Weight pruning can degrade the accuracy of large reasoning models (LRMs), yet it remains unclear whether it damages reasoning itself. We find that much of this degradation stems from a failure to stop rather than to reason. As sparsity increases, pruned LRMs increasingly fail to terminate within the token budget, often after the correct answer has appeared in their reasoning, instead falling into repetitive loops, even though they still recognize where the dense model would stop. We further find that the hidden-state difference between terminating and looping continuations is consistent across pruned checkpoints, suggesting a shared direction associated with committing to an answer. Building on this, we propose Termination-Guided Activation Steering (TGAS), a training-free method that adds this direction to the residual stream at inference time. Across six LRMs, three pruning pipelines, and math and code benchmarks, TGAS improves termination and overall accuracy while reducing repetitive reasoning and generation length, showing that this failure is actionable.
Figures & tables
Figure 1: Accuracy (left) and non-termination rate (right) on AIME-25 across pruning settings (SparseGPT with OBC calibration, 16,384-token budget, averaged over 4 runs). Accuracy decreases as non-termination becomes more frequent.
Figure 2: Gold answers in non-terminating reasoning traces of DeepSeek models on MATH-500. Left: an example of reasoning divergence, where the pruned model continues reasoning after reaching the correct answer. Right: the fraction of traces containing the gold answer and the median position of its first occurrence. n denotes the number of non-terminating samples.
Model
Ckpt
Kept
No-Term.
Wrong
Kept ∣ term
DS-Qwen -7B
obc_50
91.7 ± 0.8
3.8 ± 0.3
4.5 ± 0.6
95.3 ± 0.6
obc_60
77.8 ± 0.7
16.8 ± 0.1
5.4 ± 0.6
93.5 ± 0.7
DS-Qwen -14B
obc_50
89.2 ± 0.6
2.4 ± 1.5
8.4 ± 0.9
91.4 ± 0.8
obc_60
74.3 ± 2.8
16.4 ± 1.9
9.3 ± 0.9
88.9 ± 1.4
OT3-7B
obc_50
79.3 ± 2.9
13.1 ± 2.4
7.6 ± 1.9
91.3 ± 2.2
obc_60
63.6 ± 3.4
29.8 ± 3.3
6.6 ± 1.8
90.6 ± 2.5
Table 1: Accuracy on MATH-500 problems correctly solved by the dense model. Kept : correct; No-Term. : reasoning fails to terminate; Wrong : terminated but incorrect; Kept ∣ term : accuracy among terminated generations. From 50% to 60% sparsity, non-termination grows sharply while the terminated-but-wrong rate stays nearly flat, so accuracy among terminated generations remains high.
Figure 3: Non-termination rate and accuracy of DeepSeek models on AIME-25 under 16K- and 65K-token budgets.
DS-Qwen-7B
DS-Qwen-14B
obc_50
obc_60
obc_70
obc_50
obc_60
obc_70
AIME-25
p(</think>)
0.998
0.957
0.036
0.995
0.854
0.073
Med. rank
0
0
5.5
0
0
2
MATH-500
p(</think>)
0.996
0.924
0.040
0.998
0.907
0.130
Med. rank
0
0
7
0
0
3
Table 2: Teacher-forced stopping signal at the dense model’s STOP position: probability of </think> and its median rank (0 means </think> is the top prediction).
DS-Qwen-7B
OT3-7B
dense
obc_50
obc_60
obc_70
dense
obc_50
obc_60
obc_70
n
794
788
796
782
800
800
796
790
Δcyc
−15.3
−11.9
−9.6
−5.0
−23.1
−15.8
−13.2
−5.6
Entropy
1.55
1.51
1.41
1.44
1.03
1.22
1.33
1.60
Table 3: Log-probability ratio between cyclic and progressive continuations at decision points on AIME-25. n : number of decision points; Entropy : predictive entropy at the decision points. Less negative (larger) Δcyc values indicate a stronger preference for the repetitive continuation.
Figure 4: Progress and Cyclic behaviors illustration.
DS-Qwen-7B
DS-Qwen-14B
dense
obc_50
obc_60
obc_70
dense
obc_50
obc_60
obc_70
P (cyc ∣ cyc)
0.029
0.283
0.327
0.307
0.000
0.203
0.271
0.324
P (cyc ∣ prog)
0.026
0.043
0.064
0.085
0.029
0.033
0.059
0.081
Count (k)
1.3
51.5
85.7
113.4
0.8
47.3
68.3
136.8
Table 4: Transition dynamics between Cyclic and Progress paragraphs on AIME-25 from shared branch points. P(cyc∣cyc) measures loop persistence, while P(cyc∣prog) measures loop entry; Count : number of adjacent paragraph pairs, in thousands; dense rows have fewer pairs due to early termination, while pruned checkpoints retain reasoning divergence despite using dense correct traces.
Figure 5: Cosine similarity between termination-guided directions constructed from pruned DeepSeek models on MATH-500. We compare only checkpoints with sufficient positive-terminated and negative non-terminating samples; for example, all 1.5B checkpoints have at least 50 samples in each class. Directions constructed from different pruned checkpoints exhibit high cosine similarity.
