We study Hierarchical Reasoning Model (HRM), a representative hierarchical Transformer-based latent reasoning model with many variants, on Sudoku, Maze, and ARC-AGI-2. We mechanistically understand how HRM reasons and what information it encodes. Our analyses compare HRM against Transformer baselines with and without recurrent modules, apply causal interventions on recurrent states, and utilize linear probes against random-direction ablations, as well as sparse autoencoders with feature ablations. Our results reveal several key findings: recurrent models outperform one-pass baselines, while single-state recurrent Transformers are comparable to HRM. State interventions further show that the causal contributions of the high- and low-level states vary across task-specific checkpoints and inference stages. Selected task variables are linearly decodable from the recurrent states in HRM, yet ablating probe directions produce effects comparable to random controls. SAE ablations yield larger behavioral changes than probe-direction ablations. However, top-ranked SAE features show no stable advantage over size-matched random subsets at larger ablation sizes or across tasks; the same pattern persists in a Sudoku control with within-step BPTT. Together, we characterize that HRM is essentially implementing constraint-aware iterative refinement on a puzzle-specific solution state, in which the functional contributions of components at different levels vary without relying on a compact, causally important feature set. These results highlight the necessity of studying the different working mechanisms and the importance of developing mechanistic interpretability techniques better suited for latent-space, recursive reasoning models.
Figures & tables
Figure 1: Across all five architectures, recurrent models (HRM, Recurrent/Universal Transformer) (a) steadily reduce Hamming distance to the ground-truth solution and (b) improve cell accuracy over 16 ACT steps, while the non-recurrent baselines (Plain Transformer, GRU) remain flat at >30 mismatches and substantially lower accuracy.
Figure 2: Causal evidence for zH as a dynamic plan. (a) Per-step ablation: zeroing zH is increasingly destructive at later steps ( −6.8 at step 0 vs. −26.0 at step 15). (b) Freeze curve: preventing zH updates after step k shows progressive refinement, freezing at step 0 loses −9.1 ; by step 8, the plan is nearly complete. (c) Time-shift: transplanting later zH states into earlier positions slightly improves accuracy, confirming monotonic refinement.
Figure 3: Causal evidence for Maze’s reorganized division of labor; values are percentage-point changes. (a) All-step ablation: token accuracy moves little for either stream, but valid paths rate fall −32.5% under zH and −8.5% under zL , so token accuracy is a weak diagnostic. (b) Per-step ablation: zH matters the most at the final readout; no single-step zL ablation exceeds −4.5 . (c) Donor patching: donor- zL destroys 43 – 56% of valid paths from step 4 onward, whereas donor- zH is inert until readout ( −26 at step 15). zL is therefore the strongest patchable carrier of the intermediate path state.
Task
State / step
Target
Probe acc.
Baseline / note
Sudoku
zH , step 15
Row / col / box violation
88.3 / 87.8 / 87.5
per-cell binary
Sudoku
zL , step 15
Row / col / box violation
88.2 / 87.6 / 87.1
per-cell binary
Sudoku
zH , step 15
Per-cell correctness
82.5
per-cell binary
Sudoku
zL , step 15
Per-cell correctness
83.3
per-cell binary
Maze
zH , step 15
Exact solved / valid S–G path
77.5 / 95.7
global path features
Maze
zL , step 15
Exact solved / valid S–G path
75.8 / 92.1
global path features
Table 1: Selected linear-probe validation accuracies across tasks. Sudoku and Maze results use final-step activations from both recurrent streams where available. ARC-AGI-2 is included as a representation-level replication using zH at step 15. Expanded probe sweeps, MLP comparisons and complete target definitions are reported in Appendix A.7 , A.8 .
Figure 4: Causal effects for a representative SAE configuration. Values are mean percentage-point changes relative to the no-intervention output; negative values indicate performance degradation. Sudoku reports Δ cell accuracy, Maze reports Δ valid path rate, and ARC-AGI-2 reports Δ color-cell accuracy. Full SAE sweeps are reported in Appendix A.10 .
Figure 5: Probe readout does not imply causal relevance. (a) Task variables are linearly decodable from HRM states across Sudoku, Maze, and ARC-AGI-2. (b) Directed ablation removes the corresponding probe directions and compares the behavioral effect with rank-matched random directions. (c) Maze exhibits the mismatch most clearly: both zH and zL decode global path variables, whereas Section 4 identifies donor- zL patching as the stronger intervention on the intermediate path state.
