# Show HN: S1Code, a decision-first Rust coding agent with Jev

> Source: <https://github.com/mertcicekci0/S1Code>
> Published: 2026-09-19 17:48:19+00:00

**A native Rust coding agent with inspectable decisions and recoverable context.**

Plan with a generative model. Choose concrete actions. Review the changes. Resume the work.

S1Code brings a coding agent into your terminal: describe a task, inspect its actions and diff, approve changes, and see the actual verification result. Sessions and captured evidence survive restarts.

Its native runtime combines **deterministic policy**, **bounded decisions**, and
**generative reasoning**. Claude or OpenAI handles planning and code generation.
Optional Jev integration selects among fully specified actions and can help decide
which evidence stays in the active context. S1Code owns local tool execution.

**Experimental preview · 0.3.0-rc.8** — macOS and Linux. Source installation
is available; binary distribution details are in the [installation guide](https://github.com/mertcicekci0/S1Code/blob/main/docs/install.md).
See [implementation status](https://github.com/mertcicekci0/S1Code/blob/main/docs/BUILD_STATUS.md)
for verified paths and remaining limits.

- **Work in a real repository.** Search and read source, apply validated patches,
inspect Git changes, and run[supported verification commands](https://github.com/mertcicekci0/S1Code/blob/main/docs/project-checks.md) for Node, Python, Cargo and existing npm scripts.
- **Keep the conversation readable.** Streaming replies and tool outcomes up front;
candidates, decisions, context and diffs in an optional inspector.
- **Control execution.** Review exact patches and commands, or preapprove supported
actions for a trusted workspace. Cancel from the keyboard.
- **Pick up where you left off.** Saved sessions retain requests, artifacts and
activity. Resume without blindly repeating completed tools.
- **Recover evicted evidence.** Context eviction removes material from the working
set while preserving captured bytes for rehydration without rerunning commands.
- **Use the same engine in scripts.** Headless mode emits JSONL and persists approval
requests for later review.

For prebuilt binaries, verification and updates, see [installation](https://github.com/mertcicekci0/S1Code/blob/main/docs/install.md).
There is no npm package to install.

**Build from source:** requires Git and [Rust via rustup](https://rustup.rs/). The repository pins Rust
1.94.0. Python 3 runs the included parser demo; Windows is currently unsupported.

```
git clone https://github.com/mertcicekci0/S1Code.git
cd S1Code
cargo install --path . --locked
s1code doctor
```

On macOS, save each API key once using a hidden prompt backed by login Keychain:

```
s1code auth set claude
s1code auth set typesafe
```

Open S1Code **inside the repository you want to work on**:

```
cd /path/to/your/project
s1code
```

Enter these commands in the S1Code input field, one at a time:

```
/provider claude
/model opus
/jev typesafe
```

Then type your task directly:

```
Find why the parser tests fail, make the smallest correct fix, and run the tests.
```

Provider and model choices are remembered. Keys are reused from Keychain; never
paste them into a task. On Linux, configure `ANTHROPIC_API_KEY` and
`TYPESAFE_API_KEY` through your environment or secret manager.

**Jev is optional.** `/decision rules` uses deterministic selection without a
learned decision backend. `/eviction jev` separately enables Jev-assisted context
eviction. Jev credentials and billing are independent of the generation provider.

The separate Codex bridge uses the official installed Codex CLI and its managed login:

```
s1code login codex
s1code run "Investigate and fix the failing parser test" --mode codex
```

Codex owns execution, internal model turns and context in this mode. This is a
delegation to the official runtime, not the native S1Code loop. See
[provider setup](https://github.com/mertcicekci0/S1Code/blob/main/docs/providers.md) for CLI compatibility and permissions.

Native OpenAI generation is also available through the Responses API using an
OpenAI API key. `/claude-code` hands the terminal to the installed official Claude
Code application; that handoff does not run S1Code's native loop or Jev decisions.

```
s1code login claude
s1code account claude
s1code claude-code
```

Login stays inside the official Claude Code client. To make Jev evidence ranking
available there, connect the optional [MCP companion](https://github.com/mertcicekci0/S1Code/blob/main/docs/claude-companion.md).
Claude Code retains its tool loop and context; Jev uses a separate TypeSafe key.
Native S1Code still uses API-key generation.

| Action | Command or key | 
|---|---|
| Inspect activity, decisions, context and diff | `Ctrl+O` or`F2` during a task | 
| Review the diff from the follow-up field | `/diff` | 
| Return from the inspector | `Esc` | 
| Cancel a running task | `Esc` or`Ctrl+C` | 
| List saved tasks | `/sessions` on the home screen | 
| Resume the latest task in this workspace | `/resume` on the home screen | 
| Resume a particular task | `/resume ID` or a unique ID prefix | 
| Show setup commands | `/help` | 

Continue a task in its follow-up field to retain the conversation. A new prompt from the home screen starts an independent saved task.