MATH-500
AIME-25
LiveCodeBench
Avg
Model
Ckpt
Method
Acc ↑
Term ↑
Acc ↑
Term ↑
Acc ↑
Term ↑
Acc ↑
OT3-1.5B
dense
Base
80.2 ± 1.6
87.2 ± 1.9
32.5 ± 1.7
39.2 ± 3.2
43.2 ± 2.9
57.4 ± 3.3
52.0
obc_10
Base
79.6 ± 1.3
87.0 ± 0.5
34.2 ± 8.3
40.0 ± 2.7
43.4 ± 2.3
57.1 ± 2.6
52.4
No-Repeat
79.2 ± 1.9
86.6 ± 2.2
35.0 ± 6.9
41.7 ± 6.9
38.9 ± 2.6
55.5 ± 1.6
51.0
Forced-Answer
77.3 ± 3.0
—
29.2 ± 6.3
—
40.8 ± 2.6
—
49.1
SEAL
77.1 ± 1.9
87.0 ± 2.0
41.7 ± 5.8
52.5 ± 5.7
38.7 ± 2.0
61.3 ± 5.8
52.5
Table 5: Training-free remedies for SparseGPT-pruned LRMs with OBC calibration. Acc is accuracy (%) and Term is the percentage of generations that terminate before the 16,384 token cap. For each checkpoint, the best accuracy is in bold and the second best is underlined . TGAS rows are shaded; the ↑ subscript in Avg denotes its gain over the best competitor.
(a) RAC (SparseGPT)
AIME-25
LiveCodeBench
Ckpt
Method
Acc
Term
Acc
Term
rac_10
Base
35.8 ± 5.0
79.2 ± 4.2
56.3 ± 1.8
85.0 ± 3.8
No-Repeat
38.3 ± 5.8
80.0 ± 5.4
47.4 ± 3.1
85.8 ± 1.1
Forced-Answer
33.3 ± 4.7
—
53.7 ± 1.2
—
TGAS
39.2 ± 3.2
85.0 ± 6.9
58.7 ± 2.6
89.7 ± 3.0
Table 6: Training-free remedies for checkpoints pruned with (a) RAC and (b) C4-calibrated Wanda on DS-Qwen-7B. Acc is accuracy (%) and Term the share of generations that terminated (%).
Δcyc at decisions
DPs/1k
Model
Ckpt
U
S
U
S
DS-Qwen -7B
obc_50
−15.3± 0.5
−17.1± 0.5
9.0
5.4
obc_60
−12.0± 0.4
−15.4± 0.5
12.3
5.8
DS-Qwen -14B
obc_50
−14.7± 0.4
−17.9± 0.5
7.2
5.5
obc_60
−12.7± 0.4
−14.0± 0.4
11.5
6.8
Table 7: Δcyc at decision points, defined in Section 3.2 , on MATH-500, unsteered ( U ) vs. steered with TGAS ( S ). DPs/1k : decision points per 1,000 tokens.
Figure 13
Appendix figures & tables29 assets
Supplementary material from the paper’s appendix.
Appendix
Figure 7: Gold answers appearing in reasoning traces graded incorrect for DeepSeek-R1-Distill-Qwen models. Explicit and Conclusive denote gold-answer presence under nested detection rules.
comparison
n
agreement
κ
Conclusive vs judge
250
96.0%
0.898
Explicit vs judge
250
84.8%
0.782
Appendix
Table 9: Agreement between our automated measurements and the independent Gemini-2.5-Flash judge.
Figure 8: Prompt used to independently validate whether a reasoning trace asserts the reference answer as its own result.
MATH-500 L4–5
AIME-25
OpenR1-Math
Model
Ckpt
Prob.
BP
POS
NEG
Len
Prob.
BP
POS
NEG
Len
Prob.
BP
POS
NEG
Len
Deepseek-R1-Distill -Qwen-1.5B
obc_40
91
91
2780
132
1742
8
14
439
9
1515
112
120
3433
407
2585
obc_50
2545
367
1575
410
38
1347
2948
892
2484
obc_60
953
1959
2695
260
188
2076
1034
2806
3392
obc_70
0
2912
—
0
448
—
0
960
—
Deepseek-R1-Distill -Qwen-7B
obc_40
80
80
2554
6
1577
8
13
415
1
1822
72
103
3229
67
2729
Appendix
Table 10: Branch points and continuation outcomes across pruning ratios. Prob. denotes problems with at least one branch point; BP , the number of branch points; POS / NEG , self-terminating/non-terminating continuations; and Len , median positive-continuation length. Despite sharing prefixes from dense traces sufficient for correct answers, higher-pruned checkpoints produce more negative continuations and longer median continuation lengths.
Table 18
Figure 9: Cosine similarity between termination-guided directions from DeepSeek-R1-Distill-Qwen checkpoints. We include only checkpoints with sufficient positive and negative samples.
