NitroStack Python SDK

Build Model Context Protocol (MCP) servers in Python with a NestJS-style structure: modules, dependency injection, decorators for tools, resources and prompts, and a pipeline of guards, middleware, interceptors, pipes and exception filters. It's the Python port of the NitroStack TypeScript framework.

Docs: docs.nitrostack.ai/python
Source and issues: github.com/nitrocloudofficial/nitrostack-python-sdk
Requires: Python 3.10+

Features

Modules and dependency injection: group code with @module, inject services with @injectable(deps=[...]).
Tools, resources and prompts: plain decorated methods, with Pydantic input/output schemas, tool annotations and examples.
Execution pipeline: guards, middleware, interceptors, pipes and exception filters per tool.
Auth: API keys, JWT, and OAuth 2.1 (protected resource metadata, token introspection or JWKS, PKCE, CIMD, scopes).
Long-running tasks: tools that report progress and can be cancelled, over the MCP tasks API.
Widgets: attach an HTML UI to a tool for ChatGPT Apps and MCP Apps clients.
Transports: stdio, Streamable HTTP (plus legacy SSE), or both at once; 2025 and 2026 protocol versions.
Caching, rate limits, events and health checks as decorators.
Testing harness: call tools, resources and prompts in-process, without a transport.
CLI (nitrostack-py): scaffold, generate code, run with hot reload, validate, pack wheels, upgrade.
NitroStudio: open a project folder in NitroStudio to run, chat with and debug it.

Installation

pip install nitrostack
# or
uv add nitrostack

To work on the SDK itself (tests and tooling):

pip install -e ".[dev]"

Scaffolding a New Project (Recommended)

nitrostack-py init my-server --template python-starter

(Or via Python: python -m nitrostack.cli.main init)

Both arguments are optional; without them init asks:

Project name. Default: my-mcp-server.

Template:

Template	What it shows
`python-starter`	A calculator: tools, a widget, a health check.
`python-pizzaz`	A pizza shop finder with map and list widgets.
`python-oauth`	Flight booking with OAuth 2.1 and scope-guarded tools (see `OAUTH_SETUP.md` in the project).

Description and author.
Install dependencies (Y/n): installs the Python dependencies (uv sync when uv is on PATH, otherwise .venv + pip), then runs npm install in src/widgets if the template has one. --skip-install skips this.

init also installs agent skills for this SDK (from skills-python-sdk) into the project's coding-agent folders. --force overwrites existing skill folders.

Port flags override the defaults (3000 for MCP, 3001 for widgets):

nitrostack-py init my-server --template python-starter --port 4000 --widget 4001
nitrostack-py dev --port 4000 --widget 4001
nitrostack-py start --port 4000 --widget 4001

init and pack vendor this SDK into vendor/nitrostack and pin it with [tool.uv.sources]. The dependency name stays nitrostack in pyproject.toml and requirements.txt, so nitrostack-py install and NitroCloud still run uv sync. Do not put a file: URL in [project].dependencies; that breaks the cloud wheel metadata. The project runs that vendored copy, whatever the version spec says; nitrostack-py upgrade replaces it with another release.

Once scaffolded, follow the next steps the CLI prints.

Quick Start

1. Write your First Server

Create a file named app.py:

import asyncio
from pydantic import BaseModel, Field
from nitrostack import (
    tool,
    resource,
    injectable,
    module,
    mcp_app,
    McpApplicationFactory,
    ServerConfig,
    ExecutionContext,
)

# 1. Input Validation Schema
class AddInput(BaseModel):
    a: float = Field(description="First number")
    b: float = Field(description="Second number")

# 2. Injected Provider Service
@injectable(deps=[])
class CalculatorService:
    def add(self, a: float, b: float) -> float:
        return a + b

# 3. Controller
@injectable(deps=[CalculatorService])
class CalculatorController:
    def __init__(self, service: CalculatorService):
        self.service = service

