Correct but Unselectable: The Hidden Interface Tax in Multi-Candidate Reasoning
Organizations: Faculty of Arts and Science, Beijing Normal University, Zhuhai 519087, China · Institute of Artificial Intelligence and Future Networks, Beijing Normal University, Zhuhai 519087, Guangdong, China · School of Artificial Intelligence, Beijing Normal University, Beijing 100875, China · Guangdong Key Lab of AI and Multi-Modal Data Processing, Beijing Normal-Hong Kong Baptist University, Zhuhai 519087, China
Abstract
Small language models deployed under cost, latency, or privacy constraints cannot simply switch to larger models when reasoning fails. Multi-candidate generate-then-rerank offers a natural test-time scaling route. However, more candidates do not always yield higher accuracy. We identify a previously overlooked bottleneck between generation and selection: a correct answer may appear in the candidate pool yet fail to be selected because it is incomplete, unparseable, or misaligned with the answer mode the fixed selector expects. We term this loss the hidden interface tax and formalize a coverage-conversion decomposition that distinguishes raw oracle coverage from selectable coverage, the subset of problems where a correct candidate can actually be consumed by the selector. We propose Compatibility-Aware Candidate Construction (CACC), an interface layer between the candidate source and the fixed selector that repairs fragments, aligns answer modes, and removes selector-confusing noise without retraining either component. On numeric reasoning (GSM8K, competition_math), CACC raises both oracle coverage and final accuracy. On GPQA Diamond, CACC alone improves final accuracy from 5.6% to 19.2% (+13.6 pp); combined with proposer-side strengthening it reaches 25.8% (+20.2 pp). On MMLU-Pro, CACC exposes a coverage-conversion gap: answer-mode-compatible construction raises oracle coverage, but final accuracy decreases by 0.85 pp under the original selector; adding proposer-side strengthening improves final accuracy.
Figures & tables
| Regime | Dataset | Method | Oracle | V|O | Final | Final | Ratio |
|---|---|---|---|---|---|---|---|
| Numeric | GSM8K | Base | 0.4882 | 0.5885 | 0.2873 | ref. | 1.00 |
| Numeric | GSM8K | SP † | 0.6471 | 0.5714 | 0.3698 | +8.25pp | 1.29 |
| Numeric | GSM8K | CACC | 0.6391 | 0.5682 | 0.3632 | +7.59pp | 1.26 |
| Numeric | GSM8K | CACC + SP † | 0.6962 | 0.6017 | 0.4189 | +13.16pp | 1.46 |
| Numeric | CompMath | Base | 0.3210 | 0.4732 | 0.1519 | ref. | 1.00 |
| Numeric | CompMath | SP † | 0.4616 | 0.6014 | 0.2776 | +12.57pp | 1.83 |
| Unit | Comparison | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| MMLU-Pro | no- AMC numeric CACC | 0.0369 | 0.4767 | 0.2590 | 0.4777 | 0.0096 | 0.2277 | +43.98 | +21.87 | +21.81 |
| MMLU-Pro | CCR+ AMC CACC | 0.4775 | 0.4767 | 0.4754 | 0.4777 | 0.2270 | 0.2277 | -0.08 | +0.23 | +0.07 |
| MMLU-512 | diverse CACC | 0.4609 | 0.5547 | 0.4492 | 0.3592 | 0.2070 | 0.1992 | +9.38 | -9.00 | -0.78 |
| MMLU-512 | CACC +hygiene | 0.5547 | 0.5547 | 0.3592 | 0.4049 | 0.1992 | 0.2246 | +0.00 | +4.58 | +2.54 |
| GSM8K full | SP PS x CACC | 0.6111 | 0.7195 | 0.5943 | 0.4700 | 0.3632 | 0.3381 | +10.84 | -12.43 | -2.50 |
| Type | Dataset | Method / transition | Oracle | V|O | Final | |
|---|---|---|---|---|---|---|
| Readout | MMLU-Pro | numeric scoring MCQ reparse | 0.0369 0.4072 | 0.2590 0.5800 | 0.0096 0.2362 | +22.66pp |
| AMC | MMLU-Pro | MCQ reparse CACC | 0.4072 0.4767 | 0.5800 0.4777 | 0.2362 0.2277 | -0.85pp |
| SP | MMLU-Pro | CACC CACC + SP | 0.4767 0.5527 | 0.4777 0.5312 | 0.2277 0.2936 | +6.59pp |
| SP | GPQA | CACC CACC + SP | 0.4545 0.4899 | 0.4222 0.5258 | 0.1919 0.2576 | +6.57pp |
| Recovery | MMLU-Pro | CACC +hygiene | 0.5547 0.5547 | 0.3592 0.4049 | 0.1992 0.2246 | +2.54pp |
| Recovery | GPQA | CACC +hygiene | 0.5127 0.4931 | 0.3961 0.5386 | 0.2031 0.2656 | +6.25pp |
| Control | Evaluation | Base | CACC | ||
|---|---|---|---|---|---|
| Target verifier, canonical | GPQA | 198 | 16.16 | 24.92 | +8.75 |
| Target verifier, canonical | MMLU held-out | 11,520 | 26.40 | 27.84 | +1.44 |
| 32B candidate judge | GPQA | 198 | 6.57 | 23.23 | +16.67 |
| Mistral generator | MMLU confirm | 512 | 14.22 | 15.94 | +1.72 |
| Prospective ARC reuse | ARC-Challenge | 256 | 69.92 | 70.70 | +0.78 |
| Fresh generation | Evaluation | CACC | CD | CD+ CACC |
Appendix figures & tables21 assets
Supplementary material from the paper’s appendix.
