REVIEW 4 major objections 5 minor 14 references
The authors claim that a plain Transformer reading only sampled local subgraphs can match specialized link predictors without hand-coded heuristics or node-ID embeddings.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 05:36 UTC pith:4NWAIE6I
load-bearing objection A useful Transformer+GNN hybrid overclaimed as a plain Transformer; the empirical work is solid and the theory is fixable, but the headline claim does not survive contact with the architecture. the 4 major comments →
Plain Transformers are Surprisingly Powerful Link Predictors
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
PENCIL's central claim is that a standard encoder-only Transformer—the kind used for text, without positional encodings, structural encodings, or per-node ID embeddings—can be a top-tier link predictor if each candidate link is represented as a fixed-budget sampled subgraph with the two endpoints pinned as dedicated tokens. The model's input per token concatenates a one-hot index, the token's adjacency row within the sampled subgraph, and a role flag; random reindexing of non-query nodes makes the predictor permutation invariant in distribution. On seven benchmarks it outperforms or matches heuristic-informed GNNs and ID-embedding models, and the authors prove that with suitable weights the
What carries the argument
The load-bearing mechanism is link-centric subgraph tokenization plus a 'multiplicative residual': each sampled node is encoded as a concatenation of a one-hot slot identifier, its adjacency row inside the sampled subgraph, and a role flag, with the two query endpoints pinned to slots 0 and 1; a stack of bidirectional self-attention blocks is followed by a projection of the reconstructed adjacency matrix applied to the block output. That residual lets the plain Transformer reduce to a source-conditioned message-passing network and to sum-aggregation message passing with quasi-orthogonal token vectors, so path-based heuristics, common-neighbor counts, and SEAL-level expressivity are achieved
Load-bearing premise
The load-bearing premise, stated in Section 3.1, is that a small fixed random sample of the local neighborhood around each candidate link contains the topology needed to decide that link; on datasets where ground-truth links are determined by structure outside that budget, PENCIL's scores would drop regardless of Transformer capacity.
What would settle it
Train and evaluate PENCIL on a synthetic graph family where positive links are deliberately placed only between pairs whose local neighborhoods within the sampling budget contain no overlap—for example, cross-community edges with no short paths or common neighbors within Nmax hops. If ranking accuracy stays near chance while a full-graph or larger-budget model succeeds, the paper's central claim is falsified; if PENCIL still ranks those links correctly, the fixed-budget locality assumption is not the bottleneck.
If this is right
- PENCIL reports state-of-the-art results on cora and ogbl-ppa in the original setting and top HeaRT scores on ogbl-ppa and ogbl-ddi, using 22–146× fewer learnable parameters than the next-best competitors.
- It converges in roughly 0.5–15 epochs on ogbl-citation2, ogbl-ddi, and ogbl-ppa, versus 20–100+ epochs reported for pure GNNs.
- Theoretically, PENCIL can degenerate to a source-conditioned message-passing model that reproduces path-based scores (Katz index, Personalized PageRank, shortest-path distance, widest and most reliable path) and, under random or orthogonal token vectors, unbiased estimators of common-neighbor-type heuristics; with relational pooling it is at least as expressive as SEAL under the same sampling budg
- Depth helps: unlike MPNNs, PENCIL's performance improves with more layers on cora, pubmed, and ogbl-collab, with only mild saturation at greater depths.
- On several benchmarks structure alone is competitive with or better than using node features, suggesting node features can be weakly informative relative to local topology.
Where Pith is reading between the lines
- Inference: if the coherence argument transfers, Nmax is not just a compute knob but a correctness lever—for fixed hidden dimension d, keeping Nmax ≤ d preserves the possibility of exactly orthogonal token vectors, and pushing past d should degrade the implicit heuristic estimators; a sweep of Nmax against d on a heuristic-recovery task would isolate that effect.
- Inference: PENCIL's distributional permutation invariance provides a general template for canonicalizing query pairs in other dyadic prediction tasks (knowledge-graph completion, pair classification) without bespoke positional encodings.
- Inference: the result suggests that much of the observed gap between GNNs and ID-based methods on link prediction may be attributable to how node identity is presented to the model—a bounded set of sampled-token vectors versus a global embedding table—rather than to the need for hand-designed structural features.