Model
Top-50 SAE
Random-50 SAE
Stock HRM
−1.59
−1.37
BPTT HRM
−29.64
−29.63
Table 2: BPTT training-regime check. Values are mean changes in cell accuracy under SAE-feature ablation over 200 shared puzzles. Top-50 and random-50 ablations are statistically indistinguishable in both regimes (paired t -tests: stock, p=0.39 , d=−0.06 ; BPTT, p=0.93 , d=−0.01 ).
Appendix figures & tables26 assets
Supplementary material from the paper’s appendix.
Appendix
Model
Recur.
Hier.
Steps
Params
HRM
✓
✓
16
27M
Recurrent Trans.
✓
16
27M
Univ. Trans.
✓
16
28M
Plain Trans.
1
27M
GRU
1
25M
Appendix
Table 3: Architectures compared in this paper. Recur. denotes refinement-step recurrence; Hier. denotes hierarchical structure; Steps denotes max encoder updates per input.
Checkpoint
Cell%
Puzzle%
Hamming
Recurrent Trans. epoch 2
76.2
34.4
19.3
Recurrent Trans. epoch 4
81.4
51.4
15.1
Recurrent Trans. epoch 5
74.3
32.6
20.8
Recurrent Trans. epoch 6
71.7
25.4
22.9
Univ. Trans. epoch 3
77.0
35.4
18.6
Univ. Trans. epoch 4
80.7
48.4
15.6
Appendix
Table 4: Baseline metrics across training checkpoints on 500 held-out Sudoku puzzles. Recurrent models are evaluated at segment 15; non-recurrent models at their single forward pass. Bold marks the best checkpoint of each model.
Figure 6: Cell accuracy across segments for all five architectures on the held-out 500-puzzle test set. The recurrent models (HRM, Recurrent Transformer, Universal Transformer) improve monotonically; the non-recurrent baselines (Plain Transformer, GRU) are flat single-pass.
Figure 7: Hamming distance to the ground-truth solution across segments for the same five architectures.
Sudoku
Maze
State
Cell acc.
Token acc.
Path-cell acc.
Valid S–G
Connected
zH
−19.3
−0.8
−2.7
−32.5
−26.0
zL
−19.3
−0.4
−1.4
−8.5
−7.0
Appendix
Table 5: Combined zero-ablation results. Panel A reports all-step ablation under the previously reported task metrics. Panel B reports the task-specific primary metric at selected steps k : cell accuracy for Sudoku ( 1,000 puzzles; clean baseline 81.8% ) and valid start–goal path rate for Maze ( 200 puzzles; clean baseline 93.5% ). All values are percentage-point changes from the corresponding clean rollout. “All” zeroes the indicated stream at every recurrent step.
Task
State
1
2
4
8
12
14
15
Sudoku
zH
−6.2
−3.9
−2.0
−0.4
−0.2
0.0
0.0
Sudoku
zL
+0.2
−0.2
+0.2
+0.2
−0.1
0.0
0.0
Maze
zH
−0.5
−0.4
+0.7
+0.9
+0.6
0.0
0.0
Maze
zL
+0.5
0.0
+0.1
0.0
+0.3
0.0
0.0
Appendix
Table 6: Combined freezing results. Each stream is frozen from step k onward. Sudoku reports Δ cell accuracy ( 1,000 puzzles; clean baseline 81.8% ), while Maze reports Δ valid start–goal path rate ( 1,000 puzzles; clean baseline 92.4% ). Values are percentage points.
Task
9→2
12→2
15→2
10→13
10→14
10→15
Sudoku
+0.8
+0.7
+0.6
−0.2
−0.5
−0.6
Maze
+0.3
+0.8
+0.3
+0.2
+0.6
+0.6
Appendix
Table 7: Selected within-instance zH temporal transplants. Column labels give the donor and recipient steps as donor → recipient. Sudoku reports Δ cell accuracy and Maze reports Δ valid start–goal path rate. Both experiments use 1,000 puzzles, with clean baselines of 81.8% for Sudoku and 92.4% for Maze. Values are percentage points.
Task
Donor state
1
2
4
8
12
14
15
Sudoku
zH
−55.8
−58.8
−60.6
−61.7
−62.7
−65.4
−66.7
Sudoku
zL
−0.3
−1.5
−2.3
−0.2
−0.0
+0.3
−0.1
Maze
zH
−1.6
+0.4
−0.8
+0.8
+0.4
−4.8
−34.0
Maze
zL
−40.8
−43.6
−43.2
−44.0
−44.4
−42.8
−56.0
Appendix
Table 8: Combined cross-puzzle patching results on the common step grid. Both tasks are evaluated over 250 donor–target pairs. Sudoku reports Δ cell accuracy (clean target baseline 78.3% ). Maze reports Δ valid start–goal path rate under full-grid patching (clean target baseline 92.0% ). Values are percentage points.
