λ Lambda Harness

Build with agents. Keep the decisions yours.

A local workspace for functional programming teams, from conversation to reviewed patches.

Get started · User guide · Model backends · Contribute

---

Good software starts with a clear conversation. Lambda Harness helps you explore an idea, sharpen the requirements, and work through the change with specialist agents—all from a local website.

You approve the specification. You approve the tests. You choose what happens next.

A workflow that respects your craft

• Think before coding. Explore the repository and compare approaches, or let an interviewer challenge your assumptions until the requirements are clear.
• Tests lead the way. Review failing Cabal tests before implementation begins. Strict builds, lint, and Nix checks accompany code changes.
• Build your team. Browse agents with their own avatars, shape new specialists through chat, and share their definitions in Git.
• Get a second opinion. Review code, security, or documentation with dedicated specialists. An optional walkthrough helps you retain context.
• Keep project knowledge close. Save notes straight from chat, edit shared memory, preview what agents receive, and revisit earlier versions.
• Own every patch. Work stays in isolated local worktrees, with saved conversations and small commits. Prepare a handoff when you're ready to publish.
• Pass the work on. Export a handoff with the conversation, code, and project memory. A teammate imports it and continues from the saved step.
• Pick up from your phone. Connect privately through Tailscale to ask questions, review results, and approve the next step. Set up phone access.

Start where you need help: have a chat, explore an idea, write tests, review a PR, or take a feature through the full workflow. Every phase is a choice.

The task status reports whether a step completed, needs your input, or stopped with an error. In How to continue, choose One step at a time or All remaining steps automatically, then choose an action to start. Automatic mode still waits for approvals and stops at the first error. After a failed check, choose Ask agent to fix the failure, rerun checks, or send different instructions. Existing tasks can continue this way without restarting.

Errors support conversation too: use Ask a question to ask what failed, request commands you can run yourself, or describe an alternative. The agent can retry a supported operation with your instructions; suggested commands are displayed, not executed automatically. A failed commit also offers Continue without commits (or ask for that in chat). This keeps subsequent changes uncommitted for this task while validation and reviews continue. Existing commits and Git settings are preserved. Signing errors include a terminal unlock suggestion based on the GnuPG guidance.

flowchart LR
  Explore --> Interview
  Interview --> Spec[Approve specification]
  Spec --> Tests[Failing tests]
  Tests --> Approval[Approve tests]
  Approval --> Implement
  Implement --> Refactor
  Refactor --> Review
  Review --> Handoff[Local handoff]

Loading

Open your workspace

With Nix, run this from any directory:

nix run --refresh github:Devnull-org/lambda-harness -- \
  --state-dir ~/.local/state/lambda-harness

--state-dir is required: it chooses where your conversations, memory, and worktrees live. Reuse the same directory each time; existing users should keep their current path.

Open the printed local URL and add a Git checkout from Projects. Switch between projects from the sidebar; each keeps its own memory and saved work. Keep the server running while using the website.

Choose General chat, Explore, Plan a feature or fix, Review a PR or branch, or Write tests. Describe what you need. The harness checks your existing model logins and lets you choose when more than one backend is available.

Light, dark, or your system theme. A familiar editor, visible conversation history, and a stop button when you want to change direction.

Prefer running without Nix? See the user guide.

Your project. Your tools.

Powered by LangGraph and independent of any one model provider. The coding workflow currently supports Haskell and Nix; more functional languages are planned. Use existing agent logins or connect your own backend. Save architecture notes and invariants as shared project context for the team’s workflow.

The website runs locally. Model requests use your chosen provider. The harness creates no pull requests and pushes no code automatically.

Explore the workflow → · Configure project checks →