Quater

Most backend frameworks were designed for one main job: serve data to a frontend, then let humans click through that frontend to get work done.

That model still matters, but it is no longer enough. Moving ahead, more work is going to be done by AI agents. Asking those agents to use a product through screens, buttons, and forms is slow, fragile, and often the wrong level of access. Agents need a safe way to work with the backend directly.

That does not mean giving agents unlimited access. It means exposing the right operations, with clear inputs, clear descriptions, real auth, audit trails, and approval gates where the action is sensitive.

Quater is a Python backend framework built for this shift. You build a normal backend for people and services, and you can expose selected views directly to MCP Clients through MCP or to AI agents through the CLI. The same operation can serve the app, power an agent, and support production workflows without becoming three different pieces of code.

The goal is simple: make the backend usable by humans and operable by AI agents, without losing safety, structure, or ownership of the application logic.

Highlights

• One view serves HTTP, MCP, and CLI. You annotate the route once and all three entry points share the same logic, auth, and validation.
• Exposing a view to AI agents takes a single flag. No extra service, no separate schema file, no adapter to maintain.
• Security is on by default. Host checking, CORS, body limits, and request IDs run without you touching configuration.
• Every request carries source and entrypoint metadata, so audit logs always know weather a human or an AI agent used your backend and how it arrived.
• It gives slightly better performance than FastAPI for real workloads, with no measurable overhead when database I/O dominates.
• Its simple to use, with a small API surface and no extra configuration required.

A Small App

from quater import AuthContext, AuthRequest, HTTPError, Quater, Request


async def authenticate(ctx: AuthRequest) -> AuthContext | None:
    if ctx.headers.get("authorization") != "Bearer admin-token":
        return None
    return AuthContext(subject="admin")


app = Quater(mcp_auth=authenticate, cli_auth=authenticate)

ORDERS: dict[str, dict[str, object]] = {
    "ord_1001": {"id": "ord_1001", "status": "paid", "total": 42.5}
}


@app.get("/health")
async def health() -> dict[str, bool]:
    return {"ok": True}


@app.get(
    "/orders/{order_id}",
    tool=True,
    cli=True,
    auth=authenticate,
    description="Fetch one order by id.",
)
async def get_order(order_id: str, request: Request) -> dict[str, object]:
    order = ORDERS.get(order_id)
    if order is None:
        raise HTTPError("Order not found", status_code=404)
    assert request.auth is not None
    return {
        **order,
        "subject": request.auth.subject,
        "source": request.context.source,
        "entrypoint": request.context.entrypoint,
    }

Run it:

python -m pip install quater
quater dev main.py

If you use uv, install with uv add quater instead.

Expected server output:

[INFO] Starting granian
[INFO] Listening at: http://127.0.0.1:8000

1. Call HTTP:

curl -H "Authorization: Bearer admin-token" \
  http://127.0.0.1:8000/orders/ord_1001

{
  "id": "ord_1001",
  "status": "paid",
  "total": 42.5,
  "subject": "admin",
  "source": "api",
  "entrypoint": "server"
}

2. Call the same handler as an MCP tool:

curl http://127.0.0.1:8000/mcp \
  -H "Authorization: Bearer admin-token" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"get_order","arguments":{"order_id":"ord_1001"}}}'

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"id\":\"ord_1001\",\"status\":\"paid\",\"total\":42.5,\"subject\":\"admin\",\"source\":\"mcp\",\"entrypoint\":\"server\"}"
      }
    ],
    "isError": false
  }
}

3. Call the same handler from the local CLI without a server round trip:

export QUATER_APP=main:app
export QUATER_TOKEN=admin-token
quater actions list
quater call get_order --order-id ord_1001

{
  "id": "ord_1001",
  "status": "paid",
  "total": 42.5,
  "subject": "admin",
  "source": "cli",
  "entrypoint": "local"
}

4. For a hosted app, connect once and call the named remote, just like git:

quater connect store https://api.example.com --token admin-token
quater actions describe store get_order
quater call store get_order --order-id ord_1001

Data flow diagram

flowchart TB
    caller["Caller\nperson, service, or AI agent"]
    http["HTTP request\nGET /orders/ord_1001"]
    mcp["MCP tool call\ntools/call get_order"]
    remote_cli["Remote CLI action\nquater call store get_order"]
    adapter["Server adapter\nRSGI / ASGI / WSGI"]
    checks["Framework checks\nhost, body limit, CORS, request id"]
    router["Route metadata\nmethod, path, auth, resources"]
    mcp_surface_auth["MCP auth\nmcp_auth"]
    cli_surface_auth["CLI auth\ncli_auth"]
    route_auth["Route auth\nauth="]
    handler["Your handler\nget_order(...)"]
    response["Serialized response\nJSON, text, bytes, stream"]

    caller --> http
    caller --> mcp
    caller --> remote_cli
    http --> adapter
    mcp --> adapter
    remote_cli --> adapter
    adapter --> checks
    checks --> router
    router -->|HTTP| route_auth
    router -->|MCP| mcp_surface_auth --> route_auth
    router -->|remote CLI| cli_surface_auth --> route_auth
    route_auth --> handler
    handler --> response

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Why This Shape

Quater treats HTTP, MCP, and CLI as different ways to reach the same backend capability, not as three products you have to maintain.

• For people and services: Quater gives you normal HTTP APIs with route decorators, OpenAPI, Swagger UI, request binding, response classes, route groups, middleware, and tests.
• For MCP Clients: tool=True exposes selected routes through MCP with required descriptions, generated input schemas, transport auth, MCP docs, and audit hooks.
• For AI agents: cli=True exposes selected routes as local or remote CLI actions with discovery, dry-run, approval hooks, and JSON output for scripts.
• For the app itself: route auth, resources, app.state, lifespan hooks, and serialization stay attached to the handler instead of drifting into wrappers.
• For performance: the request path stays deliberately small with Granian/RSGI, msgspec JSON, and a native route matcher.

Benchmarks

To measure performance, we ran benchmarks on an Apple M2 with an 8-core CPU, 16 GiB RAM, macOS 26.3, Python 3.11.12, and one worker per app. Quater performed slightly better than FastAPI when real database work was involved.

For very small endpoints without database access, FastAPI can be faster because the benchmark mostly measures framework overhead. Quater still runs built-in checks such as host validation, request IDs, body limits, and security headers on every request.

In the latest run, Quater used Granian/RSGI, while FastAPI used Uvicorn with uvloop and httptools. Full setup, commands, and CSV files are available in benchmarks.

Current Status

Quater is in pre-release stage. The documented top-level imports are the public surface, but names and defaults can still change before the first stable release.

Documentation

• Quickstart: build the first app.
• Why Quater Exists: understand the problem Quater is built around.
• Manual: read the full guide and reference.

Agent Skills

Quater ships two agent skills:

• quater-apps: for operating applications built with Quater through MCP, CLI actions, and HTTP. The applications build on quater can have their own skills for operating their applications.
• quater-framework: for building and debugging applications with Quater.

Install the app-operator skill:

npx -y skills add \
  https://github.com/DevilsAutumn/quater/tree/main/agent-skills/quater-apps

Install the framework-development skill:

npx -y skills add \
  https://github.com/DevilsAutumn/quater/tree/main/agent-skills/quater-framework

Working On Quater

This repo uses uv for local development:

uv sync --group dev
uv run pytest
uv run mypy
uv run ruff format --check src tests scripts
uv run ruff check src tests scripts
uv build

Docs use VitePress:

npm install
npm run docs:reference
npm run docs:dev
npm run docs:build