jarn 0.4.2

J.A.R.N. — Just A Reliable Nerd. A TUI-first coding agent harness built on DeepAgents.

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J.A.R.N. — Just A Reliable Nerd

A TUI-first coding agent harness built on DeepAgents.

English · ภาษาไทย

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J.A.R.N. is a terminal coding agent in the spirit of Claude Code and Codex CLI, but built as its own opinionated harness on top of the DeepAgents library. Its defining trait is reliability: it plans before acting, verifies its own work, asks before doing anything risky, and never claims success on a guess.

It runs entirely in your terminal (a Web UI is on the roadmap, post-launch). Notable capabilities: AGENTS.md / CLAUDE.md interop (works out-of-the-box beside other agents), headless one-shot mode (jarn -p "..."), JSONL session transcripts, ! shell escape, OS-level execution sandbox (macOS sandbox-exec / Linux bwrap) and Docker container backend (execution.backend: docker), presets (/preset, jarn --preset) that set mode + sandbox at once, with an untrusted floor, auto-checkpoint + /undo / /redo, repo map (/map), a wiki knowledge base (/wiki), /config settings panel (interactive tabbed UI, persists to ~/.jarn/config.yaml), and per-server MCP health (/mcp status).

Status: v0.4.0 (Alpha) — released on PyPI (pip install jarn). Closes five user pain points vs other harnesses (prompt caching, plan-mode handoff, /commit + /review, background processes, macOS image paste) plus a UX-polish round (live in-place streaming, conversation /rewind, rich @-mentions, in-session /key, faster approvals). The architecture, configuration, permission engine, and terminal REPL are implemented and tested; live model calls require your own API key. See CHANGELOG.md and SECURITY.md.

Security: J.A.R.N. runs tools on your host by default (real filesystem + shell). A project's .jarn/config.yaml can declare hooks, MCP servers, and provider overrides — only trust repositories you would run code from. Untrusted projects are gated until you approve (jarn trust). Read SECURITY.md before use.

Why J.A.R.N.?

• Reliable by design — plan → act → verify is baked into the system prompt, with a self-verification loop that runs your project's build/test/lint before reporting done.
• Safe by default — a multi-layer permission system (coarse modes + fine-grained rules) sits in front of every file write and shell command, backed by a hard danger-guard that always confirms catastrophic actions — even in YOLO mode.
• Bring your own model — 13 providers (OpenRouter, Anthropic, OpenAI, Google, Mistral, Groq, DeepSeek, Together, Fireworks, xAI, Ollama, LM Studio, plus a generic OpenAI-compatible endpoint) with per-task routing so subagents can use cheaper models.
• Cost- & context-aware — live token/cost tracking (with a per-tool breakdown) and a per-session budget that can warn or hard-stop; a context-% gauge and live generation throughput (tok/s) that work for local models (LM Studio / Ollama) too, not just priced cloud ones.
• Date-aware — the current local date/time is injected into the system prompt, so "today"-relative requests don't anchor to the model's training cutoff.
• Extensible — skills, slash commands, custom subagents, lifecycle hooks, and MCP servers, all configured through plain files in ~/.jarn and .jarn/.

Install

macOS (Apple Silicon) and Linux (x64 / arm64) are supported; on Windows use WSL.

Via npm — a self-contained binary, no Python required:

npm install -g jarn-cli     # installs the `jarn` command (also available as `jarn-cli`)

Intel macs install via pip/uv instead (no npm binary is published for them).

Via pip / uv — requires Python 3.12+ and uv:

pip install jarn            # PyPI (alpha)
# or: uv tool install jarn

From source:

git clone https://github.com/chayapats/jarn && cd jarn
uv sync --extra dev
uv run jarn

uv.lock is tracked in the repo so every teammate gets the same dependency versions.

Sharing with your team

git clone <repo-url> && cd jarn
uv sync --extra dev
uv run jarn setup          # once per machine — stores API key in ~/.jarn
cd your-project
jarn doctor                # config, providers, and loaded extensions
jarn                       # trust prompt appears if the project declares hooks/MCP
jarn trust .               # pre-approve a repo you control (optional)

If a cloned project ships .jarn/config.yaml with hooks, MCP servers, or provider overrides, J.A.R.N. asks before honouring them. Decline to run safely with those settings stripped, or run jarn trust <path> after reviewing the repo. Use jarn doctor to see which skills, commands, subagents, hooks, and MCP servers would load (including shadowed or skipped files).

