules
What I would actually do if I had to edit a small model tomorrow.
1. Map first.
Run ablation and patching on the exact model, task family, prompt style, and quantization you plan to use.
2. Use a broad task suite.
The 1.5B hub moved from L26 to L14 when the suite widened. Narrow suites can give confident wrong answers.
3. Target adapters by evidence.
Atlas-guided LoRA was clearly parameter-efficient for JSON. Do not assume the same recipe wins every task.
4. Treat steering as fragile.
Layer, strength, prompt length, and quantization all matter. Add random-vector and shuffled-label controls.
5. Do not skip layers blindly.
The naive skip experiments failed hard. If you want speed, test recovery fine-tuning rather than deleting layers and hoping.
6. Log the misses.
The negative results were not cleanup. They changed the method and killed bad claims.