Appendix
| Evidence unit | Mode | Oracle | Final | V|O | Use in paper | |
|---|---|---|---|---|---|---|
| GSM8K Base | 1319 | numeric | 644/1319 | 379/1319 | 0.5885 | Numeric baseline |
| GSM8K CACC | 1319 | numeric | 843/1319 | 479/1319 | 0.5682 | Numeric main result |
| CompMath Base | 3199 | numeric | 1027/3199 | 486/3199 | 0.4732 | Numeric baseline |
| CompMath CACC | 3199 | numeric | 1419/3199 | 825/3199 | 0.5814 | Numeric main result |
| MMLU no- AMC numeric | 12032 | numeric | 444/12032 | 115/12032 | 0.2590 | Archived collapse provenance |
| MMLU MCQ reparse | 12032 | choice_letter | 4900/12032 | 2842/12032 | 0.5800 | Readout recovery |
| Component | Work and accounting boundary |
|---|---|
| Upstream generation | Audited contract: 16 sequences, at most 128 new tokens each; pool construction precedes retention. |
| Completion construction | One additional batch of 4 sequences, at most 160 new tokens each, per example. |
| Routing / normalization | Deterministic diagnostics, parsing, sanitization, and deduplication. |
| Fixed selector | Two yes/no continuation scores per retained candidate; batching determines forward-pass count. |
| Stronger proposer | Qwen3-8B replaces or augments the 1.7B source in the composition setting; accounted separately. |
| Diagnostic | Setting | Oracle | V|O | Final | Note | |
|---|---|---|---|---|---|---|
| Sample sweep | cap 8, samples 4 | 0.6422 | 0.5844 | 0.3753 | ref. | Fewer samples but high conversion |
| Sample sweep | cap 8, samples 32 | 0.6429 | 0.4517 | 0.2904 | -8.49pp | Similar oracle, lower conversion |
| Retained-cap sweep | cap 8 | 0.6391 | 0.5682 | 0.3632 | ref. | CACC canonical retained cap |
| Retained-cap sweep | cap 16 | 0.8249 | 0.3998 | 0.3298 | -3.34pp | Larger retained pool raises oracle but hurts conversion |
| Setting | Final | Note | |
|---|---|---|---|
| Canonical verifier | 0.3632 | ref. | CACC canonical selector |
| verifier128 | 0.3465 | -1.67pp | Same-pool selector sensitivity check |
| verifier512 | 0.3594 | -0.38pp | close to Canonical verifier final |
| View | Parser / scoring path | Final | Oracle | V|O | Use | |
|---|---|---|---|---|---|---|
| legacy/no- AMC | archived numeric exact match | 12032 | 115/12032 | 444/12032 | 0.2590 | Numeric scoring near-collapse |
| legacy/no- AMC | benchmark-locked MCQ reparse | 12032 | 2842/12032 | 4900/12032 | 0.5800 | Readout recovery |
| CCR+ AMC | benchmark-locked MCQ reparse | 12032 | 2731/12032 | 5745/12032 | 0.4754 | Micro-ablation reference |
| CACC | benchmark-locked MCQ reparse | 12032 | 2740/12032 | 5736/12032 | 0.4777 | Gap exposure |
| CACC + SP | benchmark-locked MCQ reparse | 12032 | 3532/12032 | 6650/12032 | 0.5311 | Gap narrowing |
| Evidence unit | Split / scope | Before | After | w/l | Statistical detail | Artifact | |
|---|---|---|---|---|---|---|---|
| Second-family recovery | MMLU confirm-512 | 102/512 (0.1992) | 115/512 (0.2246) | +2.54pp | 17/4 | McNemar ; CI [0.0078, 0.0430] | Paired JSONL |
| Second-family recovery | GPQA-128 | 26/128 (0.2031) | 34/128 (0.2656) | +6.25pp | 9/1 | McNemar ; CI [0.0156, 0.1172] | Paired JSONL |
| Selection rescue | MMLU-Pro | 0.2936 | 0.3320 | +3.84pp | 613/150 | McNemar | Paired pred. |
| Selection rescue | GPQA | 0.2576 | 0.2929 | +3.53pp | 10/3 | McNemar | Paired pred. |
| Coverage pressure | GSM8K full | 0.3632 | 0.3381 | -2.50pp | summary | Oracle +10.84pp; V|O -12.43pp | Summary only |
| Second-family smoke | GSM8K-128 | 27/128 (0.2109) | 29/128 (0.2266) | +1.56pp | 7/1 | Smoke row | Paired rows available |
| Evidence unit | Candidate pool | Prediction records | Paired statistics | Manuscript use |
|---|---|---|---|---|
| Main Base / CACC rows | available | available | row-level deltas | Frozen result rows |