Figure 8: Recipient puzzle used in Figure 9 : given clues (left) and ground-truth solution (right).
Figure 9: Sudoku cross-puzzle patching example: baseline prediction on the target puzzle (left) versus prediction after donor- zH patching (right). The patched output is rewritten toward the constraints of the donor puzzle. Blue denotes a given cell, green denotes a constraint-satisfying prediction, red denotes a constraint-violating prediction, and yellow denotes a prediction changed from the previous recurrent step. A combination of yellow and red/green denotes a cell that changed value and is constraint-violating/satisfying.
Figure 10: PCA trajectories of zH across 16 segments for 200 Sudoku puzzles. (a) Solved puzzles converge to a compact cluster. (b) Failed puzzles remain broadly dispersed, including at their endpoints. (c) Overlay of solved and failed trajectories, which occupy distinct regions of principal-component space. Green dots denote step 0 and red crosses denote step 15.
Figure 11: Population-level metrics for zH across 16 segments, separated into solved and failed Sudoku puzzles: (a) consecutive-state cosine similarity, cos(zH(t),zH(t−1)) ; (b) state norm, ∣zH(t)∣ ; (c) alignment with the final state; and (d) update magnitude, ∣ΔzH(t)∣ .
Figure 12: Maze zH trajectory analysis over 300 puzzles. (a) Trajectories projected onto the first two principal components. PC1 explains 70.6% of the variance and PC2 explains 17.7% ; the trajectories are predominantly linear and PC1-dominated, in contrast to the convergence-versus-wandering pattern observed on Sudoku. Dots denote step 0 and crosses denote step 15. (b) Per-step path metrics: valid start–goal paths increase from 24.7% at step 0 to 88.7% after the first recurrent update and subsequently plateau near 93% , quantifying the early saturation in Maze.
Figure 13: Representative linear-probe sweeps for the three tasks. Rows correspond to zH and zL . Sudoku probes report given-cell identity, per-cell correctness, and the mean of row-, column-, and box-violation probes. Maze probes report validation accuracy for four global path targets. ARC-AGI-2 shows output-grid membership, per-cell correctness, and color change. Task variables are readable from both zH and zL , even when the causal interventions in Finding 4 assign different roles to the two states.
Target
zH(0)
zH(15)
zL(0)
zL(15)
cell_digit
98.5±0.1
98.9±0.1
63.6±0.4
90.9±0.5
is_given
99.6±0.1
97.2±0.5
100.0±0.0
99.9±0.0
per_cell_correct
71.3±0.3
82.5±1.2
67.7±0.6
83.3±1.5
violated_in_row
74.1±0.6
88.3±0.6
71.4±0.8
88.2±0.7
violated_in_col
74.1±0.6
87.8±0.6
70.9±0.6
87.6±0.8
violated_in_box
74.0±0.5
87.5±0.7
70.3±0.2
87.1±0.8
Appendix
Table 9: Complete Sudoku linear-probe endpoint results. Values are validation accuracy (%), reported as mean ± standard deviation over five seeds. Each probe uses 32,400 training cells and 8,100 validation cells.
Target
zH(0)
zH(15)
zL(0)
zL(15)
Global path targets
exact_solved
85.7±3.3
77.5±1.5
76.1±3.1
75.8±3.7
connects_start_goal
78.6±3.9
94.5±2.6
61.2±3.2
92.3±3.0
valid_sg_path
84.6±2.2
95.7±2.1
68.3±4.6
92.1±1.7
valid_optimal_path
83.9±3.4
86.6±1.8
72.5±2.6
84.9±1.7
path_f1
n/r
n/r
n/r
n/r
Appendix
Table 10: Complete Maze linear-probe endpoint results. Categorical targets report validation accuracy (%) as mean ± standard deviation over five puzzle-disjoint seeds. Global probes use 800/200 train/test mazes; local probes use 102,400/25,600 train/test cells. n/r denotes continuous targets whose R2 is explicitly marked unreliable because the evaluation target is nearly constant.
Target
zH(0)
zH(15)
zL(0)
zL(15)
Per-cell targets
per_cell_correct
90.2±0.6
91.6±0.5
92.6±0.4
97.7±0.3
input_inside_grid
99.9±0.1
99.7±0.2
100.0±0.0
99.9±0.0
output_inside_grid
99.9±0.1
99.8±0.1
100.0±0.0
99.9±0.0
colour_changed
96.5±0.2
97.2±0.3
98.6±0.1
99.7±0.1
same_as_input
96.0±0.2
97.6±0.3
98.7±0.0
99.7±0.1
Appendix
Table 11: Complete ARC-AGI-2 linear-probe endpoint results. Reportable categorical targets give validation accuracy (%) as mean ± standard deviation over five seeds, except shape_correct at step 15 ( n=4 valid seeds). Dashes denote one-class step-0 targets for which a classifier cannot be fit. n/r denotes near-constant continuous targets with unreliable R2 .