Quick start

jarn setup        # first-run wizard: pick a provider, store your API key, choose defaults
cd your-project
jarn init         # create a JARN.md project-context file (optional but recommended)
jarn              # launch the TUI
jarn doctor       # diagnose config / providers / keys / extensions at any time

On first launch with no config, J.A.R.N. runs the setup wizard automatically.

Non-interactive / scripting

jarn -p "summarise the open TODOs"          # one-shot: print reply and exit
echo "what changed?" | jarn -p -            # read prompt from stdin
jarn -p "do X" --json                        # emit JSON: {result, tokens, cost, turns}
jarn -p "do X" --model anthropic/claude-opus-4-8  # override model for this run
jarn -p "do X" --permission-mode auto-edit  # allow file writes without prompting
jarn -p "do X" --cwd /path/to/project       # set working directory

Fail-closed safety: the default modes (ask / plan) refuse any tool that would normally prompt for approval and exit non-zero. Pass --permission-mode auto-edit or yolo to allow unattended tool use — the danger-guard still blocks catastrophic commands in every mode.

The interface: native inline

jarn            # start a session
jarn --resume   # pick a previous session to resume on launch

J.A.R.N. renders the conversation straight to your terminal's normal buffer — no alternate screen. The whole transcript lives in your terminal's native scrollback: one scroll gesture scrolls everything and native selection/copy works across the entire history, exactly like Claude Code. Assistant replies stream live and render as Markdown; tool calls, approvals, and a per-turn diff preview appear inline.

Using J.A.R.N.

┌ toolbar: model · mode · queue · ctx · cost ─────────────────────────────┐
│                                                                            │
│   conversation stream (assistant output, tool calls, approvals)           │
│                                                                            │
├────────────────────────────────────────────────────────────────────────┤
│ › your message…                                                          │
└────────────────────────────────────────────────────────────────────────┘

• Type a message and press Enter to send (Shift+Enter / Ctrl+J for a newline).
• Start a line with / for a command (see below). @ references a file path.
• ↑ / ↓ navigate input history.
• Tab accepts the highlighted completion (/command or @file).
• Shift+Tab cycles the permission mode (plan → ask → auto-edit → yolo); the new mode flashes on the input border and stays in the status bar.
• Ctrl+O (or /expand) opens the last turn's full tool output in the pager.
• Ctrl+V (macOS) pastes an image/screenshot from the clipboard — it's saved under .jarn/pastes/ and inserted as an @path the agent reads on send.
• Esc cancels the running turn. Ctrl+C cancels a turn / clears the input, and twice in a row exits (Claude Code-style). Ctrl+Q also quits.
• Copy text: the terminal owns selection — just drag to select and ⌘C (or your terminal's copy), and scroll with your terminal's native scrollback, exactly like Claude Code.

Assistant replies render as Markdown (headings, lists, syntax-highlighted code).

/model, /mode, and /resume with no argument open an arrow-key picker (↑/↓ + Enter; Esc cancel). /model also offers a custom ref prompt.

While a turn is running, submitted lines are queued (shown in the toolbar as queue N); manage them with /queue, /queue clear, /queue cancel <n>, or /queue move <from> <to>.