| MMLU no- AMC numeric view | archived | available | provenance audit | Scoring/readout-path provenance |
| MMLU MCQ reparse | archived scoreboard | available | provenance audit | MCQ Readout recovery |
| Second-family closure | available | available | McNemar + bootstrap | Frozen paired recovery |
| Conversion-aware rescue | available | available | McNemar where available | Gap narrowing evidence |
| Power Sampling x CACC | summary + pool | summary metrics | aggregate diagnostic | Diagnostic summary-only |
| Method | Task | Correct / | Final | Extra cost | Artifact status |
|---|---|---|---|---|---|
| Self-Refine | GSM8K full | 643/1319 | 0.4875 | 5.0 calls/example | Full row available |
| Self-Refine | CompMath numeric | 789/3199 | 0.2466 | 17.0 calls/example | Full row available |
| Self-Refine | MMLU-Pro | 2838/12032 | 0.2359 | 17.0 calls/example | Full row available |
| Self-Refine | GPQA Diamond | 24/198 | 0.1212 | 17.0 calls/example | Full row available |
| Self-Refine | GSM8K-128 | 51/128 | 0.3984 | eval-128 | Small reference row |
| ToT-lite | GSM8K-128 | 37/128 | 0.2891 | eval-128 | Small search baseline row |
| Component | Diagnostic trigger | Online action | Frozen output |
|---|---|---|---|
| CCR | partial derivation, missing final answer, fragmented numeric form | repair final-answer span and recover parseable candidate text | candidate text plus parser status |
| AMC | Answer-mode mismatch, missing choice letter, Wrapper drift | map candidate to benchmark-locked mode and protect choice-letter wrapper | mode-aligned candidate |
| SCH | duplicated scaffold, template residue, selector-attractive noise | remove residue, deduplicate, and keep retained cap stable | hygienic retained pool |
| Routing policy | diagnostic vector and retained cap | select rule sequence without gold access | frozen |
| Error class | Diagnostic signal | Expected effect |
|---|---|---|
| Oracle miss | No candidate parses to gold | Low oracle |
| Fragment / partial | Reasoning stops before final answer | Low oracle / Low V|O |
| Answer-mode mismatch | Numeric answer in MCQ or missing choice letter | Low MCQ final |
| Invalid final | Malformed final channel | Parser failure |
| Scaffold residue | Instruction or template leakage | Selector confusion |
| Wrapper drift | Second-family answer wrapper differs | Transfer loss |
| Example id | Dataset | Gold | Audited failure / recovery pattern |
|---|---|---|---|
| gsm8k-test-00595 | GSM8K | 13 | Oracle miss; CACC verifier selects 15 although gold is 13 |
| competition-math-numeric-test-02878 | CompMath | 13 | raw oracle hit exists, but verifier overrules first candidate and selects 11 |
| mmlu-pro-test-08795 | MMLU-Pro | A | no- AMC numeric view misses a choice-form solution; MCQ-aware CACC + SP later recovers final answer A |
| gsm8k-test-00462 | GSM8K | 71 | CACC repairs Fragment / partial candidate into a boxed final answer |
| gpqa-diamond-train-recNFJjE5PPTqVJGv | GPQA | B | selected wrong candidate contains scaffold / instruction residue, illustrating hygiene pressure |
| mmlu-pro-test-08026 | MMLU-Pro | D | stronger proposer adds/selects D after CCR/no- AMC misses the answer |
| Interface item | Constraint | Reproducibility role |
|---|---|---|
| CCR prompt template | no gold access; repair only observable fragments | reproduces Fragment / partial candidate repair |
| AMC answer wrapper | numeric tasks keep exact numeric form; MCQ tasks hard-lock to choice_letter | fixes answer-mode alignment and readout path |
| Numeric parser normalization | normalize whitespace, delimiters, final numeric span, and wrapper variants | keeps open-ended scoring stable across candidate sources |
