✳ Aster

An open-source coding agent you run yourself, with the model you choose.

Aster is a terminal agent that reads your code, answers questions, edits files, runs commands, and reviews your changes. It works with any OpenAI-compatible provider: OpenRouter, OpenAI, Groq, Anthropic, or a model running on your own machine.

Three ideas shape it:

• You own the whole thing. Your key, your model, your machine. Sessions, memory, and skills are plain files on your disk. There is no hosted control plane, no vector database, and no telemetry. Point it at a local model and it works with no network at all.
• A harness, not a prompt. Tools, permissions, history, memory, and retrieval are shared infrastructure. Chat, review, and fix all inherit them, so a new capability is a small addition rather than another agent rebuilding the same scaffolding.
• Make it prove things. An agent that sounds right is not the same as one that is right. Aster spends extra model effort trying to disprove its own findings before showing them to you, and it tells you when it ran out of room instead of quietly wrapping up.

Status: early, building in the open. Chat, review, memory, skills, permissions, and MCP have landed. Expect rough edges.

Install

curl -fsSL https://withaster.dev/install | sh

Or build it yourself (Rust 1.85 or newer):

git clone https://github.com/zfinix/aster && cd aster
cargo install --path crates/aster-cli

Start here

aster init     # pick a provider, paste your key, done
cd your-repo
aster          # opens the chat

aster init writes ~/.aster/aster.yaml and stores your key in ~/.aster/.env, so it applies to every repo. Prefer environment variables? Skip init and export these instead:

export ASTER_API_KEY=sk-...
export ASTER_BASE_URL=https://openrouter.ai/api/v1
export ASTER_MODEL=anthropic/claude-sonnet-5

Then just talk to it:

❯ where does the retry logic live?
❯ add a test for the empty-input case
❯ why is that finding critical?

Aster reads files, searches the repo, runs commands, and edits code when you let it. Ask it something outside a repo and it still works, it just has less to look at.

The chat

Key	What it does
enter	Send. esc interrupts a running turn.
esc esc	Quit (two presses, so a stray one does not).
shift+tab	Step to the next permission mode.
ctrl+j	Newline without sending.
@	Mention a file from this repo.
↑	Step back through what you have sent.

Type / for commands:

Command	What it does
/model	Switch model, or pick from what the provider serves.
/provider	Switch to a different endpoint, then pick a model.
/mode	Choose how freely the agent edits (also shift+tab).
/effort	Reasoning budget: off, low, medium, high.
/resume	Reopen one of this repo's earlier sessions.
/clear	Start fresh.
/help	Everything above, in the terminal.

Outside the TUI:

aster chat "why is finding 2 critical?"   # one answer, then exit
echo "explain this repo" | aster          # piped input is the prompt
aster chat --continue                     # pick up the last session
aster chat --resume                       # choose a session from a list

How much it is allowed to do

One setting decides how freely the agent edits. Change it with shift+tab mid-chat, --permission-mode for one run, or permissions.mode in aster.yaml.

Mode	What it does
plan	Explores and proposes. Never edits.
manual	Asks before every edit.
auto	Edits what is safe, stops to ask about anything risky.
edit	Edits without asking (the default).

Whatever the mode, commands run in a sandbox: writes are limited to the repo and temp directories, and secrets are stripped from the environment. (/yolo turns the sandbox off. It asks three times first, and turns the chat red.) Writes to .git/, workflow files, and hooks are blocked, and reads of key and env files are blocked. Widen or narrow any of it under permissions in aster.yaml.example.

Review your changes

Review is Aster's most developed capability. It does not just ask a model to skim a diff, it tries to disprove what the model claims.

aster review                              # the current branch
aster review --range main..HEAD           # an explicit range
git diff HEAD~1 | aster review --diff -   # a diff on stdin
aster review --pr 42                      # a GitHub PR (run `aster login` first)
aster review --pr 42 --comment            # post the findings as PR comments
aster review --tui                        # browse findings interactively
aster review --json | aster fix --apply   # let it fix what it found

A finding looks like this:

  ✳ 1 finding worth your attention.

  HIGH  correctness  1/1  74%
  Unchecked index can panic on an empty slice
  crates/aster-index/src/grep.rs:58

With --json, the same thing comes out as data for CI or another tool:

[
  {
    "file_path": "crates/aster-index/src/grep.rs",
    "line": 58,
    "severity": "high",
    "category": "correctness",
    "title": "Unchecked index can panic on an empty slice",
    "suggestion": "Handle the PoisonError instead of unwrapping.",
    "confidence": 0.74
  }
]

How review works

flowchart LR
    A[HYPOTHESIZE] --> B[RETRIEVE] --> C[VERIFY] --> D[SHAPE]

Loading

1. Hypothesize. A cheap model over-produces candidate defects from the diff. A candidate without a concrete failure scenario is dropped.
2. Retrieve. Pull only the evidence that candidate needs: the changed hunk, a window of source, the enclosing symbol, and references from a local SQLite and FTS5 symbol index. No repo-wide walk.
3. Verify. A second call, prompted to refute, kills plausible-but-wrong findings. A candidate survives only above --min-confidence (default 0.5). Point ASTER_VERIFY_MODEL at a stronger model for a real second opinion.
4. Shape. Deduplicate, rank by severity times confidence, and emit.