- Inference: the fast convergence on ogbl-citation2 and ogbl-ppa and slower learning on Planetoid point to a testable prediction: pretraining PENCIL's input projection on structural heuristic-regression tasks should reduce the small-data gap; the paper leaves this untested.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes PENCIL, a Transformer-encoder architecture for link prediction. For each candidate pair (u,v), it samples a fixed-budget local subgraph, tokenizes the nodes with one-hot subgraph indices, adjacency rows, and role flags, and appends two task tokens for the endpoints. The model stacks BERT-style blocks; after each block, Eq. (2) adds a multiplicative residual P_k(Ã Z^(k)) built from the adjacency matrix reconstructed from the input tokenization. The paper reports strong results on Planetoid and OGB benchmarks under both the original and HeaRT protocols, claims state-of-the-art performance on several datasets, and provides a theoretical analysis connecting PENCIL to NBFNet, MPLP, heuristic estimation, and SEAL. A stated limitation in Sec. 7 is that a tight characterization of full-attention Transformer link predictors is still lacking, which is relevant when evaluating the theoretical claims.
Significance. If the empirical results hold, PENCIL is a meaningful contribution: it shows that a relatively simple architecture — a Transformer encoder over sampled subgraphs plus an explicit adjacency residual — can match or beat more complex heuristic- and ID-based link predictors on several large benchmarks, with far fewer parameters and good convergence behavior. The code release, multi-benchmark evaluation including HeaRT, and the small reported variances are strengths. However, the central 'plain Transformer' claim is not supported by the architecture as presented, and the theoretical reductions contain load-bearing mistakes. The work is therefore significant as a Transformer-plus-minimal-structural-prior link predictor, but the contribution as stated in the title and abstract is narrower and needs to be reframed.
major comments (4)
- [§3.2, Eq. (2); Appendix C.3, Table 9] The paper's central claim that PENCIL is a 'plain Transformer' that 'replaces hand-crafted priors' is not supported. Eq. (2) defines H^(k) = Z^(k) + P_k(Ã Z^(k)), an explicit adjacency-propagation branch that is a GCN-style message-passing operation, not a standard Transformer component. The ablation in Table 9 shows that removing this multiplicative residual degrades performance sharply (e.g., pubmed MRR drops from 38.28 to 21.49; ogbl-collab Hits@20 from 53.47 to 49.52), and Appendix C.3 states that 'an explicit structural prior is necessary for every layer.' The title and abstract therefore overstate the architecture. The authors should either reframe the contribution as a Transformer augmented with a multiplicative adjacency residual, or present a version without the residual that supports the 'plain Transformer' claim.
- [Appendix B.3, B.6; Propositions 4.2, 4.5] The proofs of Propositions 4.2 and 4.5 set T_k to map tokens to zeroes. Under Eq. (2), this makes Z^(k)=0 and hence H^(k)=P_k(÷0)=0, so the layerwise update does not reduce to any nontrivial source-conditioned MPNN. The intended choice appears to be T_k=Id, which is the setting used in the proof of Theorem 4.9 in Appendix B.8. Additionally, B.6 claims e_i can be chosen orthonormal when d≥|V|, but PENCIL operates on at most Nmax sampled tokens, with Nmax≪|V|; the relevant condition is d≥Nmax, not d≥|V|. These existence claims do not follow as written and must be corrected.
- [Appendix C.2; Table 2] For ogbl-ppa under HeaRT, the text says the authors used the optimal checkpoint identified in the original benchmark setting for the final HeaRT evaluation. Because HeaRT changes the validation negatives, selecting a checkpoint on the original validation split is not equivalent to selecting on the HeaRT validation split and can bias the reported MRR of 45.43. This matters because the paper's claim of top HeaRT performance on ogbl-ppa rests on this number. The authors should either rerun with HeaRT-appropriate checkpoint selection or provide evidence that the difference has no effect.
