Spine compiled coding agents' subtask trees into context and cut total cost 32.4 percent while raising SWE-Milestone Score 11.8 percent
AspireAI community · arXiv (Southern University of Science and Technology)
*Long-horizon coding agents usually append interactions to one growing history, which inflates context and degrades performance; the common alternative, recursive subagent delegation, conflates subtask granularity with execution choice.* Spine is an interface and runtime that aligns model context with subtask nesting and completion. An agent declares explicit subtask boundaries through work scopes, so nested subtasks can run inline while delegation is reserved for independent parallel work. The runtime compiles the tree into context just in time, keeping unfinished subtasks' interactions and replacing completed ones with their results, while preserving prefix-cache reuse.
Integrated into Codex as SpineCodex with GPT-5.6 Sol, it improved SWE-Milestone Score by 11.8 percent while reducing total cost by 32.4 percent, lifted the average ProgramBench test-pass rate by 17.3 percent, and improved Mean@5 on eight of ten FrontierSWE tasks. It also matches Recursive Subagent's score at 73.7 percent lower cost. A separate integration into Pi shows the same direction with Gemini 3.8 Flash and Qwen 3.8 27B.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.