    @tool(
        name="add",
        description="Add two numbers together",
        input_schema=AddInput
    )
    async def add(self, input: AddInput, context: ExecutionContext) -> float:
        context.logger.info(f"Adding {input.a} and {input.b}")
        return self.service.add(input.a, input.b)

    @resource(
        uri="calc://info",
        name="Calculator Info",
        description="Metadata about this calculator"
    )
    async def get_info(self, context: ExecutionContext) -> str:
        return "Simple Add Calculator v1.0.0"

# 4. Modules
@module(
    name="calculator",
    controllers=[CalculatorController],
    providers=[CalculatorService]
)
class CalculatorModule:
    pass

@module(
    name="app",
    imports=[CalculatorModule]
)
class AppModule:
    pass

# 5. Application Entrypoint
@mcp_app(
    module=AppModule,
    server=ServerConfig(name="math-server", version="1.0.0")
)
class App:
    pass

async def main():
    app = await McpApplicationFactory.create(App)
    await app.start()

if __name__ == "__main__":
    asyncio.run(main())

Run it with python app.py (stdio), or MCP_TRANSPORT_TYPE=http python app.py for Streamable HTTP on http://127.0.0.1:3000/mcp.

Scaffolded projects split the same pieces into files: main.py calls McpApplicationFactory.create(AppModule), app_module.py holds the root module, and each feature lives in modules/<name>/. When the server's name is generic (app or mcp-server, as with a root @module(name="app") and no @mcp_app), the name and description reported to clients come from SERVER_NAME / SERVER_DESC.

2. Configure Environment Variables

The SDK reads settings from the process environment and the project's .env file. The process environment wins: a variable already set when the server starts (for example PORT=3001 from NitroStudio or your shell) is not overwritten by .env. .env fills in what isn't set, and ConfigModule.for_root(defaults=...) fills in the rest. (Before 0.4.1, .env overwrote the environment.)

Environment Variable	Description
`HOST`	Bind address for HTTP/SSE (default: `127.0.0.1`). Set `0.0.0.0` to listen on all interfaces (containers).
`TRUSTED_PROXIES` / `MCP_TRUSTED_PROXIES`	Comma-separated IPs, CIDRs, or hostnames allowed to send `X-Forwarded-Host` / `X-Forwarded-Proto`. Unset means those headers are ignored. `X-Forwarded-For` is never used to decide trust.
`PORT` / `MCP_SERVER_PORT`	The port to bind for HTTP/SSE transport (default: `3000`). Overridden by `nitrostack-py --port`.
`WIDGETS_DEV_PORT`	Widget Next.js port (default: `3001`). Overridden by `nitrostack-py --widget`.
`SERVER_NAME` / `SERVER_DESC`	Server name and description reported to clients when the configured name is generic (`app`, `mcp-server`). `init` sets `SERVER_NAME` to the project folder name.
`MCP_TRANSPORT_TYPE`	Transport selection: `stdio`, `http`, or `dual` (combining stdio + HTTP/SSE).
`NODE_ENV`	If set to `production`, defaults to `dual` transport. Otherwise defaults to `stdio`.
`MCP_MAX_SESSIONS`	Cap on concurrent Streamable HTTP sessions; new sessions beyond the cap get an HTTP `429`. Unset = unlimited.
`MCP_SESSION_TIMEOUT_MS`	Idle timeout (ms) for stateful HTTP sessions; sessions with no activity for this long are terminated automatically. Unset = no timeout.
`MCP_GRACEFUL_SHUTDOWN_TIMEOUT_MS`	How long (ms) the HTTP transport waits for in-flight requests to finish when shutting down (default: `10000`).
`NITRO_MCP_PROTOCOL_VERSION`	Protocol era (case-insensitive): `auto` / `both` / `dual` / `dual-spec` (default when unset or unknown), `modern` / `latest` / `2026` / `2026-07-28`, or `legacy` / `2025` / `2025-06-18` / `2025-11-25`. `auto` is not the same as `modern`. Wins over `ServerConfig.protocol_era`.
`MCP_STATELESS`	Explicit override: `true` forces `modern` (stateless HTTP), `false` forces `legacy` (sessionful). Wins over `NITRO_MCP_PROTOCOL_VERSION` and `ServerConfig.protocol_era`.
`MCP_ALLOWED_HOSTS` / `MCP_ALLOWED_ORIGINS`	Comma-separated allow-lists for DNS-rebinding protection, used only when CORS is disabled.
`NITROSTACK_APP_MODE`	Widget metadata flavour: `universal` (default), `openai`, or `mcp-app`. See Widgets.
`OAUTH_REQUIRED`	With `OAuthGuard`: `true` rejects calls without a valid token. Unset lets token-less calls through, for local development.
`NITROSTACK_LOG_FILE`	Destination file for logs (default: `nitrostack.log`).
`NITROSTACK_LOG_LEVEL`	Log level (`DEBUG`, `INFO`, `WARNING`, `ERROR`).
`NITROSTACK_LOG_TO_STDOUT`	Set to `true` to allow logging to stdout under stdio transport (Caution: may corrupt protocol stream).
`NITROSTACK_HTTP_DEBUG`	Set to `1` to log every HTTP request (method, path, headers, body) and response status to stderr. Use when diagnosing a client that fails to connect: uvicorn's access log shows neither headers nor timestamps.

