Delta

Delta is a minimal coding agent harness in Python built around a small, inspectable model–tool loop. It provides filesystem and shell tools, multi-provider model support, customizable tools, skills, prompts and hooks, and clean interfaces for extending agent behavior without changing the core runtime. Delta can be used interactively from the terminal, scripted for one-shot tasks, or embedded into larger agent and evaluation workflows. Requires Python 3.12 or newer.

Install

The package name on PyPI is deltaa; once installed it gives you a delta command.

Fastest path — one script, works on a machine with nothing set up yet:

macOS / Linux:

curl -LsSf https://raw.githubusercontent.com/kartikg-gk/delta/main/scripts/install.sh | sh

Windows (PowerShell):

irm https://raw.githubusercontent.com/kartikg-gk/delta/main/scripts/install.ps1 | iex

No admin/root privileges needed. If uv isn't found on your machine, the script grabs it first, then puts Delta in its own isolated environment and runs delta --version at the end so you know it worked. Read either script under scripts/ before running it if you'd rather not pipe-to-shell blind.

If you already manage Python packages some other way, pick one:

uv tool install deltaa      # isolated, recommended if you have uv
pipx install deltaa         # isolated, no uv needed
python -m pip install deltaa

Confirm it landed:

delta --version

When a new version ships:

uv tool upgrade deltaa

Quickstart

Delta needs a model provider. Set one up once:

delta config

Now cd into whatever project you want it working on and launch it:

cd my-project
delta

With no arguments, delta drops you into the terminal UI — start typing and hit Enter, same as any chat interface:

what does the auth module in this repo do?

Need it non-interactively instead — for a script, a Makefile target, CI, whatever — pass -p and it prints the answer and exits:

delta -p "list every route this API exposes"
delta --model gpt-5 -p "where is the retry logic for failed requests?"

If you'd rather stay in a plain scrolling terminal than the full-screen UI:

delta --repl

Providers and configuration

Built-in providers: OpenAI, Anthropic, OpenRouter, and Ollama for local models.

delta config walks you through picking a provider, a model, and an API key. It can also run without prompts:

delta config --provider openai --model gpt-5
delta config show        # saved settings, keys masked
delta config path        # where the file lives

Settings are stored in ~/.delta/config.toml, outside your project, with owner-only permissions. Environment variables take priority over the file:

Variable	Purpose
OPENAI_API_KEY / ANTHROPIC_API_KEY / OPENROUTER_API_KEY	Provider credentials
DELTA_API_KEY	API key for delta config without an interactive prompt
OPENAI_BASE_URL / ANTHROPIC_BASE_URL	Point a provider at another compatible endpoint
DELTA_PROVIDER / DELTA_MODEL	Force the active provider or model
DELTA_HOME	Move ~/.delta (sessions, logs, skills, prompts, extensions)
DELTA_CONFIG_DIR	Move the config file

Inside a session, /model and /provider switch on the fly.

Commands

From your shell:

delta [prompt]                           # terminal UI, or a one-shot prompt
delta -p "..."                           # print mode: stream the answer, then exit
delta -r <session-id>                    # resume a saved session
delta config [show|path]                 # provider setup
delta provider list|setup|select <name>  # inspect or switch providers
delta session list                       # saved sessions
delta session export <id> [-f text|json|jsonl]
delta session stats [id]                 # token and cost totals

Useful flags: --model/-m, --provider, --system-prompt, --max-turns, --session-dir, --no-session, --repl, --verbose/-v.

Inside a session, slash commands give live control:

Area	Commands
Session	/name /continue /export /stats
History	/compact /branch /branches /rewind
Model	/model /provider /think /plan
Workspace	/shell /skills /create-skill /reload
Info	/help /version /diag /quit

The terminal UI also has /new and /session for starting and switching sessions.

Skills, prompts, and extensions

Project instructions. Delta reads AGENTS.md from, in increasing priority: ~/.delta/, ~/.agents/, <project>/.delta/, <project>/.agents/, and the project root. All files found are combined.

Skills are markdown instructions the agent can pull in when relevant. Put one at <project>/.delta/skills/<name>/SKILL.md (or under .agents/skills/, or the same paths in your home directory), list them with /skills, and invoke one directly with /skill:<name> [args]. /create-skill <name> <text> writes a new one for you.

Prompt templates live in .delta/prompts/ (project or home). Run one by typing /<template-name>.

Extensions add capabilities without a restart: drop a .py file that defines a register function into ~/.delta/extensions/ or <project>/.delta/extensions/, then run /reload.

Project structure

Three packages, layered so dependencies only point inward: delta_harness imports neither of the others, delta_model imports the harness, and delta_app imports both.

src/
  delta_harness/          # core: the agent loop and its vocabulary
    contracts/            #   transcript entries, tool specs, event types, JSON values
    provider/             #   the model-provider interface and raw stream events
    session/              #   durable JSONL records, storage, replay, index, summaries
    engine.py             #   the agent loop
    driver.py             #   transcript-owning harness around the loop

  delta_model/            # providers: everything that talks to a model API
    claude.py  _claude/   #   Anthropic Messages API
    oai_compatible.py     #   OpenAI-compatible endpoints (OpenAI, OpenRouter, Ollama, ...)
    _oai/                 #   request building, response parsing, normalization, transport
    transport/            #   HTTP client, retry with backoff, error classification
    scripted.py           #   a deterministic fake provider for tests
    settings.py           #   environment-driven provider settings
    limits.py             #   context and output limits

  delta_app/              # application: the parts you actually use
    cli/                  #   entry point, argument parsing, `delta config`
    tui/                  #   terminal UI: transcript, composer, completions, pickers
    ui/render.py          #   headless output (plain, JSON, transcript)
    config/               #   config schema, TOML store, setup wizard, hidden key input
    tools/                #   file tools, shell tool, shared helpers
    runtime.py            #   session construction shared by every frontend
    conversation.py       #   the session object the CLI and UI both drive
    directives.py         #   slash-command registry and dispatch
    instructions.py       #   system-prompt assembly
    skillset.py           #   markdown skills
    prompts.py            #   prompt templates
    discovery.py          #   where skills, prompts, and sessions are found
    plugins.py            #   file-based extensions with hot reload
    refresh.py            #   `/reload` wiring
    context/budget.py     #   token accounting and compaction thresholds
    safety.py             #   tool approval and output scrubbing
    planning.py           #   plan mode
    reasoning.py          #   thinking modes
    sessions.py           #   create, resume, list, export
    logs.py               #   per-session run logs

tests/                    # one test module per concern
scripts/                  # install scripts

Development

git clone https://github.com/kartikg-gk/delta.git
cd delta
uv sync
uv run delta --version
uv run pytest
uv run ruff check .

To point the global delta command at your checkout instead of a PyPI release:

uv tool install --editable --force .

Code changes in the checkout take effect right away with this setup. What doesn't update on its own is the tool's dependency list and version metadata — those are snapshotted at install time, so after pulling changes that touch pyproject.toml (a new dependency, a version bump), rerun the command above to pick them up.

License

MIT