Figure 14: MLP-minus-linear probe gains on Maze at step 15. Nonlinear probes provide modest gains for zH and larger gains for several zL path targets, particularly start–goal connectivity and valid-path status. These gains indicate nonlinear accessibility rather than causal relevance.
Figure 15: MLP-minus-linear probe gains on Sudoku at step 15. No target gains more than 1.9 points from a nonlinear readout, and the mean gain is +0.68 for zH and −0.08 for zL . The Sudoku constraint variables are therefore predominantly linearly accessible from both streams.
Step
0
4
8
12
15
zH
V–V
+0.921
+0.947
+0.953
+0.953
+0.953
zH
C–V
−0.485
−0.369
−0.346
−0.342
−0.330
zH
PC1 (%)
93.3
95.0
95.5
95.5
95.4
zH
Rel.
0.74
0.73
0.72
0.71
0.71
zL
V–V
+0.966
+0.964
+0.970
+0.967
+0.969
Appendix
Table 12: Sudoku probe geometry across recurrent steps. V-V is the mean cosine between violation directions; C-V is the mean cosine between correctness and violation directions; PC1 is computed from the four centered, unit-normalized directions; Rel. is mean same-target agreement across seeds. Values are means over five seeds. Random-direction nulls: ∣cos∣=0.035 and PC1 =35.9% .
zH
zL
Target
Acc. (%)
Rel.
Acc. (%)
Rel.
is_wall
100.0
0.79
100.0
0.84
is_free
100.0
0.76
100.0
0.86
on_optimal_path
98.6
0.77
98.9
0.84
off_path_passable
98.5
0.47
98.4
0.85
is_corner_on_path
81.0
0.19
88.2
0.81
Appendix
Table 13: Maze probe geometry at step 15. Panel A reports validation accuracy from the geometry refits and same-target seed reliability. Panel B reports pairwise cosines among the four stable directions and PC1 for the stable set and all nine directions. Values are means over five seeds.
zH
zL
Direction pair
step 0
step 15
step 0
step 15
changed–copied
−0.873
−0.780
−0.916
−0.883
correct–copied
+0.573
+0.328
+0.274
+0.020
correct–changed
−0.551
−0.139
−0.234
+0.016
changed–boundary
+0.386
+0.256
+0.079
+0.057
correct–in-grid
+0.028
+0.257
+0.143
+0.257
Appendix
Table 14: ARC-AGI-2 geometry for six probe directions. Changed, copied, correct, boundary, in-grid, and eos denote colour_changed , same_as_input , per_cell_correct , is_object_boundary , input_inside_grid , and is_eos .
Task
n
Top-50
Rand.-50
p
Sudoku
300
−3.63
−3.42
0.22
Maze
300
−2.99
−3.35
0.18
ARC-AGI-2
150
+1.27
+1.28
0.94
Appendix
Table 15: SAE causal effects used in Figure 5 (b). Mean metric change on the primary metric of each task (i.e., cell accuracy for Sudoku, valid path rate for Maze, color-cell accuracy for ARC-AGI-2). p tests top-50 = random-50.
k
Top- k
Random- k
Top–Random
p
16
−3.08
−2.13
−0.95
2.9×10−4
32
−12.12
−12.37
+0.25
0.45
50
−3.63
−3.42
−0.21
0.23
64
−3.92
−4.44
+0.52
0.085
Appendix
Table 16: Sudoku top- k SAE sensitivity. Values are mean Δ cell accuracy (%).
Feat.
Best target
Peak step
rbest
rnext
263
box violation
0
+0.41
0.40
597
is-given
13
+0.39
0.33
1809
is-given
–
+0.39
0.21
1542
per-cell correct
0
−0.37
0.34
654
is-given
–
+0.35
0.31
211
is-given
2
−0.35
0.33
Appendix
Table 17: Most-specialized Sudoku SAE features ( D=2048 , λ=0.01 ). rbest is the Pearson correlation between the feature and the best target; rnext is the largest absolute correlation between the feature and any other target.
Model
Top-50
Random-50
paired p
Cohen’s d
Stock HRM
−1.59
−1.37
0.39
−0.06
BPTT HRM
−29.64
−29.63
0.93
−0.01
Appendix
Table 18: BPTT SAE causal-ablation details. Values are mean Δ cell accuracy (%).