Read your own settings through ConfigService:

from nitrostack import ConfigModule, ConfigService, injectable, module

@injectable(deps=[ConfigService])
class WeatherService:
    def __init__(self, config: ConfigService):
        self.units = config.get("WEATHER_UNITS", "metric")
        self.api_key = config.get_or_throw("WEATHER_API_KEY")  # KeyError if unset

@module(
    name="app",
    imports=[ConfigModule.for_root(env_file_path=".env", defaults={"WEATHER_UNITS": "metric"})],
    providers=[WeatherService],
)
class AppModule:
    pass

Building Blocks

The examples below build on each other (later ones reuse LookupInput and NoInput) and are methods on a controller (an @injectable class listed in a module's controllers). Every tool takes (self, input, context), where input is an instance of its input_schema model and context is the ExecutionContext (logger, auth, task, request metadata).

Tools

from pydantic import BaseModel, Field
from nitrostack import tool, initial_tool, ToolAnnotations, ToolExamples, ExecutionContext

class LookupInput(BaseModel):
    city: str = Field(description="City name")

class Weather(BaseModel):
    city: str
    temp: float
    units: str

class NoInput(BaseModel):
    pass

class WeatherController:
    @tool(
        name="get_weather",
        title="Get Weather",
        description="Current weather for a city",
        input_schema=LookupInput,
        output_schema=Weather,  # optional; adds structuredContent
        annotations=ToolAnnotations(read_only_hint=True, destructive_hint=False, idempotent_hint=True),
        examples=ToolExamples(input={"city": "Paris"}, output={"city": "Paris", "temp": 21, "units": "metric"}),
    )
    async def get_weather(self, input: LookupInput, context: ExecutionContext) -> dict:
        return {"city": input.city, "temp": 21, "units": "metric"}

    @tool(name="whats_new", description="Shown when a client connects", input_schema=NoInput)
    @initial_tool  # clients that support it call this on connect
    async def whats_new(self, input: NoInput, context: ExecutionContext) -> str:
        return "Try get_weather."

A tool with no arguments still needs an input model; use an empty BaseModel subclass like NoInput (not BaseModel itself). visibility="hidden" keeps a tool callable but out of tools/list. Raise an exception to return an MCP error to the client.