Native patches and processes require approval by default. For a trusted workspace,
`/permissions full-access` preapproves **supported actions** for the session;
`/permissions manual` restores prompts. Denied actions remain denied. This is not
an OS sandbox: approved tests and build scripts execute repository code with your
user's filesystem and network access.

```
Observe → construct concrete candidates → filter by policy → select
   ↑                                                        ↓
Verify and update ← persist evidence ← execute ← revalidate
```

The generator supplies plans, complete tool arguments, patches or answers. Forced transitions run directly; ambiguous candidate selection can use Jev. Deterministic policy and workspace checks apply again before execution, regardless of which backend selected the action. Completion requires current verification evidence.

The active context is a working set over durable artifacts. Budget-triggered
**reversible eviction** preserves pinned constraints and dependency groups; evicted
content can be retrieved exactly as captured. Historical snapshots stay historical.
Rehydration preserves evidence, not a guarantee that a model will ask for it.

Claude prompt-cache usage, provider calls, tool actions and verification outcomes are recorded when exposed. Lower cost, greater speed and better task performance are hypotheses to evaluate, not promises.

```
s1code demo --offline --workspace /tmp/s1code-parser-demo
s1code context-demo --workspace /tmp/s1code-context-demo
```

Use empty destination directories. The parser demo uses a labeled **OFFLINE
SIMULATION** for generation, with real reads, patch approvals and failing/passing
tests. The context demo stores, evicts and rehydrates actual evidence. Neither is
a live model demonstration.

For a disposable, budgeted **live Claude + Jev** task, run this from the source
checkout:

```
python3 scripts/try_live.py task
```

The launcher shows its request allowance before any API calls and reuses saved
macOS keys. See the [demo guide](https://github.com/mertcicekci0/S1Code/blob/main/docs/demo.md) for preparation and interpretation.

```
s1code run "Fix the failing parser test" --provider claude --decision jev --headless
s1code resume <session-id> --headless --approve <exact-candidate-id>
```

Stdout contains machine-readable JSONL; diagnostics go to stderr. Approval is bound to the exact action and its preconditions. A run does not automatically commit or push your changes.

```
cargo fmt --check
cargo clippy --locked --all-targets -- -D warnings
cargo test --locked
cargo build --locked --release
python3 scripts/release_check.py
```

Default tests need no credentials or network inference. `scripts/check.sh` runs the
broader local checks; [evaluation](https://github.com/mertcicekci0/S1Code/blob/main/docs/evaluation.md) explains fixture validation,
live trials and matched-policy comparisons. Fixture success is not agent success.
Provider-restricted results stay private pending clearance.

- [Quickstart](https://github.com/mertcicekci0/S1Code/blob/main/docs/quickstart.md) ·[Providers and authentication](https://github.com/mertcicekci0/S1Code/blob/main/docs/providers.md)
- [Architecture](https://github.com/mertcicekci0/S1Code/blob/main/docs/architecture.md) ·[Implementation status](https://github.com/mertcicekci0/S1Code/blob/main/docs/BUILD_STATUS.md)
- [Demo guide](https://github.com/mertcicekci0/S1Code/blob/main/docs/demo.md) ·[Evaluation methodology](https://github.com/mertcicekci0/S1Code/blob/main/docs/evaluation.md)
- [Security](https://github.com/mertcicekci0/S1Code/blob/main/SECURITY.md) ·[Privacy](https://github.com/mertcicekci0/S1Code/blob/main/PRIVACY.md) ·[Contributing](https://github.com/mertcicekci0/S1Code/blob/main/CONTRIBUTING.md)
- [Changelog](https://github.com/mertcicekci0/S1Code/blob/main/CHANGELOG.md) ·[Release process](https://github.com/mertcicekci0/S1Code/blob/main/docs/releasing.md)

New project code is [Apache-2.0](https://github.com/mertcicekci0/S1Code/blob/main/LICENSE). Dependencies retain their own licenses;
see [third-party notices](https://github.com/mertcicekci0/S1Code/blob/main/THIRD_PARTY_NOTICES.md). No vendor affiliation or endorsement
is claimed. S1Code is a working name; naming availability and trademark clearance
have not been established.