The expensive model is spent only on challenging what survived, never on the whole diff. Full write-up in docs/ALGORITHM.md.

One caveat worth knowing: the confidence gate filters the verifier's self-reported confidence. That is a useful heuristic, not a calibrated probability, so treat the number as a ranking signal rather than odds.

The rest of the commands

Command	What it is for
aster memory	Facts Aster should keep between sessions. add, list, show, remove.
aster sessions	Past conversations. list, show, delete, prune.
aster skills	Reusable instructions the agent loads on demand. add, list, find, bundled, remove.
aster mcp	MCP servers that give the agent more tools. list, enable, disable.
aster web	Fetch a page or crawl a site as Markdown.
aster fix	Turn review findings into edits. Dry run unless you pass --apply.
aster login	Link GitHub, for reviewing and commenting on PRs.

Two flags work everywhere, before or after the subcommand: --json for machine-readable output, and --effort for the reasoning budget.

aster --json sessions list
aster --effort high review

Memory

aster memory add "we deploy from the release branch, never main"
aster memory add "prefers terse replies" --title tone
aster memory list
aster memory remove tone

Short facts go into project memory, which is always in context. Named blocks are listed by title and read in full only when the agent needs them.

Skills

Skills are folders with a SKILL.md telling the agent how to do something specific. Aster reads the titles and loads the body only when it is relevant.

Nine core skills ship built in and are always available: git and GitHub workflows, verification before reporting done, build triage, shell batching, CLI craft, context economy, taking corrections, and security hygiene. Nine more are bundled but off by default (debugging, refactoring, tests, dependency upgrades, supply-chain safety, background processes, and others); aster skills bundled lists them. Installing any skill with the same name overrides its built-in.

aster skills find react          # search GitHub for skills
aster skills add owner/repo      # install from a repo
aster skills add ./my-skill -p   # install into this project only
aster skills add claude-code     # import from another agent on this machine
aster skills bundled             # list the optional built-in skills
aster skills bundled write-tests # turn one on
aster skills list

Any agent key from the skills registry works as a source, so skills already installed for Claude Code, Cursor, Codex, Gemini CLI, and the rest come across as-is. aster skills add with no source lists the ones it finds installed.

MCP servers

MCP servers add tools like a browser or a GitHub client. Declare them under mcp.servers in aster.yaml, then:

aster mcp list              # what is configured, and what it offers
aster mcp enable chrome     # turn one on
aster mcp disable chrome    # turn it off, keep the config

Tools are injected progressively, so a large server does not eat your context. See docs/MCP.md.

Configuration

Order of precedence: CLI flags, then environment, then aster.yaml, then defaults. API keys are read from the environment only, never from the yaml.

aster.yaml is read from your repo root, then ~/.aster/aster.yaml for everything else. Copy aster.yaml.example to get started.

Env var	What it does	Default
ASTER_API_KEY	Your provider key (required)	none
ASTER_BASE_URL	Any OpenAI-compatible endpoint	https://openrouter.ai/api/v1
ASTER_MODEL	The model to use	openai/gpt-4o-mini
ASTER_EFFORT	Thinking budget: off, low, medium, high	low
ASTER_MAX_TOOL_ROUNDS	Tool calls in one turn before it must answer	60
ASTER_COMMAND_TIMEOUT	Seconds a single command may run	300
ASTER_VERIFY_MODEL	A stronger model for review's verify pass	same as ASTER_MODEL
ASTER_HYPOTHESIS_MODEL	A cheap model for review's first pass	same as ASTER_MODEL
ASTER_MAX_TOKENS	Cap generated tokens (off disables)	8000
ASTER_SEED	Fixed sampling seed (off disables)	0

.env.example lists every variable with notes.

Going deeper

The three ideas at the top are the short version. The long version, with the diagrams and the reasoning behind each decision, lives in the docs:

Doc	What it covers
ARCHITECTURE.md	How the crates fit together and what a turn does end to end.
ALGORITHM.md	The review pipeline and its cost model.
HARNESS.md	Sessions, memory, approvals, and delegation.
MEMORY.md	What Aster remembers, and how it is disclosed.
MCP.md	Progressive tool injection: one bridge, schemas on demand.
ANALYZERS.md	Wiring semgrep and ast-grep into review.
ROADMAP.md	What is next.

Repository layout

crates/
  aster-cli/         the `aster` command-line interface
  aster-ai/          provider-agnostic OpenAI-compatible chat client
  aster-harness/     the verification-first review pipeline
  aster-index/       code index: SQLite + FTS5 + ripgrep
  aster-analyzers/   static analysis (semgrep / ast-grep)
  symbol-extractor/  tree-sitter symbol extraction (14 languages)
  aster-persist/     sessions and project memory
  aster-skills/      on-demand instructions
  aster-agents/      specialized agent definitions
  aster-policy/      read, write, and command permissions
  aster-mcp/         progressive MCP tool injection
  aster-models/      shared domain types
desktop/             the desktop app (Tauri)
editors/vscode/      the VS Code extension
docs/                architecture, algorithm, memory, MCP, roadmap

Contributing

Contributions are welcome. CONTRIBUTING.md has the dev setup and the three checks CI runs: fmt, clippy, test. For security issues, see SECURITY.md.

License

Apache-2.0. See NOTICE.