Why SpineCodex

SpineCodex gives your Codex a SpineTree to work on: long-running, multi-step work is broken into owned Work Units, persisted by the runtime as SpineBranches, and refined, delegated, and completed as the tree evolves — without forcing the entire process into one ever-growing transcript.

Get started

Install it in your existing Codex environment and run it directly. The current development branch targets upstream OpenAI Codex 0.153.4. Published builds are listed in the release history:

npm install -g @spinejit/spine-codex@latest
spine-codex

Spine Spawn is enabled by default. Run /experimental to enable the optional Memory Projection surface, then save and start a new conversation. Set spine_spawn.max_concurrent_threads_per_session in ~/.codex/config.toml to configure the total per-session thread limit, including the root thread.

What this enables

Compared with Codex, SpineCodex resolves 89% more tasks at 27% lower total cost on SWE-Milestone and extends the effective working context by up to 10×. It also improves the average score by 10.8 points on ProgramBench and the mean score by 9.2 points on FrontierSWE.

| Linear context | SpineCodex | | -------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | ❌Run out of context? | ✅256K → 2.5M Effective Working ContextSpineJIT compiles completed branches into semantic Node Memory, extending effective working context beyond the native window. | | ❌Drift after repeated compaction? | ✅Minimum Effective Context. Maximum Focus.Spine Runtime maintains the SpineTree and projects only the context required by the current Work Unit, keeping the agent focused. | | ❌Lose patience and focus on long tasks? | ✅Recursive Subagent Scaling on Demand.SpineJIT lets the agent recursively unfold into specialized subagents on demand, bringing divide-and-conquer structure and greater reasoning depth to complex problems. |

What's new

Upcoming

• PI and DeepSeek Harness plugins — SpineSDK integrations in development.
• App Spine UI — Inspect and operate the SpineTree, Work Units, and runtime state from the Codex App. In development.

Moves the upstream Codex baseline to 0.153.4. Unbinds the source-ledger history cap from the one-turn visible item limit, and the 1MiB synthetic context-plan cap, so long sessions are no longer killed at 4096 raw history items or 1048576 bytes of Node Memory.

Improves paginated-session recovery: incompatible historical records no longer block a valid lineage, while broken files and lineage-boundary errors remain fatal. Adds regression coverage for malformed rate-limit records during replay.

Restores the upstream Codex compatibility identity (0.147.0) while keeping the SpineCodex product version independent. Also hardens update-cache isolation, resumed Spawn visibility, and release metadata checks.

Separates the SpineCodex update cache from upstream Codex installations so product updates cannot collide with the upstream client.

Moves Spine onto a sampling-boundary runtime: Work Units, recursive Spawn, Node Memory, replay, and projection are coordinated by SpineSDK and surfaced through the native Codex experience. The runtime owns tree state and context projection while the agent focuses on the current unit.

Introduces the public SpineJIT design: compile a linear message stream into a SpineTree, replace completed branches with Node Memory, and support recursive subagent scaling. Spine Spawn and Memory Projection were the first experimental surfaces of that design.

How Spine works

LLMs consume a linear context, but work unfolds recursively.

• Agent: works on the current Work Unit.
• Spine Runtime: persists Work Units as SpineBranches and composes them into a SpineTree.
• SpineJIT: projects the current branch into the linear context required by the next model sample.

Work Unit -> SpineBranch -> SpineTree -> current-branch Context

The agent manages work; Spine maintains the recursive state behind the existing workflow.

Long-horizon performance

Across three long-horizon coding benchmarks, SpineCodex delivers stronger outcomes: 1.89× resolved tasks at 27% lower total cost on SWE-Milestone, +10.80pp average score on ProgramBench, and +9.2pp mean score on FrontierSWE.

SWE-Milestone (ICML 2026)

Long-horizon software development · 80 milestones · GPT-5.6 · sol high

| System | Resolved | Total cost | | -------------------- | -----------: | ----------------: | | BaseCodex | 9 | $764.18 | | SpineCodex | 17 | $556.46 |

1.89× resolved tasks at 27% lower total cost.

ProgramBench

Whole-repo program reconstruction · Random sample: 50 of 200 tasks · GPT-5.6 · Sol high · conservative cost estimate

| System | Avg. score | Tasks scoring >95% | Cost | | -------------------- | ---------------: | -----------------: | ----------------: | | BaseCodex | 62.55% | 2/50 | $188.12 | | SpineCodex | 73.35% | 7/50 | $475.10 |

+10.80pp average score and 3.5× high-scoring tasks.

FrontierSWE

Ultra-long-horizon coding · 9-task evaluation · GPT-5.6 · high · estimated API cost per trial

| System | Mean score | Best score | Cost | | -------------------- | -------------: | -------------: | ---------------: | | BaseCodex | 33.5 | 37.9 | $20.16 | | SpineCodex | 42.7 | 46.8 | $37.29 |

+9.2pp mean score and +8.9pp best score.

How SpineJIT works

Agent Morphogenesis: Each task shapes its own context and execution through just-in-time context-tree compilation and recursive subagent scaling.

TL;DR: SpineJIT replaces the live suffix of a context with shorter memory, while keeping the prefix unchanged so it can continue to hit the prompt cache.