| MCQ parser configuration | preserve benchmark option set and final choice-letter endpoint | supports MMLU-Pro / GPQA reparse provenance |
| SCH sanitizer rules | remove Scaffold residue, duplicated wrappers, and selector-attractive debris | stabilizes verifier input distribution |
| Manifest binding | every prompt/parser/sanitizer version is tied to run id, seed, split, and scorer | enables rerunning table and figure generation scripts |
| Claim | Evidence unit | Paper use |
|---|---|---|
| CACC improves numeric open-ended reasoning | GSM8K + competition_math | Direct final-gain evidence |
| MCQ benefits from AMC / scoring-path alignment | MMLU-Pro readout chain | Answer-mode alignment evidence |
| Second-family compatibility is recoverable | MMLU confirm-512 / GPQA-128 | Paired recovery evidence |
| CACC + SP narrows MCQ gap | MMLU-Pro / GPQA | Proposer-side composition evidence |
| Conversion-aware rescue improves final selection | MMLU-Pro / GPQA | Selection-side composition evidence |
| Power Sampling x CACC diagnoses Coverage pressure | aggregate summary | Coverage–conversion diagnostic |
| Symbol | Meaning |
|---|---|
| , , | input instance, ground-truth answer, evaluation dataset |
| benchmark-locked answer mode, fixed by registry | |
| raw candidate pool of size produced by the generator | |
| , , | mode-aware candidate construction, retained-cap controller, retained pool of size |
| gold-free diagnostic feature vector (parseability, Answer-mode mismatch, residue, …) | |
| , | frozen rulebook ( ) and routing policy |
| Benchmark | Split / | Mode | Retained cap | Parser / scoring endpoint |
|---|---|---|---|---|
| GSM8K | clean test, | numeric | 8 | numeric exact match on final boxed span |
| competition_math:numeric | numeric subset, | numeric | 8 | numeric exact match on normalized final span |
| MMLU-Pro | test, | MCQ | 8 | benchmark-locked choice-letter reparse |
| GPQA Diamond | full, | MCQ | 8 | benchmark-locked choice-letter reparse |
| MMLU-Pro confirm-512 | confirm split, | MCQ | 8 | second-family paired closure run |
| GPQA-128 | subset, | MCQ | 8 | second-family paired closure run |
| Evaluation | Seed | Base | CACC | 95% CI | |||
|---|---|---|---|---|---|---|---|
| GPQA | 198 | 14251 | 15.66 | 24.24 | +8.59 | [2.53,15.15] | .0137 |
| GPQA | 198 | 20260724 | 16.67 | 26.26 | +9.60 | [3.54,15.66] | .00432 |
| GPQA | 198 | 314159 | 16.16 | 24.24 | +8.08 | [1.52,14.65] | .0259 |
| MMLU held-out | 11,520 | 14251 | 26.35 | 28.07 | +1.73 | [1.04,2.43] | |
| MMLU held-out | 11,520 | 20260724 | 26.52 | 28.15 | +1.63 | [0.94,2.34] | |
| MMLU held-out | 11,520 | 314159 | 26.35 | 27.31 | +0.96 | [0.26,1.68] | .00799 |
| Evaluation | Cell | |||
|---|---|---|---|---|
| GPQA ( ) | Base | 20.71 | 82.93 | 17.17 |
| CACC | 51.01 | 45.54 | 23.23 | |
| CD | 75.25 | 39.60 | 29.80 | |
| CD+ CACC | 81.82 | 35.19 | 28.79 | |
| MMLU confirm ( ) | Base | 31.45 | 72.05 | 22.66 |
| CACC | 48.83 | 60.00 | 29.30 |
| Evaluation | Selector | Base | CACC | |||
|---|---|---|---|---|---|---|
| GPQA | 198 | Majority | 16.67 | 19.70 | +3.03 | .2101 |
| GPQA | 198 | 8B option-logprob | 17.68 | 28.28 | +10.61 | .00460 |
| GPQA | 198 | 1.7B candidate judge | 5.56 | 18.69 | +13.13 | |
| GPQA | 198 | 32B candidate judge | 6.57 | 23.23 | +16.67 | |
| MMLU full | 12,032 | Majority | 21.17 | 18.48 | -2.69 | |
| MMLU confirm | 512 | 8B option-logprob | 32.62 | 32.62 | 0.00 | 1.000 |
| Condition | |||
|---|---|---|---|
| Base | 85.16 | 82.11 | 69.92 |
| CACC | 91.02 | 77.68 | 70.70 |
Efficient Language Model Reasoning
Efficient language model reasoning studies how to improve problem-solving accuracy while reducing inference, memory, latency, and training costs.
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