AIME25
LCB
MATH-500 L4–5
family
level
n kept
kept
n kept
kept
n kept
kept
DeepSeek-R1-Distill-Qwen-1.5B (OBC)
dense
27.0 ± 0.0
100
133.0 ± 2.8
100
197.0 ± 1.4
100
sparse10
19.5 ± 0.7
72.2 ± 2.6%
107.5 ± 3.5
80.8 ± 0.9%
175.0 ± 2.8
88.8 ± 2.1%
sparse50
15.0 ± 1.4
55.6 ± 5.2%
51.0 ± 4.2
38.3 ± 2.4%
133.5 ± 6.4
67.8 ± 2.7%
sparse60
1.5 ± 0.7
5.6 ± 2.6%
0.0 ± 0.0
0.0 ± 0.0%
39.5 ± 9.2
20.0 ± 4.5%
DeepSeek-R1-Distill-Qwen-7B (OBC)
dense
47.0 ± 5.7
100
223.5 ± 2.1
100
223.0 ± 2.8
100
Appendix
Table 13: Dense-correct generations retained by the pruned model. n kept : correct for both models; kept : fraction of dense-correct generations also solved by the pruned model. Values are mean ± std.
AIME25
LCB
MATH-500 L4–5
family
level
(a)
(b)
(c)
kept ∣ term.
(a)
(b)
(c)
kept ∣ term.
(a)
(b)
(c)
kept ∣ term.
DeepSeek-R1-Distill-Qwen-1.5B (OBC)
sparse10
19.5 (72)
2.5 (9)
5.0 (19)
79.7 ± 5.2%
107.5 (81)
4.5 (3)
21.0 (16)
83.7 ± 0.5%
175.0 (89)
5.5 (3)
16.5 (8)
91.4 ± 0.5%
sparse50
15.0 (56)
6.5 (24)
5.5 (20)
73.1 ± 4.4%
51.0 (38)
50.5 (38)
31.5 (24)
61.8 ± 1.4%
133.5 (68)
38.0 (19)
25.5 (13)
83.9 ± 3.3%
sparse60
1.5 (6)
22.5 (83)
3.0 (11)
35.0 ± 21.2%
0.0 (0)
132.5 (100)
0.5 (0)
0.0%
39.5 (20)
142.5 (72)
15.0 (8)
72.0 ± 8.5%
DeepSeek-R1-Distill-Qwen-7B (OBC)
sparse10
33.0 (71)
4.0 (9)
10.0 (21)
77.5 ± 9.5%
198.0 (89)
4.5 (2)
21.0 (9)
90.4 ± 1.3%
208.0 (93)
3.0 (1)
12.0 (5)
94.5 ± 1.3%
sparse50
30.0 (64)
8.5 (18)
8.5 (18)
78.1 ± 3.5%
145.0 (65)
25.5 (11)
53.0 (24)
73.2 ± 1.0%
204.5 (92)
8.5 (4)
10.0 (4)
95.3 ± 0.6%
Appendix
Table 14: Breakdown of dense-correct generations under pruning. Each generation is (a) correct , (b) reasoning divergence (no </think> ), or (c) terminated-wrong ( </think> emitted but the final answer is incorrect). Values show mean count per run with the corresponding percentage in parentheses. Kept ∣ term. denotes accuracy among terminated generations. Italics indicate fewer than 30 terminated dense-correct items.
Model
Level
Token Limit
Non-termination Rate
Accuracy
Accuracy ∣ Terminated
Recovered Traces
Recovered Correct
7B
dense
16,384
20.0%
42.5%
53.1%
—
—
7B
dense
65,536
3.3%
42.5%
44.0%
20 / 24 (83.3%)
0
7B
obc_10
16,384
20.0%
31.7%
39.6%
—
—
7B
obc_10
65,536
2.5%
31.7%
32.5%
21 / 24 (87.5%)
0
7B
obc_50
16,384
42.5%
30.0%
52.2%
—
—
7B
obc_50
65,536
12.5%
30.8%
35.2%
36 / 51 (70.6%)
2
Appendix
Table 15: Effect of increasing the generation budget from 16,384 to 65,536 tokens on the DeepSeek-R1-Distill-Qwen 7B and 14B models on AIME-25. A 65,536 row continues each generation non-termination at 16,384 from its own tokens; recovered = non-termination at 16,384 and terminated by 65,536.
AIME-25
MATH-500 L4–5
Model
Ckpt
p( </think> )
Med. rank
logp STOP
logp </think> OTHER
Δclose
Δstop
p( </think> )
Med. rank
logp STOP
logp </think> OTHER
Δclose
Δstop
Deepseek-R1-Distill -Qwen-1.5B
obc_10
0.9999
0
−0.000
−22.71
+22.71
+0.377
0.9999
0
−0.000
−23.11
+23.11
+0.374
obc_50
0.9804
0
−0.020
−27.28
+27.26
+0.400
0.9918
0
−0.008
−26.51
+26.50
+0.428
obc_60
0.7273
0
−0.379
−26.56
+26.18
+0.134
0.6301
0
−0.771
−26.06
+25.29
−0.226
obc_70
0.0351
44
−9.192
−24.19
+14.99
−8.431
0.0154
157.5
−11.076
−23.84
+12.76
−10.229
Deepseek-R1-Distill -Qwen-7B
obc_10
1.0000
0
−0.000
−23.30
+23.30
+0.361
0.9999
0
−0.000
−24.15
+24.15
+0.349
Appendix
Table 16: Teacher-forced evaluation of the stopping signal at the dense model’s </think> position. STOP denotes the position whose next token is the dense </think> , while OTHER includes all trace positions more than 50 tokens before STOP. Logp STOP is the mean log-probability of </think> at STOP. Logp </think> OTHER is the mean log-probability of </think> over OTHER positions, and Δclose = logp STOP − logp </think> OTHER. Δstop = logp STOP − the mean log-probability of the token the dense model actually generated at OTHER positions. Rank 0 indicates that </think> is the model’s top prediction at STOP. All quantities are computed by teacher-forcing dense-model traces through each checkpoint.