- [Appendix B.8; Theorem 4.9] The proof of Theorem 4.9 equates the sampled subgraph with the full m-hop enclosing neighborhood N_m(u,v). But PENCIL's fixed-budget sampler (e.g., (1,150) on ogbl-ppa) is not guaranteed to contain all nodes at distance ≤m; it is a random subsample. Consequently, the kϕ-kρ-m parameters m and the set over which the 1-|N_m|-WL guarantee is claimed need to be defined in terms of the actual sampled set, and the comparison to SEAL under the 'same sampling constraint' does not follow as written. The paper should also make explicit that this result concerns an LRP-augmented variant of PENCIL, not the model evaluated in Sec. 5.
minor comments (5)
- [§3.2, Fig. 3] The text says 'We omit the default sequential PE,' but the input includes one-hot node identifiers and role flags. Please clarify that these are not sequential positional encodings but token-identity and role signals.
- [Tables 5, 6] The entry '# Epochs 0.5' for ogbl-citation2 is confusing; report optimization steps or early-stopping checkpoints instead.
- [Figure 1] The caption contains placeholder star glyphs ('⋆⋆⋆...'); the figure should be re-rendered with the intended marker.
- [Appendix C.3] The sentence 'an explicit structural prior is necessary for every layer' should be reconciled with the abstract; as written, it confirms that the architecture is not plain.
- [Sec. 7] The conclusion correctly notes that a tighter theoretical characterization of full-attention Transformer link predictors is lacking; consider moving this caveat earlier, since the theoretical claims in Sec. 4 are mostly existence results.
Circularity Check
No significant circularity: PENCIL's empirical results are benchmark-based and the theoretical expressivity claims are transparent containment proofs relying on external theorems.
full rationale
The paper's central empirical claim is supported by direct comparisons on standard benchmarks (Tables 1-2) against external baselines; no fitted constant or trained parameter is subsequently relabeled as a prediction. The pairwise-heuristic study (Sec. 3.3) is explicitly a supervised regression probe with full-graph targets, not a claimed prediction of unseen links. The theoretical chain is also non-circular. Prop. 4.2 shows that by zeroing the attention branch, Eq. (2) degenerates to a source-conditioned MPNN; Cor. 4.3 then imports the generalized Bellman-Ford theorem from Zhu et al. (2021); Prop. 4.5 imports MPLP's unbiased-estimator results (Dong et al., 2024); Thm. 4.9 imports Lachi et al.'s kϕ-kρ-m framework and explicitly states it is a lower-bound/degeneration result, with the full-attention characterization left as future work. These cited theorems are external, parameter-free, and their assumptions do not include PENCIL's benchmark outcomes, so they count as independent support. The paper's self-citations (LPFormer, HeaRT, ContextGNN, GiGL) appear as baselines/context, not as the load-bearing justification for the main result. The mismatch between the 'plain Transformer' label and the multiplicative residual in Eq. (2), including the ablation showing the residual is necessary (Table 9), is a substantive correctness/novelty concern but not circularity: no derived quantity is equal to an input by construction. Therefore, no significant circularity is found.
Axiom & Free-Parameter Ledger
free parameters (3)
- Sampling configuration (depth, neighbors) =
(2,20) on cora/citeseer, (1,150) on ogbl-ppa, (1,350) on ogbl-ddi, etc.
- Model hyperparameters (layers, hidden size, heads, batch size, LR, epochs) =
Varies by dataset; see Tables 5 and 6
- Katz damping beta and PageRank alpha in heuristic regression =
beta=0.005, alpha=0.85
axioms (5)
- domain assumption Sampled subgraphs of budget Nmax are sufficient for the link prediction tasks
- domain assumption The input projection's random vectors for the Nmax tokens are sufficiently well-separated (small mutual coherence) to preserve structural identity
- standard math Known expressivity results of NBFNet, MPLP, and relational pooling transfer to PENCIL's encoding
- ad hoc to paper There exists a parameter setting with T_k = identity (or zero, as written) and a chosen input projection that makes PENCIL behave as an MPNN
- domain assumption HeaRT and original splits, and quoted baseline numbers, are comparable to PENCIL's runs
invented entities (2)
-
Multiplicative residual connection P_k(ÃZ(k))
independent evidence
-
Random token vectors induced by the frozen input projection W0
independent evidence
read the original abstract
Link prediction is a core challenge in graph machine learning, demanding models that capture rich and complex topological dependencies. While Graph Neural Networks (GNNs) are the standard solution, state-of-the-art pipelines often rely on explicit structural heuristics or memory-intensive node embeddings -- approaches that struggle to generalize or scale to massive graphs. Emerging Graph Transformers (GTs) offer a potential alternative but often incur significant overhead due to complex structural encodings, hindering their applications to large-scale link prediction. We challenge these sophisticated paradigms with PENCIL, an encoder-only plain Transformer that replaces hand-crafted priors with attention over sampled local subgraphs, retaining the scalability and hardware efficiency of standard Transformers. Through experimental and theoretical analysis, we show that PENCIL extracts richer structural signals than GNNs, implicitly generalizing a broad class of heuristics and subgraph-based expressivity. Empirically, PENCIL outperforms heuristic-informed GNNs and is far more parameter-efficient than ID-embedding--based alternatives, while remaining competitive across diverse benchmarks -- even without node features. Our results challenge the prevailing reliance on complex engineering techniques, demonstrating that simple design choices are potentially sufficient to achieve the same capabilities. Our code is publicly available at https://github.com/quang-truong/pencil.