Resources and prompts

from typing import List
from nitrostack import resource, prompt, PromptArgument, PromptMessage, ExecutionContext

class WeatherController:
    @resource(uri="weather://units", name="Units", description="Units in use", mime_type="text/plain")
    async def units(self, context: ExecutionContext) -> str:
        return "metric"

    @prompt(
        name="forecast_brief",
        description="Write a short forecast",
        arguments=[PromptArgument(name="city", description="City", required=True)],
    )
    async def forecast_brief(self, args: dict, context: ExecutionContext) -> List[PromptMessage]:
        return [PromptMessage(role="user", content=f"Write a short forecast for {args['city']}.")]

Guards, interceptors, pipes, middleware and filters

Each is a small class with one async method; attach it with a decorator below @tool. Generate stubs with nitrostack-py generate guard|pipe|interceptor|filter <Name>.

Stage	Method	Attach with	Runs
Guard	`can_activate(context) -> bool`	`@use_guards(...)`	first; `False` rejects the call ("Access denied by guard")
Pipe	`transform(value, metadata)`	`@use_pipes(...)`	on the tool input, after guards
Middleware	`use(context, next_fn)`	`@use_middleware(...)`	around interceptors and the tool
Interceptor	`intercept(context, next_fn)`	`@use_interceptors(...)`	around the tool; can change its result
Exception filter	`catch(error, context)`	`@use_filters(...)`	on errors; its return value becomes the result

from nitrostack import tool, use_guards, use_interceptors, use_filters, ExecutionContext

class OfficeHoursGuard:
    async def can_activate(self, context: ExecutionContext) -> bool:
        return context.metadata.get("office_open", True)

class TimingInterceptor:
    async def intercept(self, context: ExecutionContext, next_fn):
        result = await next_fn()
        context.logger.info(f"{context.tool_name} finished")
        return result

class FriendlyErrors:
    async def catch(self, error: Exception, context: ExecutionContext):
        return {"error": str(error)}

class WeatherController:
    @tool(name="get_forecast", description="Forecast for a city", input_schema=LookupInput)
    @use_guards(OfficeHoursGuard)
    @use_interceptors(TimingInterceptor)
    @use_filters(FriendlyErrors)
    async def get_forecast(self, input: LookupInput, context: ExecutionContext) -> dict:
        return {"city": input.city, "forecast": "sunny"}

Authentication

Register an auth module in the root module, then guard tools. Guards fill context.auth (subject, scopes, client_id, claims, ...).

Module	Guard	Configured by
`ApiKeyModule.for_root(keys_env_prefix="API_KEY", hashed=False)`	`ApiKeyGuard`	`x-api-key` header checked against `API_KEY`, `API_KEY_1`, `API_KEY_2`, ... (SHA-256 hashes when `hashed=True`)
`JWTModule.for_root(secret_env_var="JWT_SECRET", audience=..., issuer=...)`	`JwtGuard`	`Authorization: Bearer <jwt>`
`OAuthModule.for_root(resource_uri=..., authorization_servers=[...], scopes_supported=[...], token_introspection_endpoint=... or jwks_uri=...)`	`OAuthGuard`	OAuth 2.1 access tokens, audience-bound; `OAUTH_REQUIRED=true` to require one

from nitrostack import (
    ApiKeyGuard, ApiKeyModule, ExecutionContext, OAuthGuard, module, require_scopes, tool, use_guards,
)

class AccountController:
    @tool(name="get_balance", description="Account balance", input_schema=NoInput)
    @use_guards(ApiKeyGuard)
    async def get_balance(self, input: NoInput, context: ExecutionContext) -> dict:
        return {"balance": 42}

    @tool(name="close_account", description="Close the account", input_schema=NoInput)
    @use_guards(OAuthGuard)
    @require_scopes("admin")  # needs context.auth with every listed scope
    async def close_account(self, input: NoInput, context: ExecutionContext) -> dict:
        return {"closed": True}

@module(name="app", imports=[ApiKeyModule.for_root()], controllers=[AccountController])
class AppModule:
    pass

Register ApiKeyModule when you use ApiKeyGuard: without it, the guard falls back to the API_KEY variable, and with neither set it lets every call through.

has_scope, has_any_scope and has_all_scopes check context.auth inside a tool. For OAuth, start from the python-oauth template and its OAUTH_SETUP.md; PKCE (generate_pkce_params, verify_pkce), client ID metadata documents (resolve_cimd) and generate_www_authenticate_header are exported for custom flows.

