Chassis

Chassis is a production-grade Python agent harness.

It owns the runtime environment in which agents execute — plugins, capabilities, scoped resources, reversible effects, immutable runtime generations, policy, budgets, secrets, configuration, diagnostics, and observability metadata — and hands an immutable view of that environment to an execution engine.

LangGraph is the first-class execution engine mounted within Chassis. Chassis is not a graph framework.

Install

pip install chassis-harness

Python 3.12+. The import package is chassis.

Quickstart

from chassis import MODEL, Harness
from chassis.langgraph import AgentDefinition, GraphBuildInputs, LangGraphAgent
from chassis.runtime import HarnessRunContext

harness = Harness(name="quickstart")
harness.provide(MODEL, my_chat_model)                      # a langchain-core chat model
harness.register_agent(
    LangGraphAgent(
        AgentDefinition(name="research-agent", version="1", state_schema=ChatState, build=build_agent),
        checkpointer=InMemorySaver(),
    )
)

async with harness:
    result = await harness.agents.invoke(
        "research-agent", {"messages": [HumanMessage("hi")]}, thread_id="thread-1"
    )
    print(result.text, result.generation_id)        # hello … gen_0001

A run acquires one immutable generation and keeps it: swapping a provider publishes a new generation without mutating the environment underneath an in-flight run.

Runnable end to end — no credentials, scripted model, asserts its own output:

uv run python examples/quickstart.py

Full walkthrough: docs/getting-started.md.

A plugin in ten lines

from chassis import DATABASE, PluginContext, plugin
from chassis.tools import ToolPolicy

@plugin(name="web-search", version="1.0.0", provides={"tools": "1.0.0"}, requires={"database": ">=1,<2"})
async def web_search(ctx: PluginContext) -> None:
    store = ctx.require(DATABASE)                       # resolved for this composition
    ctx.tools.register(search_tool, policy=ToolPolicy(permissions=("network.fetch",)))
    ctx.create_task(warm_cache(store), name="cache-warmer")

No activation logic, no deregistration calls, no task bookkeeping: the harness orders plugins by declared capabilities, owns every effect through the plugin's scope, and cancels its tasks on unload.

The core proposition

Chassis allows agent runtime composition to change over time while active runs retain a coherent environment, dependencies react correctly, resources have explicit ownership, and obsolete components are disposed only when they are no longer reachable.

Guarantees and deliberate absences states what that buys you, and what Chassis refuses to promise.

What Chassis is not

• not a graph engine — LangGraph owns graph execution and durability;
• not a replacement for langchain-core models, tools, or runnables;
• not a replacement for LangSmith tracing or experiment management;
• not a sandbox — in-process Python plugins are trusted code, and policy is not presented as isolation;
• not a system that claims deterministic replay of arbitrary clocks, networks, databases, or external services.

Status

Pre-1.0 (0.1.0). The surface covered by tests/test_public_api.py may break in a minor release; every break is recorded in CHANGELOG.md.

Development

Chassis uses uv as its canonical project manager. Python ≥ 3.12 is required.

uv sync
uv run pytest
uv run ruff check .
uv run ruff format --check .
uv run pyright
uv build

pyproject.toml and uv.lock are the canonical dependency state. Do not introduce alternative project managers or parallel requirements.txt files.

Releasing

1. record the change in CHANGELOG.md under a ## [x.y.z] heading;
2. bump version in pyproject.toml to the same number;
3. tag and push: git tag vX.Y.Z && git push origin vX.Y.Z.

.github/workflows/release.yml runs the full check suite, then refuses to build unless the tag, the project version, and the changelog agree; it builds the distribution, installs the wheel into a clean environment, runs the quickstart against it, and publishes through PyPI trusted publishing (no token is stored). workflow_dispatch verifies all of that without publishing.

Examples

uv run python examples/quickstart.py                  # smallest useful app
uv run python examples/basic_agent.py                 # LangGraph agent end to end
uv run python examples/reactive_cascade.py            # database → memory → extension
uv run python examples/safe_provider_replacement.py   # generations across a provider swap

Each example asserts what it prints, so running it verifies the behaviour. The test suite runs all of them.

Documentation

• docs/getting-started.md — install and first run.
• docs/recipes.md — behind a web service, per-tenant composition, hot provider swaps, budgets, durable runs.
• docs/troubleshooting.md — symptom, cause, and the exact diagnostics output for each.
• docs/ — lifecycle, plugin authoring, LangGraph, observability, security assumptions, replay limitations, configuration, and design guarantees.

Layout

src/chassis/
  core/          scopes, effects, generations, errors
  capabilities/  versioned contracts, provider registry, snapshots
  plugins/       manifests, author API, resolver, registry
  hooks/         scope-owned hook registry
  tasks/         scope-owned background work
  tools/         tool registry and the execution boundary
  policy/        permissions and the evaluation boundary
  budget/        hierarchical budget governor
  secrets/       providers and redaction
  langgraph/     agent definitions, graph cache, LangGraph runtime
  telemetry/     instrumentation protocol, LangSmith, recording
  persistence/   canonical hashing and runtime snapshots
  replay/        bounded record/replay
  config/        declarative configuration and reconciliation
  testing/       TestHarness and fakes

Contributing

Issues and pull requests are welcome. Start with CONTRIBUTING.md for the gates a change must pass and what a reviewable commit looks like. Questions belong in Discussions; security reports go through private advisories instead of a public issue — see SECURITY.md for what is in scope.

License

Apache-2.0 — see LICENSE.