Figures
Reference graph
Works this paper leans on
-
[2]
URL https: //doi.org/10.24963/ijcai.2021/291
doi: 10.24963/IJCAI.2021/291. URL https: //doi.org/10.24963/ijcai.2021/291. Adamic, L. A. and Adar, E. Friends and neighbors on the web.Soc. Networks, 25:211–230, 2003. doi: 10.1016/ S0378-8733(03)00009-1. Alon, U. and Yahav, E. On the bottleneck of graph neural networks and its practical implications. InIn- ternational Conference on Learning Representations,
-
[3]
URL https://openreview.net/forum? id=i80OPhOCVH2. Baras, J. S. and Theodorakopoulos, G. Path problems in net- works.Synthesis Lectures on Communication Networks, 3:1–77, 2010. Bodnar, C., Frasca, F., Wang, Y ., Otter, N., Montufar, G. F., Li´o, P., and Bronstein, M. Weisfeiler and Lehman go topological: Message passing simplicial networks. In Meila, M. an...
Pith/arXiv arXiv 2010
-
[4]
Coupette, C., Wayland, J., Simons, E., and Rieck, B
URL https://openreview.net/forum? id=Ua6zuk0WRH. Coupette, C., Wayland, J., Simons, E., and Rieck, B. No metric to rule them all: Toward principled evaluations of graph-learning datasets. InForty-second International Conference on Machine Learning, 2025. URL https: //openreview.net/forum?id=XbmBNwrfG5. Dao, T., Fu, D. Y ., Ermon, S., Rudra, A., and R´e, C...
Pith/arXiv arXiv 2025
-
[9]
M¨uller, L., Galkin, M., Morris, C., and Ramp ´aˇsek, L
URL http://proceedings.mlr.press/ v97/murphy19a.html. M¨uller, L., Galkin, M., Morris, C., and Ramp ´aˇsek, L. Attending to Graph Transformers.ArXiv preprint, abs/2302.04181, 2023. URL https://arxiv.org/ abs/2302.04181. Newman, M. E. J. Clustering and preferential at- tachment in growing networks.Phys. Rev. E, 64: 025102, Jul 2001. doi: 10.1103/PhysRevE.6...
Pith/arXiv arXiv 2023
-
[2009]
Here, N(u) denotes the set of one-hop neighbors of node u, and deg(w) :=|N(w)| denotes the degree of node w
discount common neighbors w with large degree, thereby emphasizing rarer shared neighbors. Here, N(u) denotes the set of one-hop neighbors of node u, and deg(w) :=|N(w)| denotes the degree of node w. The summations range over the shared neighbors w∈ N(u)∩ N(v). Since these scores are non-negative and typically right-skewed, a log transformation s7→log(1 +...
1953
-
[2017]
Kreuzer, D., Beaini, D., Hamilton, W
URL https://openreview.net/forum? id=SJU4ayYgl. Kreuzer, D., Beaini, D., Hamilton, W. L., L ´etourneau, V ., and Tossou, P. Rethinking graph transformers with spec- tral attention. InAdvances in Neural Information Process- ing Systems 34: Annual Conference on Neural Informa- tion Processing Systems 2021, NeurIPS 2021, December 6-14, 2021, virtual, pp. 216...