Caching and rate limits

from nitrostack import cache, injectable, rate_limit, tool

@injectable()
class WeatherService:
    @cache(ttl=60)  # seconds; keyed on the arguments
    async def lookup(self, city: str) -> dict:
        return {"city": city, "temp": 21}

class WeatherController:
    @tool(name="ping", description="Ping", input_schema=NoInput)
    @rate_limit(max=10, window=60)  # 10 calls per 60 s, across all callers
    async def ping(self, input: NoInput, context: ExecutionContext) -> str:
        return "pong"

Put @cache on a service method with plain, hashable arguments (strings, numbers, tuples). It can't key on a tool's Pydantic input object.

Long-running tasks

Set task_support="optional" (or "required") and the client can run the tool as an MCP task: it polls tasks/get, can cancel it, and gets live notifications/progress when it sends a progressToken. context.task is None when the tool is called normally.

import asyncio
from pydantic import BaseModel
from nitrostack import tool, ExecutionContext

class ReportInput(BaseModel):
    steps: int = 5

class ReportController:
    @tool(name="build_report", description="Build a report", input_schema=ReportInput, task_support="optional")
    async def build_report(self, input: ReportInput, context: ExecutionContext) -> dict:
        for step in range(1, input.steps + 1):
            if context.task:
                context.task.update_progress(f"Step {step}/{input.steps}")
                context.task.throw_if_cancelled()
            await asyncio.sleep(1)
        return {"steps": input.steps}

Events and health checks

from typing import List
from nitrostack import EventEmitter, health_check, injectable, on_event

@injectable()
class OrderService:
    async def place(self, order_id: str) -> None:
        await EventEmitter.get_instance().emit("order.placed", {"id": order_id})

@injectable()
class AuditLog:
    def __init__(self):
        self.entries: List[str] = []

    @on_event("order.placed")
    def record(self, payload: dict) -> None:
        self.entries.append(payload["id"])

    @health_check("audit")
    def healthy(self) -> bool:
        return True

Listeners and health checks are found on any provider or controller in a module. When any health check exists, the server exposes the health://status resource ({"audit": "healthy"}). Keep health checks synchronous.

Transport Options

NitroStack apps can run over three transports, selected via MCP_TRANSPORT_TYPE (or ServerConfig(transport_type=...)):

stdio (default outside production): JSON-RPC over stdin/stdout — the standard mode for desktop MCP clients (Claude Desktop, Cursor, etc.).
http: Streamable HTTP + legacy SSE over a real network port, for cloud/remote deployments. Exposes:
- GET / — documentation landing page (connection setup for Cursor/Claude/ChatGPT plus the registered tool catalog). Same page TypeScript NitroStack serves on Open URL.
- POST/GET/DELETE /mcp — Streamable HTTP (session-based JSON-RPC + SSE streaming). /mcp and /mcp/ are equivalent; the server does not 307 between them (MCP Inspector needs the no-slash URL for its SSE GET).
- GET /sse + POST /mcp/messages/ — legacy HTTP+SSE for older clients (trailing slash required so messages aren't swallowed by the Streamable HTTP /mcp mount)
- GET /mcp/health — health check (status, active session count, uptime)
- Per-session isolation, idle-session timeouts, and DNS-rebinding protection are provided by the underlying mcp SDK's StreamableHTTPSessionManager; NitroStack adds CORS, a concurrent-session cap, and the health endpoint on top.
- Task-mode tools that call context.task.update_progress(...) push a live notifications/progress event over the session's SSE stream (in addition to always being pollable via tasks/get) whenever the client sends a _meta.progressToken on the tools/call request.
dual (default in production): runs stdio and http concurrently as asyncio tasks in the same process/event loop — not separate threads — so both share the same DIContainer singletons, and uvicorn's signal-based graceful shutdown works correctly (it only installs signal handlers on the main thread). Shutdown is coordinated: either transport stopping (STDIO hitting EOF, or HTTP receiving a termination signal) cleanly stops the other.

