langctl

Scaffold, run, and deploy production LangChain agents — frontend and agent in one command.

langctl new my-agent      # backend + chat UI, wired together
cd my-agent
langctl dev               # one command, one URL, already talking to each other

langctl dev starts the LangGraph Agent Server and your Next.js app, waits for the agent's health endpoint before booting the UI, proxies the agent behind the frontend's own origin, and tears both down cleanly on Ctrl-C. Like next dev, but the backend is an agent.

Why the proxy

The browser only ever talks to /api/agent/... on the frontend's origin:

localhost:3000                      127.0.0.1:2024
┌────────────────────────┐          ┌──────────────────┐
│ Next.js                │          │ langgraph dev    │
│  /            chat UI  │          │  /threads /runs  │
│  /api/agent/* ─proxy───┼─────────▶│  /assistants /ok │
└────────────────────────┘          └──────────────────┘
        same origin ⇒ no CORS, ever

Three things fall out of this:

• CORS never applies. There is no cross-origin request to preflight.
• The API key stays on the server. The proxy runs in a route handler and attaches x-api-key there. Nothing secret reaches the browser.
• Dev and production differ by one variable. AGENT_PROXY_TARGET unset means the local agent; langctl deploy sets it to the deployed Agent Server URL. The frontend source does not change.

Chat UI

langctl new scaffolds one of three UIs. All share the same proxy, streaming contract, and langctl dev behaviour — only web/app/components/ differs.

No template contains hand-written CSS. globals.css is @import "tailwindcss"; and nothing else, except ai-elements, which adds a @theme token block because its components reference semantic classes. A test enforces this.

ai-elements is experimental and currently fails npm run build. Its generated components pull streamdown, which resolves two incompatible copies of shiki; npm overrides do not dedupe them. Our own files typecheck clean — the failure is entirely in the vendored components. It is excluded from the interactive wizard and reachable only via an explicit --ui ai-elements.

Commands

Configuration

agent.yaml is the single source of truth; langgraph.json is generated from it. sync merges rather than overwrites, so hand-added keys (auth, checkpointer, dockerfile_lines, …) survive, and drift in owned keys is reported instead of silently clobbered.

Install

uv tool install -e .        # editable: source edits take effect immediately
langctl --version

Install editable. A non-editable uv tool install . is cached by version, so after changing the source uv tool install --force . reports Audited … and keeps running the old code — the version string has to change for the cache to miss. If you already hit that: uv tool install --force --reinstall -e .

When debugging which copy is running, call it by absolute path (~/.local/bin/langctl): an activated project venv shadows the global binary.

Development

uv venv && . .venv/bin/activate
uv pip install -e ".[dev]"
pytest                    # unit + process-level integration
ruff check src tests

The suite spawns real child processes rather than mocking Popen: the failures that matter here — orphaned grandchildren, ports left held, signals that never arrive — do not exist at the mock level. See tests/test_supervisor.py and tests/test_signals.py.

Two heavier checks need a scaffolded project with dependencies installed:

export LANGCTL_E2E_PROJECT=/path/to/scaffolded/project
tests/e2e/dev_runtime.sh     # health gate, proxy, thread creation, clean teardown
tests/e2e/sse_streaming.sh   # asserts SSE arrives incrementally, not buffered

sse_streaming.sh measures arrival times, not just the final body — a buffering proxy passes every status-code assertion and still ruins the product.

Status

Phase 1 (scaffold + unified dev runtime) works end to end. Deploy providers (langsmith_cloud, vercel) are next — see plan/.

License

Apache-2.0. Generated projects carry no license obligation to this tool.