AIME-25
LCB
MATH-500 L4–5
Model
Ckpt
ndec
nctrl
Dec.
Ctrl.
Ent.
ndec
nctrl
Dec.
Ctrl.
Ent.
ndec
nctrl
Dec.
Ctrl.
Ent.
DS-Qwen 1.5B
dense
796
200
−13.0
−13.2
1.60
720
200
−16.7
−15.5
1.46
684
200
−14.9
−21.9
1.55
obc_10
794
200
−13.4
−13.4
1.62
734
200
−16.0
−12.5
1.44
690
200
−15.3
−23.3
1.57
obc_50
786
194
−8.4
−11.7
1.32
788
192
−8.2
−5.3
1.48
686
200
−10.2
−15.3
1.35
obc_60
762
182
−3.8
−4.5
1.33
756
172
−2.7
−3.2
1.33
762
198
−5.6
−6.8
1.32
obc_70
524
154
1.6
−1.2
1.48
556
158
2.2
−0.9
1.60
506
160
1.0
−0.8
1.54
Appendix
Table 17: Log-probability ratio between cyclic and progressive continuations on AIME-25, LCB and MATH-500. Δcyc=logP(CYCLIC)−logP(PROGRESS) , scored over the next 32 tokens. Dec. is measured at decision points, Ctrl. at randomly sampled paragraph in the same traces; Ent. is the predictive entropy at the decision points. ndec is the number of decision points and nctrl the number of control points. More negative means the model prefers the progressive continuation more strongly.
MATH-500 L4–5
AIME-25
Model
Ckpt
P (cyc ∣ cyc)
P (cyc ∣ prog)
Count
P (cyc ∣ cyc)
P (cyc ∣ prog)
Count
DeepSeek-R1-Distill-Qwen 7B
dense
0.157
0.025
4094
0.029
0.026
1296
obc_40
0.206
0.027
155830
0.230
0.031
38947
obc_50
0.257
0.047
207714
0.283
0.043
51479
obc_60
0.325
0.086
434020
0.327
0.064
85675
obc_70
0.307
0.105
596073
0.307
0.085
113353
Appendix
Table 18: Transition dynamics between cyclic and progressive states on MATH-500 Levels 4–5 and AIME-25, measured on continuations from a shared dense branch point. P(cyc∣cyc) and P(cyc∣prog) measure loop persistence and loop entry over adjacent paragraph pairs; Count is the number of such pairs.
Table 26
(a) OpenR1-Math
Model
Levels
Transfer
Own
Shuffled
p
DS-Qwen-1.5B
40 → 50
0.973
0.976
0.839 / 0.950
0.005
DS-Qwen-1.5B
40 → 60
0.974
0.977
0.848 / 0.945
0.005
DS-Qwen-1.5B
50 → 40
0.971
0.970
0.833 / 0.940
0.005
DS-Qwen-1.5B
50 → 60
0.977
0.977
0.851 / 0.956
0.005
DS-Qwen-1.5B
60 → 40
0.960
0.970
0.827 / 0.933
0.010
Appendix
Table 21: Held-out ROC AUC for separating terminating from non-terminating continuations when a direction constructed at one sparsity level (%) is applied to another. Transfer : AUC of the source direction on the target’s held-out continuations; Own : AUC of the target’s own direction; Shuffled : mean / 95th percentile of the transfer AUC when the source labels are permuted (200 permutations), with permutation p -value p .
Figure 10: PCA of continuation representations at the steering layer on OpenR1-Math. For each model, PCA is fitted jointly on the continuations of all sparsity levels. Terminating continuations form a compact cluster, while non-terminating ones spread widely along the same PC1 at every sparsity level.
Accuracy (%)
Under TGAS (%)
Model
Ckpt
#NT
Base
Oracle
TGAS
NT → Correct
Wrong → Correct
Kept
DS-Qwen-1.5B
dense
22
75.2 ± 0.5
75.8 ± 1.4
77.1 ± 5.4
19.6 ± 4.0
43.2 ± 9.5
90.9 ± 1.5
obc_10
22
73.3 ± 2.7
73.7 ± 2.7
76.7 ± 0.0
29.0 ± 9.5
42.5 ± 6.5
90.7 ± 4.8
obc_50
72
54.8 ± 2.4
57.4 ± 3.0
61.8 ± 4.9
33.7 ± 1.9
42.2 ± 13.9
82.2 ± 2.3
obc_60
204
15.5 ± 3.0
32.6 ± 3.0
30.9 ± 1.1
25.8 ± 0.3
9.2 ± 5.5
65.8 ± 3.9
DS-Qwen-7B
dense
4
85.1 ± 1.1
85.1 ± 1.1
86.3 ± 1.9
16.6 ± 23.5
44.0 ± 3.7
94.2 ± 2.5
Appendix
Table 22: Headroom and recovery on MATH-500. #NT : number of non-terminating Base generations. Oracle : Base accuracy if every non-terminating generation that already states the gold answer explicitly were counted as correct. NT → Correct and Wrong → Correct : share of non-terminating and terminated-wrong Base generations that become correct under TGAS. Kept : share of Base-correct generations that remain correct.