Example:

server = ServerConfig(name="my-server", transport_type="http", max_sessions=100, session_timeout_ms=1_800_000)

ServerConfig.protocol_era is used only when MCP_STATELESS and NITRO_MCP_PROTOCOL_VERSION are both unset. Unknown tokens become auto, same as an unknown env value.

CLI (`nitrostack-py`)

The CLI is installed with the SDK (nitrostack-py, or python -m nitrostack.cli.main). Run nitrostack-py --help to list commands.

Project lifecycle

nitrostack-py init my-server
nitrostack-py dev          # hot-reload development server
nitrostack-py start        # production server (no reload)
nitrostack-py register --name my-mcp-server --file main.py

Generate components

nitrostack-py generate tool add_numbers
nitrostack-py generate module payments
nitrostack-py generate guard MyGuard
nitrostack-py generate pipe Validation
nitrostack-py generate interceptor Transform
nitrostack-py generate filter HttpException
nitrostack-py generate service Email

Generated files:

Command	Output
`generate tool <name>`	`{name}_tool.py` in the current directory
`generate module <name>`	`{name}_module.py` in the current directory
`generate guard <Name>`	`guards/<name>.py`
`generate pipe <Name>`	`pipes/<name>.py`
`generate interceptor <Name>`	`interceptors/<name>.py`
`generate filter <Name>`	`filters/<name>.py`
`generate service <Name>`	`services/<name>.py`

Attach generated pipeline classes with @use_guards, @use_pipes, @use_interceptors, or @use_filters. Register services in a module's providers list.

Pack a deployable wheel

nitrostack-py pack --dry-run    # list files; does not write an artifact
nitrostack-py pack              # write dist/*.whl
nitrostack-py pack --include-env  # also pack .env files (use with caution!)

pack builds a wheel with setuptools (the same backend as this SDK), refreshes requirements.txt from pyproject.toml when possible, and always includes .env.example. .env, .env.local, and .env.production are excluded unless you pass --include-env. Temporary build directories are deleted afterwards.

Upgrade, install, validate

nitrostack-py upgrade                 # move the project to the latest nitrostack on PyPI
nitrostack-py upgrade --version 0.4.1 # move it to exactly this version
nitrostack-py upgrade --dry-run       # print the change; do not edit files
nitrostack-py upgrade --allow-downgrade --version 0.1.0  # required to pin an older version

nitrostack-py install                 # install project + development dependencies
nitrostack-py install --production    # skip optional extras and requirements-dev.txt

nitrostack-py validate                # lint deps, @mcp_app imports, and @module() refs

init writes pyproject.toml, .python-version, and uv.toml, then runs uv lock when uv is on PATH. install prefers uv sync in that case. If requirements.txt pins nitrostack as a local path (-e /path/to/nitrostack-python-sdk), install uses pip install -r requirements.txt instead so unpublished SDK testing still works. The uv equivalent is:

[tool.uv.sources]
nitrostack = { path = "/path/to/nitrostack-python-sdk", editable = true }

then uv lock / uv sync. Without uv, install falls back to .venv + pip.

upgrade in a project with vendor/nitrostack (every project init creates) replaces the vendored copy with the target release: the wheel is downloaded from PyPI and checked against its published sha256, or copied from the running CLI when it is that release. The dependency stays the plain name nitrostack. In other projects, upgrade updates the nitrostack spec in pyproject.toml's dependencies (and requirements.txt when it already pins nitrostack): --version X writes nitrostack==X, and without --version the latest release is written as nitrostack>=latest. Either way, run nitrostack-py install afterwards, and a target older than the current version is rejected unless --allow-downgrade is passed. validate reports missing/conflicting dependencies, @mcp_app modules that fail to import, and @module() imports/exports that are not real classes.