To control this suffix replacement precisely, SpineJIT is implemented as a just-in-time compilation and context-mapping pipeline:

$$ \text{context messages} \rightarrow \text{Spine tokens} \rightarrow \text{SpineTree (ParseStack)} \rightarrow \text{new context} $$

The pipeline has two main stages.

1. JIT-compile context into a SpineTree

SpineJIT treats a context $C$---a message list, or simply a sentence whose characters are messages---as a stream to compile.

At each sampling boundary, it turns newly appended messages and control events into Spine tokens and updates a live LR(0) ParseStack:

SpineJIT uses four token kinds:

$$ \Sigma_{\mathrm{Spine}} = {\mathrm{Message},\ \mathrm{Open},\ \mathrm{Close},\ \mathrm{SpineSpawnNode}} $$

Message represents a raw context item. Open, Close, and SpineSpawnNode are special tokens emitted by SpineJIT at the corresponding sampling boundaries.

$$ \begin{aligned} \mathrm{SpineTree} &\to \mathrm{Nodes}\ \mathrm{End} \ \mathrm{Nodes} &\to \mathrm{Node} \mid \mathrm{Nodes}\ \mathrm{Node} \ \mathrm{Node} &\to \mathrm{Message} \mid \mathrm{SpineTreeNode} \ \mathrm{SpineTreeNode} &\to \mathrm{Open}\ \mathrm{Nodes}\ \mathrm{Close} \mid \mathrm{SpineSpawnNode} \end{aligned} $$

End is only the logical end of a session; a live session never emits it. Therefore, the ParseStack is the live SpineTree, and the reduction Open Nodes Close -> SpineTreeNode turns a closed subtree into one node.

In short, SpineJIT uses LR(0) JIT compilation to map context $C$ to a Spine Tree $PS$:

$$ PS = \mathrm{compile}(C) $$

2. Map the SpineTree into a new context

The structured SpineTree can now be mapped into a shorter context while preserving its stable prefix. For ParseStack $PS$, define:

$$ C' = f(PS) = \prod_{i=0}^{n} h(PS[i]) $$

$$ h(X) = \begin{cases} \prod_{x \in X} h(x), & X = \mathrm{Nodes} \ \mathrm{raw}(X), & X = \mathrm{Message} \ \mathrm{memory}(X), & X = \mathrm{SpineTreeNode} \ \mathrm{spine\_node\_desc}(X), & X = \mathrm{Open} \end{cases} $$

Here, $\prod$ means ordered concatenation.

The mapping is deliberately small:

• Message keeps its original content through $\mathrm{raw}(X)$.
• A closed SpineTreeNode is replaced by its shorter $\mathrm{memory}(X)$.
• An unmatched Open is represented by a concise $\mathrm{spine\_node\_desc}(X)$, helping the LLM delimit the currently live Spine node.

As parsing progresses, completed work in the context suffix is reduced into a SpineTreeNode and then projected as memory. Earlier context remains unchanged:

$$ \mathrm{prefix} \cdot \mathrm{suffix} \longrightarrow \mathrm{prefix} \cdot \mathrm{memory} $$

This is the central idea of SpineJIT: compress the context where work has finished, without invalidating the reusable prefix.

3. How SpineJIT inserts Spine control tokens

The LLM decides when to open or close a SpineTreeNode from the current context. The guiding objective is to maximize the average relevance of the remaining context to the current task.

Here, a sampling means one complete processing cycle for a model response: the response itself together with any tool calls it produces.

SpineJIT exposes Spine tools to let the LLM express these decisions. After a successful tool call in a sampling step, SpineJIT inserts the corresponding control token at a precise boundary:

| Tool call | Inserted token | Position | | --------------- | ------------------- | ------------- | | spine.open | Open | Pre-sampling | | spine.close | Close | Pre-sampling | | spine.next | Close Open | Pre-sampling | | spine.spawn | SpineSpawnNodes | Post-sampling |

These tokens connect the model's task-boundary decisions to the LR(0) parser, which continuously updates the ParseStack and therefore the context seen by the next sampling step.

Citation

A technical report on SpineJIT will be released soon.

If you use SpineCodex in your research, please cite this repository:

@software{xiang2026spinecodex,
  title = {Agent Morphogenesis: Just-in-Time Context Tree Compilation for Cost-Efficient Recursive Subagent Scaling},
  author = {Jiahong Xiang and Kunqiu Chen and Yuqun Zhang},
  year = {2026},
  url = {https://github.com/GhabiX/SpineCodex}
}

Project

SpineCodex is an independently maintained OpenAI Codex CLI (upstream 0.153.4), maintained by Jiahong Xiang and Kunqiu Chen.

SpineCodex is licensed under the Apache-2.0 License. OpenAI Codex and other derived components retain their attribution in NOTICE.

Contributing

We welcome bug reports, issue discussions, and ideas for new features. If you have a feature or PR idea, please open an issue and reach out to us first so we can confirm the direction, scope, and compatibility with the current SpineSDK and upstream Codex version. Please report any bugs through GitHub Issues; we will follow up promptly and work toward a fix. You are also welcome to reach out to me directly.