MATH-500
AIME-25
LiveCodeBench
Model
Ckpt
Method
Acc
Term
Acc
Term
Acc
Term
Deepseek-R1-Distill -Qwen-1.5B
dense
Base
75.2 ± 0.5
91.6 ± 0.0
22.5 ± 4.2
71.7 ± 5.8
34.5 ± 1.6
76.1 ± 1.8
No-Repeat
71.0 ± 0.0
91.2 ± 0.5
22.5 ± 4.2
74.2 ± 7.4
27.6 ± 1.6
80.8 ± 3.9
Forced-Answer
74.2 ± 2.4
—
22.5 ± 4.2
—
35.3 ± 1.8
—
SEAL
73.7 ± 2.1
95.8 ± 0.6
18.3 ± 1.9
76.7 ± 6.1
35.0 ± 3.7
84.7 ± 3.3
TGAS
77.1 ± 5.4
92.4 ± 1.3
25.8 ± 4.2
75.8 ± 5.7
36.8 ± 2.6
86.3 ± 3.7
Appendix
Table 23: Training-free remedies for SparseGPT-pruned LRMs with OBC calibration DeepSeek-R1-Distill-Qwen models. Acc. and Term. denote accuracy and termination rate, respectively. Forced-Answer’s Term. is not reported, since forcing ends every generation by design.
MATH-500
AIME-25
LiveCodeBench
Model
Ckpt
Method
Acc
Term
Acc
Term
Acc
Term
Deepseek-R1-Distill -Llama-8B
dense
Base
82.4 ± 2.7
97.9 ± 0.8
34.2 ± 5.7
70.0 ± 11.2
58.7 ± 2.5
85.5 ± 1.8
No-Repeat
83.0 ± 0.8
97.7 ± 0.0
35.0 ± 4.3
71.7 ± 3.3
44.7 ± 3.3
85.0 ± 1.3
Forced-Answer
81.3 ± 1.6
—
30.8 ± 1.7
—
53.2 ± 1.1
—
SEAL
80.2 ± 1.8
97.7 ± 0.3
32.5 ± 5.7
87.5 ± 4.2
57.9 ± 2.7
91.6 ± 2.3
TGAS
84.0 ± 2.2
97.3 ± 0.0
38.3 ± 5.8
80.8 ± 3.2
60.8 ± 2.2
87.9 ± 1.8
Appendix
Table 24: Training-free remedies for SparseGPT-pruned LRMs with OBC calibration DeepSeek-R1-Distill-Llama and OpenThinker3 models. Acc. and Term. denote accuracy and termination rate, respectively. Forced-Answer’s Term. is not reported, since forcing ends every generation by design.
MATH-500
AIME-25
LiveCodeBench
Model
Ckpt
Method
Acc
Term
Acc
Term
Acc
Term
Deepseek-R1-Distill -Qwen-1.5B
obc_50
Base
54.8 ± 2.4
72.5 ± 0.5
13.3 ± 2.7
46.7 ± 4.7
13.4 ± 2.2
32.1 ± 4.3
Forced-Answer
59.4 ± 3.0
—
13.3 ± 2.7
—
16.6 ± 3.1
—
TGAS
61.8 ± 4.9
85.1 ± 1.9
15.8 ± 1.7
58.3 ± 5.8
16.1 ± 1.0
51.8 ± 6.9
Masked-LoRA
60.3 ± 3.1
82.1 ± 1.4
11.7 ± 5.8
52.5 ± 5.7
15.3 ± 1.8
30.8 ± 4.3
obc_60
Base
15.5 ± 3.0
21.9 ± 1.9
1.7 ± 1.9
6.7 ± 4.7
0.0 ± 0.0
0.5 ± 0.6
Appendix
Table 25: Training-free remedies vs. Masked-LoRA for SparseGPT-pruned LRMs with OBC calibration DeepSeek-R1-Distill-Qwen models. Forced-Answer’s Term is not reported, since forcing ends every generation by design.
Δcyc at decisions
DPs/1k
Model
Ckpt
U
S
U
S
Deepseek-R1-Distill -Qwen-1.5B
dense
−14.9± 0.5
−16.4± 0.6
8.1
5.6
obc_10
−15.3± 0.5
−16.5± 0.6
8.0
5.3
obc_50
−10.2± 0.4
−12.7± 0.5
9.2
5.2
obc_60
−5.6± 0.3
−6.3± 0.4
23.7
10.5
Deepseek-R1-Distill -Qwen-7B
dense
−18.2± 0.6
−20.5± 0.6
7.1
4.7
Appendix
Table 26: Δcyc at decision points on MATH-500 L4–5 with and without TGAS. U : unsteered traces; S : steered traces scored with the steering hook. More negative values indicate stronger preference for moving on. DPs/1k : decision points per 1,000 generated tokens.