Widgets (UI tools)

Bind an HTML template to a tool with @widget and return domain JSON from the handler:

from nitrostack import tool, widget, ExecutionContext

class ShopController:
    @tool(name="show_card", description="Product card", input_schema=CardInput)
    @widget("card")
    async def show_card(self, input: CardInput, context: ExecutionContext) -> dict:
        return {"name": "Widget", "price": 9.99}

Place HTML at widgets/out/{route}.html (e.g. widgets/out/card.html). The SDK registers ui://widget/card.html as an MCP resource and sets mode-gated _meta on tools/list and tools/call results.

NITROSTACK_APP_MODE (default universal):

Mode	Tool `_meta`	Resource MIME
`universal` (default)	Both OpenAI and MCP Apps keys	`text/html;profile=mcp-app`
`openai`	`openai/outputTemplate`, `ui/template`	`text/html`
`mcp-app`	`_meta.ui` (`resourceUri`, `visibility`, CSP)	`text/html;profile=mcp-app`

Object form for CSP and border options:

from nitrostack import WidgetOptions, WidgetCsp, widget

@widget(WidgetOptions(
    route="chart",
    prefers_border=True,
    csp=WidgetCsp(connect_domains=["https://api.example.com"]),
))
async def show_chart(self, input, context):
    ...

nitrostack-py init copies widgets/out/{route}.html for every @widget. Widget HTML is generated in Python from the tool's structuredContent (one iframe, N cards).

Custom widget HTML: data and host bridge

The tool result is passed to the widget as a JSON-compatible object. The shared host bridge normalizes the following sources, in order:

window.openai.toolOutput.structuredContent (or toolResult).
A JSON application/json content block, or a JSON object in a text/plain content block.
The #nitrostack-tool-data embedded JSON script used by Python-rendered pages.

The normalized object is passed to window.__nitroWidgetRender(data). Keep the tool return shape stable and render from that object. NITROSTACK_APP_MODE changes the advertised metadata and resource MIME type, not this data shape:

openai uses OpenAI output-template metadata and the OpenAI host data channel.
mcp-app uses MCP Apps _meta.ui metadata; tool-result notifications are normalized by the bridge.
universal (the default) publishes both forms.

A hand-written file found at widgets/out/{route}.html is loaded as-is. The loader does not inject the host bridge into that file. The built-in route templates created by init do include the bridge, so custom HTML must include the equivalent bridge/render hook itself. The generated files under examples/widgets/out/ are the reference implementation.

Minimal render hook (used after the bridge has been included):

<div id="value">Waiting for tool data…</div>
<script>
  window.__nitroWidgetRender = function (data) {
    const value = data && data.value;
    document.getElementById("value").textContent =
      value == null ? "—" : String(value);
  };
</script>

Widget CSP metadata

WidgetCsp describes the origins a widget needs; NitroStack publishes this as metadata for the host to enforce. The SDK does not build or attach a response Content-Security-Policy header itself.

Field	Meaning	MCP Apps metadata	OpenAI metadata
`connect_domains`	Network connections made by the widget	`connectDomains`	`connect_domains`
`resource_domains`	External resources such as images, scripts, or styles	`resourceDomains`	`resource_domains`
`frame_domains`	Origins the widget may embed in frames	`frameDomains`	`frame_domains`

Whether a scheme such as data: is accepted is host-dependent. Listing data: in resource_domains requests/advertises that resource origin; it is not a guarantee that every host will allow data URIs.

MCP Inspector: use HTTP + stateless, then the Apps tab. tools/call also embeds the data-filled HTML. Do not use widgets/preview.html as the live result — that file is a static helper. Live preview: http://localhost:3000/widgets/preview.