Pith/arXiv arXiv 2021
-
[2018]
Wang, X., Yang, H., and Zhang, M
URL https://openreview.net/forum? id=rJXMpikCZ. Wang, X., Yang, H., and Zhang, M. Neural common neigh- bor with completion for link prediction. InThe Twelfth International Conference on Learning Representations, ICLR 2024, Vienna, Austria, May 7-11, 2024, 2024. Welch, L. Lower bounds on the maximum cross correlation of signals (corresp.).IEEE Transactions...
Pith/arXiv arXiv 2024
-
[2019]
Murphy, R
URL https://openreview.net/forum? id=Syx72jC9tm. Murphy, R. L., Srinivasan, B., Rao, V . A., and Ribeiro, B. Relational pooling for graph representations. In Chaudhuri, K. and Salakhutdinov, R. (eds.),Proceed- ings of the 36th International Conference on Ma- chine Learning, ICML 2019, 9-15 June 2019, Long Beach, California, USA, volume 97 ofProceedings of...
2019
-
[2020]
URL https://openreview.net/forum? id=S1ldO2EFPr. Paszke, A., Gross, S., Massa, F., Lerer, A., Bradbury, J., Chanan, G., Killeen, T., Lin, Z., Gimelshein, N., Antiga, L., Desmaison, A., K¨opf, A., Yang, E., DeVito, Z., Rai- son, M., Tejani, A., Chilamkurthy, S., Steiner, B., Fang, L., Bai, J., and Chintala, S.PyTorch: an imperative 11 Plain Transformers ar...
arXiv 2019
-
[2021]
doi: 10.1186/S12859-021-04082-Y. Abboud, R., Ceylan, ˙I. ˙I., Grohe, M., and Lukasiewicz, T. The surprising power of graph neural networks with ran- dom node initialization. In Zhou, Z. (ed.),Proceedings of the Thirtieth International Joint Conference on Artifi- cial Intelligence, IJCAI 2021, Virtual Event / Montreal, Canada, 19-27 August 2021, pp. 2112–2...
-
[2022]
Truong, Q
URL https://openreview.net/forum? id=7UmjRGzp-A. Truong, Q. and Chin, P. Weisfeiler and lehman go paths: Learning topological features via path complexes. In Proceedings of the AAAI Conference on Artificial Intelli- gence, volume 38, pp. 15382–15391, 2024. Vaswani, A., Shazeer, N., Parmar, N., Uszkoreit, J., Jones, L., Gomez, A. N., Kaiser, L., and Polosu...
2024
-
[2023]
Zhou, T., L¨u, L., and Zhang, Y .-C
URL https://proceedings.mlr.press/ v202/zhou23n.html. Zhou, T., L¨u, L., and Zhang, Y .-C. Predicting missing links via local information.The European Physical Journal B - Condensed Matter and Complex Systems, 71:623–630, 10 2009. doi: 10.1140/epjb/e2009-00335-8. Zhu, Z., Zhang, Z., Xhonneux, L. A. C., and Tang, J. Neural bellman-ford networks: A general ...
-
[2024]
URL https://proceedings.mlr.press/ v235/fey24a.html. Fey, M., Sunil, J., Nitta, A., Puri, R., Shah, M., Stojanoviˇc, B., Bendias, R., Barghi, A., Kocijan, V ., Zhang, Z., He, X., Lenssen, J. E., and Leskovec, J. Pyg 2.0: Scal- able learning on real world graphs. InTemporal Graph Learning Workshop @ KDD 2025, 2025. URL https: //openreview.net/forum?id=DHHL...
Pith/arXiv arXiv 2025
-
[8856]
Ma, L., Lin, C., Lim, D., Romero-Soriano, A., Dokania, P
URL https://openreview.net/forum? id=pHCdMat0gI. Ma, L., Lin, C., Lim, D., Romero-Soriano, A., Dokania, P. K., Coates, M., Torr, P. H. S., and Lim, S. Graph inductive biases in transformers without message passing. InInternational Conference on Machine Learning, ICML 2023, 23-29 July 2023, Honolulu, Hawaii, USA, volume 202, pp. 23321–23337, 2023. Ma, L., ...
arXiv 2023
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.