AIME-25
LiveCodeBench
Model
Ckpt
Base
D =4096
D =8192
D =16384
Base
D =4096
D =8192
Deepseek-R1-Distill -Qwen-1.5B
dense
22.5 ± 4.2
19.2 ± 1.7
22.5 ± 4.2
22.5 ± 4.2
34.5 ± 1.6
35.0 ± 1.8
35.3 ± 1.8
obc_10
21.7 ± 4.3
16.7 ± 4.7
21.7 ± 6.9
21.7 ± 4.3
32.9 ± 2.6
33.7 ± 1.9
33.4 ± 3.4
obc_50
13.3 ± 2.7
12.5 ± 3.2
14.2 ± 1.7
13.3 ± 2.7
13.4 ± 2.2
19.2 ± 4.6
16.6 ± 3.1
obc_60
1.7 ± 1.9
2.5 ± 3.2
2.5 ± 3.2
2.5 ± 3.2
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
obc_70
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
Appendix
Table 27: Effect of the Forced-Answer budget. Accuracy (%) after forcing an answer at D reasoning tokens, compared with the unforced checkpoint ( Base ). Generations finishing before D are kept unchanged, and at D=16,384 only capped generations are forced.
AIME-25
LiveCodeBench
Base
n =8
n =32
Base
n =8
n =32
Model
Ckpt
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
DS-Qwen -1.5B
dense
22.5 ± 4.2
71.7 ± 5.8
7.5 ± 4.2
68.3 ± 7.9
22.5 ± 4.2
74.2 ± 7.4
34.5 ± 1.6
76.1 ± 1.8
8.9 ± 1.4
79.7 ± 1.0
27.6 ± 1.6
80.8 ± 3.9
obc_10
21.7 ± 4.3
71.7 ± 1.9
6.7 ± 2.7
66.7 ± 4.7
24.2 ± 5.7
70.0 ± 8.6
32.9 ± 2.6
76.6 ± 3.1
8.9 ± 2.6
77.9 ± 4.8
30.5 ± 2.3
82.4 ± 3.0
obc_50
13.3 ± 2.7
46.7 ± 4.7
3.3 ± 0.0
46.7 ± 9.8
17.5 ± 5.7
51.7 ± 7.9
13.4 ± 2.2
32.1 ± 4.3
1.8 ± 1.0
40.3 ± 5.0
9.5 ± 2.6
40.8 ± 6.2
obc_60
1.7 ± 1.9
6.7 ± 4.7
0.0 ± 0.0
42.5 ± 13.4
2.5 ± 5.0
23.3 ± 4.7
0.0 ± 0.0
0.5 ± 0.6
0.0 ± 0.0
31.6 ± 3.6
0.0 ± 0.0
7.4 ± 0.9
Appendix
Table 28: No-Repeat n -gram size. Acc is accuracy (%) and Term the share of generations that terminated (%). All three columns of a dataset come from the same evaluation pass, so Base here is that pass’s unblocked run.
α=−2
α=−1.5
α=−1
α=0
α=+1
α=+1.5
α=+2
Model
Test
Ckpt
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Deepseek-R1-Distill -Qwen-7B
AIME
dense
31.7 ± 4.3
70.0 ± 6.1
37.5 ± 5.0
83.3 ± 7.2
37.5 ± 4.2
81.7 ± 1.9
42.5 ± 5.7
80.0 ± 7.2
41.7 ± 6.9
79.2 ± 8.8
42.5 ± 6.9
82.5 ± 3.2
36.7 ± 4.2
83.3 ± 5.0
obc_10
32.5 ± 3.2
70.0 ± 4.7
37.5 ± 9.2
76.7 ± 3.8
37.5 ± 4.2
80.0 ± 6.1
31.7 ± 5.8
80.0 ± 7.2
40.0 ± 6.1
76.7 ± 8.2
43.3 ± 3.2
85.0 ± 9.6
36.7 ± 2.7
88.3 ± 7.2
obc_50
27.5 ± 6.3
33.3 ± 7.2
25.8 ± 5.7
45.8 ± 10.7
30.8 ± 6.9
55.0 ± 4.3
30.0 ± 4.7
57.5 ± 7.4
30.0 ± 4.7
63.3 ± 4.7
25.8 ± 4.2
64.2 ± 4.2
25.8 ± 3.2
72.5 ± 4.3
obc_60
10.0 ± 2.7
13.3 ± 4.7
14.2 ± 4.2
16.7 ± 4.7
15.8 ± 5.7
25.8 ± 3.2
22.5 ± 6.3
43.3 ± 8.6
22.5 ± 1.7
49.2 ± 4.2
20.8 ± 7.4
55.0 ± 3.3
20.0 ± 3.2
63.3 ± 8.2
LCB
dense
55.3 ± 2.2
73.4 ± 2.0
57.4 ± 2.2
80.5 ± 3.7
55.8 ± 2.7
81.1 ± 1.5
59.2 ± 3.8
83.7 ± 1.4
59.7 ± 2.2
89.2 ± 3.0
59.7 ± 1.3
90.0 ± 3.3
56.1 ± 1.4
91.3 ± 1.3
Appendix
Table 29: Effect of steering strength α on accuracy and termination. Each model is steered with its main direction scaled by α , absolute steering coefficient; α=0 is the unsteered checkpoint and negative α pushes against the direction. Acc is accuracy (%) and Term the share of generations that terminated (%).