Turn Authentication off in Inspector for the starter and pizzaz templates (they have no OAuth). If Auth is on, Inspector POSTs /register and you will see Cannot POST /register / Unexpected token '<'. Connect Streamable HTTP to http://localhost:3000/mcp (no trailing slash).

For open pizza shops only, call show_pizza_list with {"openNow": true} or show_pizza_map with {"filter": "open_now"}. Omitting those fields returns every shop, including closed ones (Pizzeria Delfina).

Example server: examples/widgets_example.py with templates in examples/widgets/out/. For MCP Inspector over HTTP, use stateless mode:

cd examples
MCP_TRANSPORT_TYPE=http MCP_STATELESS=true NITROSTACK_APP_MODE=universal python widgets_example.py

Using with Clients

NitroStudio

Open the project folder in NitroStudio. It recognises a Python NitroStack project by main.py plus nitrostack in requirements.txt or pyproject.toml, starts it, and lets you call tools, preview widgets, chat with it, and run OAuth. Studio passes the port it picks through PORT, which wins over .env (0.4.1 and later), so several projects can run side by side.

Claude Desktop

nitrostack-py register --name my-mcp-server --file main.py

(If your scripts folder is not in PATH, use: python -m nitrostack.cli.main register --name my-mcp-server --file main.py)

This finds the Claude Desktop config (standard and Windows Store installs), points it at your virtualenv's Python, and writes the server entry. Restart Claude Desktop afterwards.

Testing

Testing Harness

Call tools, resources and prompts in-process with NitroTestingModule; no transport, no subprocess:

import asyncio
from nitrostack.testing import NitroTestingModule
from app import AppModule

async def test_add():
    harness = await NitroTestingModule.create(AppModule)
    assert await harness.call_tool("add", {"a": 5, "b": 10}) == 15.0
    assert await harness.read_resource("calc://info") == "Simple Add Calculator v1.0.0"

if __name__ == "__main__":
    asyncio.run(test_add())

call_tool takes the tool's arguments as an MCP client sends them and returns the decoded result (JSON is parsed; errors come back as their message). get_prompt(name, args) returns the prompt messages.

Running the SDK's tests

pip install -e ".[dev]"
pytest tests

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NitroStack Python SDK

Features

Installation

Scaffolding a New Project (Recommended)

Quick Start

1. Write your First Server

2. Configure Environment Variables

Building Blocks

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Resources and prompts

Guards, interceptors, pipes, middleware and filters

Authentication

Caching and rate limits

Long-running tasks

Events and health checks

Transport Options

CLI (`nitrostack-py`)

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Generate components

Pack a deployable wheel

Upgrade, install, validate

Widgets (UI tools)

Custom widget HTML: data and host bridge

Widget CSP metadata

Using with Clients

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Running the SDK's tests

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Name		Name	Last commit message	Last commit date
.github/workflows		.github/workflows
examples		examples
nitrostack		nitrostack
scripts		scripts
tests		tests
.gitignore		.gitignore
.python-version		.python-version
MANIFEST.in		MANIFEST.in
README.md		README.md
pyproject.toml		pyproject.toml
requirements.txt		requirements.txt
tox.ini		tox.ini
uv.lock		uv.lock
uv.toml		uv.toml

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NitroStack Python SDK

Features

Installation

Scaffolding a New Project (Recommended)

Quick Start

1. Write your First Server

2. Configure Environment Variables

Building Blocks

Tools

Resources and prompts

Guards, interceptors, pipes, middleware and filters

Authentication

Caching and rate limits

Long-running tasks

Events and health checks

Transport Options

CLI (nitrostack-py)

Project lifecycle

Generate components

Pack a deployable wheel

Upgrade, install, validate

Widgets (UI tools)

Custom widget HTML: data and host bridge

Widget CSP metadata

Using with Clients

NitroStudio

Claude Desktop

Testing

Testing Harness

Running the SDK's tests

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CLI (`nitrostack-py`)