Built-in commands

Command	Description
/help	Show available commands and shortcuts.
/init	Create a JARN.md project context file.
/config	View or edit settings: /config, /config get , /config set (persists).
/model [/ref|refresh]	Show or switch the active model; /model refresh re-queries local endpoints.
/mode [plan|ask|auto-edit|yolo]	Show or switch the permission mode (plan/ask/auto-edit/yolo).
/sandbox [on|off]	Show or toggle the execution backend (local/sandbox).
/key [<key>]	Set or replace the API key for the current provider (stored in the keychain).
/preset [<preset-name>]	Show or apply a preset — a shortcut that sets mode + sandbox at once.
/profile [<preset-name>]	Deprecated alias of /preset (kept working).
/cost	Show session token usage and cost.
/compact	Summarize and compact the conversation context.
/expand	Open the last turn's full tool output in the pager (same as Ctrl+O).
/clear	Clear the conversation and start a fresh thread.
/sessions	List and resume previous sessions.
/resume	Pick a previous session to resume.
/rewind	Rewind the conversation to an earlier turn and continue (forks a new thread).
/skills	List available skills.
/memory [search|show|add|update|delete|dump] ...	List, search, show, add, update, delete, or dump long-term memory.
/permissions	Show current permission rules and allowlist.
/mcp [status]	Show configured MCP servers with per-server health and last error.
/trust	Trust this project root and lift the untrusted review-only floor.
/queue [clear|cancel <n>|move <from> <to>]	Show or manage queued input lines (while a turn is running).
/undo	Revert the last agent turn's file changes.
/redo	Re-apply the last undone agent turn's file changes.
/abort	Cancel the running turn and roll back its file changes.
/commit	Draft a commit message from the current diff and commit (with approval).
/review	Review the current working-tree diff for bugs and quality (read-only).
/checkpoints	List recent auto-checkpoints.
/ps [kill <id>]	List or kill background processes (from run_in_background).
/quit	Exit J.A.R.N.
/map [focus] [--refresh]	Show the ranked repo map (codebase overview).
/wiki [search <q>|list]	Search or list wiki knowledge-base pages.
/doctor	Diagnose configuration, providers, and keys.

Permission modes

Mode	File reads	File writes	Shell	Network
plan	✅	❌	❌	❌
ask (default)	✅	ask	ask	ask
auto-edit	✅	✅ in-scope	ask	✅ (read-only)
yolo	✅	✅	✅	✅

In plan mode the agent researches read-only, then presents a concrete plan (exit_plan_mode). Approve it and J.A.R.N. escalates the mode (default auto-edit, configurable via plan.exit_mode; the picker also offers ask) and carries the plan out in the same turn — no manual mode switch. Untrusted projects stay clamped to plan.

The danger-guard overrides all modes: rm -rf (incl. rm -r -f / --recursive --force), force-push, git reset --hard, mkfs, fork bombs, out-of-scope writes, etc. always require explicit confirmation (or are blocked outright). Esc/Ctrl+C cancels a turn and kills any shell it spawned. See docs/PERMISSIONS.md.

Untrusted repos: a project's .jarn/config.yaml can declare hooks, MCP servers, and providers — capabilities that can run code or read secrets. J.A.R.N. asks you to trust a project before honoring those keys (once per repo); decline and they're ignored while the session continues safely.

Configuration

Two tiers, both YAML, merged together (project overrides global):

~/.jarn/config.yaml      global: providers, keys (by reference), defaults, budget
.jarn/config.yaml        per-project: MCP servers, hooks, permission rules (committed)
JARN.md                  per-project context, auto-loaded into the system prompt

API keys are referenced, never inlined — ${ENV_VAR} or keychain:jarn/<provider>. Project config is gated by a trust prompt (see above). See docs/CONFIGURATION.md for the full reference.

Extending

Drop files into ~/.jarn/{skills,commands,agents} (global) or .jarn/{...} (project):

• Skills (skills/*.md) — reusable knowledge/workflows, auto- or manually-triggered.
• Commands (commands/*.md) — custom /slash prompt templates.
• Subagents (agents/*.md) — specialist agents the main loop can delegate to.
• Hooks (config) — shell commands run on lifecycle events (lint after edit, test before commit).
• MCP servers (config) — connect external tool servers (stdio or HTTP).

See docs/EXTENDING.md (quick start) and examples/.

Documentation

• Architecture — how the subsystems fit together
• Configuration — every config key explained
• Permissions — modes, rules, danger-guard, approvals
• Extending — skills, commands, subagents, hooks, MCP
• Contributing — dev setup, tests, conventions
• Roadmap — what's in v1 / v1.x and what's next
• Web UI — planned, post-launch design
• Open-core — licensing & business model
• SPEC.md — the original design specification

Development

uv sync --extra dev
uv run pytest                 # 1166 tests: logic + mocked-agent + packaging gate
uv run ruff check src tests   # lint
uv run mypy src/              # type-check (CI-gated)
uv run jarn doctor            # sanity-check your environment (add --json for machine output)

License

Apache-2.0. See LICENSE.

Built on DeepAgents, LangGraph, prompt_toolkit, Rich, and Textual (onboarding wizard only).