n50
n75
n100
Full
Model
Test
Ckpt
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Deepseek-R1-Distill -Qwen-7B
AIME
dense
38.3 ± 6.4
82.5 ± 9.2
36.7 ± 9.0
79.2 ± 5.7
42.5 ± 4.2
80.0 ± 3.8
41.7 ± 6.9
79.2 ± 8.8
obc_10
35.8 ± 3.2
83.3 ± 8.6
38.3 ± 4.3
80.8 ± 3.2
35.8 ± 1.7
80.8 ± 3.2
43.3 ± 4.7
80.8 ± 1.7
obc_50
31.7 ± 4.3
69.2 ± 3.2
28.3 ± 6.4
62.5 ± 5.0
30.8 ± 8.8
69.2 ± 4.2
31.7 ± 3.3
68.3 ± 6.9
obc_60
18.3 ± 4.3
55.0 ± 5.8
20.0 ± 3.8
57.5 ± 3.2
25.0 ± 6.4
62.5 ± 3.2
22.5 ± 3.2
54.2 ± 3.2
LCB
dense
58.2 ± 2.3
87.4 ± 3.1
57.9 ± 2.3
86.3 ± 0.9
57.9 ± 2.3
88.4 ± 2.6
59.7 ± 2.2
89.2 ± 3.0
Appendix
Table 30: TGAS with directions built from different numbers of samples per class. Acc is accuracy (%) and Term the share of generations that terminated (%). n50 , n75 and n100 use 50, 75 and 100 samples per class; Full is the direction used in the main experiments. We use the same α values as in the main experiments for each configuration.
Model
SEAL
Layer mean
Unit-avg
DS-Qwen-1.5B
dense
0.172
0.108
obc_50
0.245
0.269
DS-Qwen-7B
dense
0.283
0.187
obc_50
0.306
0.265
DS-Qwen-14B
dense
0.269
0.221
obc_50
0.244
0.214
Appendix
Table 31: Cosine between the steering directions of SEAL and TGAS, averaged over decoder layers. Layer mean : cosine of the unnormalised layer averages; Unit-avg : each layer normalised before averaging.
AIME-25
LiveCodeBench
Model
Pruner
Ckpt
Method
Acc
Term
Acc
Term
Deepseek-R1-Distill -Qwen-7B
RAC
10%
Base
35.8 ± 5.0
79.2 ± 4.2
56.3 ± 1.8
85.0 ± 3.8
No-Repeat
38.3 ± 5.8
80.0 ± 5.4
47.4 ± 3.1
85.8 ± 1.1
Forced-Answer
33.3 ± 4.7
—
53.7 ± 1.2
—
TGAS
39.2 ± 3.2
85.0 ± 6.9
58.7 ± 2.6
89.7 ± 3.0
50%
Base
24.2 ± 5.7
32.5 ± 5.7
32.6 ± 1.7
37.4 ± 2.2
Appendix
Table 32: Training-free remedies on checkpoints pruned with SparseGPT-pruned LRMs with RAC calibration and with C4 calibration (Wanda prune). Acc is accuracy (%) and Term the share of generations that terminated (%). Forced-Answer’s Term is not reported, since forcing ends every generation by design. Best accuracy per checkpoint and dataset in bold.
Figure 40
AIME-25
LiveCodeBench
SEAL (dense)
SEAL (50%)
TGAS
SEAL (dense)
SEAL (50%)
TGAS
Model
Ckpt
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
Acc
Term
DS-Qwen-1.5B
dense
18.3 ± 1.9
76.7 ± 6.1
19.5 ± 2.9
75.8 ± 8.3
25.8 ± 4.2
75.8 ± 5.7
35.0 ± 3.7
84.7 ± 3.3
35.5 ± 2.2
86.6 ± 3.0
36.8 ± 2.6
86.3 ± 3.7
obc_10
21.7 ± 4.3
80.8 ± 7.4
25.0 ± 6.9
76.7 ± 8.6
26.7 ± 4.7
75.0 ± 6.9
34.5 ± 4.6
85.5 ± 3.0
33.4 ± 3.6
81.3 ± 2.2
37.1 ± 2.6
83.2 ± 2.9
obc_50
15.8 ± 3.2
60.0 ± 6.7
16.7 ± 2.7
57.5 ± 5.7
15.8 ± 1.7
58.3 ± 5.8
13.2 ± 3.5
46.3 ± 4.0
15.3 ± 1.8
45.8 ± 5.0
16.1 ± 1.0
51.8 ± 6.9
obc_60
5.0 ± 6.4
27.5 ± 7.4
3.3 ± 2.7
24.2 ± 7.4
5.0 ± 1.9
40.0 ± 4.7
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
0.0 ± 0.0
2.4 ± 1.8
Appendix
Table 34: SEAL with steering vectors built on the dense model or on the 50% checkpoint, compared with TGAS. Acc : accuracy (%); Term : termination rate (%). Best accuracy per checkpoint and benchmark in bold.
AIME-25
LiveCodeBench
Model
Sparsity
TGAS
Plan-CoT dense
Plan-CoT 50%
TGAS
Plan-CoT dense
Plan-CoT 50%
Accuracy (%)
DS-Qwen-7B
Dense
41.7±6.9 (+1)
37.5±3.2 ( − 0.2)
39.2±5.0 ( − 0.2)
59.7±2.2 (+1)
58.7±2.8 ( − 0.1)
60.3±3.0 ( − 0.2)
10%
43.3±4.7 (+1.25)
40.0±2.7 ( − 0.1)
38.3±4.3 ( − 0.2)
60.3±3.4 (+1.25)
60.3±2.3 ( − 0.1)
57.9±3.5 ( − 0.2)
50%
31.7±3.3 (+0.5)
29.2±3.2 ( − 0.1)
31.7±7.9 ( − 0.2)
41.8±2.3 (+0.5)
40.3±1.3 ( − 0.1)
40.0±3.1 ( − 0.2)
60%
22.5±3.2 (+1.25)
22.5±3.2 ( − 0.1)
25.0±1.9 ( − 0.1)
19.5±3.3 (+1.25)
15.5±1.8 ( − 0.2)
13.7±1.2 ( − 0.2)
Appendix
Table 35: TGAS at its best steering strength α (shown in parentheses) compared with Plan-CoT at best steering coefficient, using a steering vector extracted from the dense model (Plan-CoT dense ) or from the 50%-sparse model (Plan-CoT 50% ). Best value per row and benchmark in bold.
Large Language Models (LLMs) have achieved remarkable success in complex reasoning tasks through Chain-of-Thought (CoT) prompting. However, these models often exhibit "computational overthinking," generating redundant reasoning steps that increase latency and cost without improving accuracy. Recent studies suggest that CoT trajectories can be significantly pruned, yet existing methods often rely on forcing a static thinking budget, heuristic filtering, sub-optimal early exit via classification, or expensive re-training. In this paper, we introduce OS-Pruner, a lightweight plug-in framework that formulates chain-of-thought pruning as an optimal stopping problem. Given a reasoning prefix, OS-Pruner learns whether further reasoning is worth its token cost by optimizing an explicit utility that trades off final-answer accuracy against generated length. Our novel formulation enables the model to dynamically assess the sufficient point of termination for a reasoning chain. OS-Pruner is designed to be lightweight during both training and inference, and to provide users with fine-grained control over the reasoning-effort vs. accuracy trade-off. On diverse reasoning benchmarks and base models, OS-Pruner achieves 20-60% reduction in generation length with minimal accuracy sacrifice.
Mohammed Ehab, Aymane El Gadarri, Vivek F. Farias +2
Large Reasoning Models (LRMs) achieve strong performance on complex tasks through extended chain-of-thought generation, but incur substantial computational costs during inference. In production settings, batched inference is essential for high throughput, yet the existing training-free adaptive pruning methods we evaluate severely degrade in this regime. Because a batch must share a single pruning mask, these methods aggregate activations across samples and then apply threshold-based selection; the threshold, calibrated offline on unaggregated activations, no longer matches the aggregated distribution, so the realized sparsity ratio drifts and accuracy on reasoning tasks collapses under batched inference. In this work, we propose a training-free adaptive pruning method designed specifically for batched inference in LRMs, built on two components. First, we replace threshold-based selection with periodic top-k selection over the aggregated importance scores, which is unaffected by the shift that aggregation induces in the activation distribution, and which runs selection once per update period rather than at every token, preserving the speedup. Second, based on the observation that important neurons re-fire periodically during long reasoning generation, we introduce an activation memory that accumulates importance across update phases so that recurring neurons are retained. Experiments on diverse reasoning benchmarks demonstrate that our method outperforms the previous state-of-the-art adaptive pruning method by 39.7 percentage points in average accuracy at batch size 4 with 50% target sparsity on DeepSeek-R1-Distill-Qwen-7B, and reaches 1.40x speedup over dense inference at 50% actual sparsity.
Large reasoning models (LRMs) generate long chain-of-thought traces before answering, creating significant inference overhead. Pruning can reduce this cost, but its effectiveness depends on the calibration data used to estimate parameter importance. Recent work calibrates on the model's own rollouts instead of generic dataset, but treats all reasoning tokens uniformly, regardless of whether they contribute to successful reasoning. As a result, pruning protects weights by statistical salience rather than by their contribution to correct reasoning, so weights behind erroneous computation survive as readily as those behind correct computation. These erroneous patterns then get carried into the pruned model, degrading reasoning quality, producing both lower accuracy and longer reasoning traces. We propose Outcome-Based Calibration for Large Reasoning Model Pruning (OBC-Prune) to close this gap. OBC first constructs difficulty-matched pairs of correct and incorrect rollouts from problems the model answers inconsistently. It then estimates the causal importance of each reasoning sentence through intervention-based analysis, quantifying how removing its influence affects subsequent predictions. These causal importance scores are converted into per-token weights that rescale the calibration activations used by one-shot pruning methods (SparseGPT, Wanda, ALPS), without modifying the underlying pruning algorithms. Experiments on DeepSeek-R1-Distill-Qwen 1.5B, 7B, and 14B models at 40% and 50% sparsity demonstrate consistent improvements over state-of-the-art calibration baselines across most model sizes and sparsity levels on MATH500, LiveCodeBench, and AIME 2025. These results indicate that preserving causally important reasoning circuits is a substantially more effective pruning objective than uniformly preserving observed activations.