micromcp

A minimum-viable MCP server for the stateless 2026-07-28 protocol revision. Pure standard library — no runtime dependencies, no framework. Apache-2.0.

pip install micromcp            # Python 3.11+  (first release pending; until then:
pip install "micromcp[django]"  #  pip install git+https://github.com/mhalle/micromcp)

Prefer a single file you can vendor? python tools/bundle.py regenerates build/micromcp.py from the package; CI regenerates it and runs every suite against it.

One core, two transports:

• Server — a WSGI app (Django, Flask, gunicorn, waitress, wsgiref)

• ASGIServer — a native ASGI app (Starlette, FastAPI, Litestar, uvicorn)

Handlers may be def or async def on either. Sync handlers run on a thread pool, so a slow one neither serializes requests nor blocks the event loop.

Quick start

from micromcp import MCP, ASGIServer

mcp = MCP("newton-civic", "1.0.0")

@mcp.tool
def crash_count(street: str, since: int = 2020) -> dict:
    """Count crashes on a street since a given year."""
    return {"street": street, "count": 11}

app = ASGIServer(mcp)          # uvicorn micromcp_app:app

Type hints become JSON Schema, docstrings become tool descriptions, and a returned dict becomes both content text and structuredContent.

Related MCP server: Golf

Tools

@mcp.tool(title="Crash count by street", read_only=True, idempotent=True)
def crash_count(street: str, since: int = 2020) -> CrashCount: ...

@mcp.tool(name="severity-breakdown")     # override the wire name
def _internal(street: str | None = None) -> dict: ...

@mcp.tool(guards=[lambda principal: principal is not None])
def protected(x: int) -> dict: ...

read_only / destructive / idempotent become the standard annotation hints clients use to decide what to auto-approve. outputSchema is inferred from a TypedDict or dataclass return annotation, or passed explicitly via output_schema=.

Guard failures return isError: true in-band rather than an HTTP error, which is what lets a model reason about them.

Arguments are validated against the generated inputSchema before the handler runs: a wrong type, a missing key, or an unexpected one is -32602 (the schema closes additionalProperties, nested objects included). Validated JSON is then converted to what the signature declares — date, Enum, set, tuple, Path, dataclass, pydantic model — so the handler receives real objects, not strings and lists. When a tool declares an outputSchema, its structuredContent is checked against it and a mismatch is an in-band isError; dataclass and pydantic return values are converted to dicts first. Defaults that are not JSON (dates, enums, paths) are rendered as strings. Literal, Enum, tuple[int, str], set[str], dict[str, int], NewType, dates, paths, and pydantic models all map to the right schema, and the validator enforces what those schemas say — including the minimum, pattern, const, format, and $ref constraints pydantic emits, with nested $defs namespaced per parameter so two models' inner classes cannot collide. Conversion errors are answered with a constant -32602; the validator's text stays in the server log. A Google- or NumPy-style Args: section in the docstring becomes per-parameter descriptions and is removed from the tool description the model reads.

Registration is strict where silence would hurt: a duplicate name raises unless you pass replace=True, a name outside [A-Za-z0-9_-]{1,64} (the grammar LLM tool-calling APIs enforce) raises, positional-only parameters raise, and *args/**kwargs are left out of the schema.

Resources and prompts

@mcp.resource("schema://crash", mime_type="application/json")
def crash_schema() -> str: ...

@mcp.resource("crash://{crash_id}")      # template; crash_id coerced to int
def one_crash(crash_id: int) -> str: ...

@mcp.prompt
def review(street: str, year: int = 2025) -> list:
    return [{"role": "user", "content": {"type": "text", "text": "..."}}]

Exact URIs win over templates; among templates the longest literal prefix wins. Templates are reported via resources/templates/list. Placeholders must be plain identifiers — RFC 6570 operators ({?q}, {+path}) and duplicate names are rejected at registration — and matching is linear in the URI length. int and bool parameters are coerced strictly (+12, 12, banana are -32602, not silently 12 or False). A handler returning bytes is delivered as a base64 blob.

Prompt arguments arrive as strings and are coerced to the handler's int, float, or bool hints; Principal and Context parameters are injected exactly as for tools, and docstring Args: become argument descriptions.

UI apps (MCP Apps / MCP-UI)

A host that renders UI needs three things from a server, and all three are plain registration options:

from micromcp import MCP, ASGIServer, result, embedded_resource

HTML = open("widget.html").read()          # static: no guards, no templates, no user data

@mcp.resource("ui://crash-widget",         # mime defaults to text/html;profile=mcp-app
              meta={"ui": {"prefersBorder": True, "csp": {"resourceDomains": []}}})
def crash_widget() -> str:
    return HTML

@mcp.tool(meta={"ui": {"resourceUri": "ui://crash-widget"}})   # published as the tool's _meta
def show_crashes(street: str) -> dict:
    by_year = lookup(street)
    if by_year is None:
        return result([{"type": "text", "text": f"no data for {street}"}], is_error=True)
    return result([{"type": "text", "text": f"Crashes on {street}"}],
                  structured={"street": street, "by_year": by_year})

meta= on a tool, resource, template, or prompt is validated at registration (a dict, JSON-serializable, valid _meta keys, no reserved io.modelcontextprotocol/ prefix) and published as its _meta; a resource's meta also rides on its read contents. result(content, structured=, is_error=, meta=) is the explicit way to return a finished tool result; its blocks are checked against the five content-block types before they leave, and its _meta is merged under the server's identity stamp. A plain dict is always data and is wrapped as JSON text, so client-derived data can never be mistaken for a result. embedded_resource(uri, text=|blob=) builds the block that MCP-UI-style hosts render from the result itself; MCP Apps hosts instead load the widget from the ui:// resource the tool's _meta names. A declared outputSchema still requires structured.

Widgets must be static. Hosts fetch a ui:// resource under their own identity, with none of the end user's credentials, and cache it per connector: a guarded widget is simply never rendered, and a templated one would interpolate URI text into HTML the host runs. Registration warns about both. Every piece of per-user data belongs in the guarded tool's result, which the host delivers to the widget as ui/notifications/tool-result. Inside a tool, ctx.client_capabilities exposes what the client declared, so a handler can check for extensions["io.modelcontextprotocol/ui"] and return a text-only result to hosts without UI. A tool's _meta.ui.visibility is passed through for the host to honor; guards= remains the only server-side enforcement.

Verified 2026-09-14 in the Claude desktop chat via a custom connector (examples/mcp_app_modal.py, deployed on Modal), twice: first with an early widget, then with the current one, which completes the 2026-01-26 handshake, answers ping and ui/resource-teardown, renders only host-delivered results, and reports its size from a ResizeObserver. On a fresh connector the host prefetches the ui:// widget before its first tools/call, calls the tool, delivers the result to the iframe as ui/notifications/tool-result, and renders it. Two host behaviors worth knowing: the connector validator negotiates by sending initialize, taking the -32022 refusal, and retrying with server/discover; and the host caches a connector's widget after its first fetch, so a changed widget needs a new connector identity to be picked up. The reference servers still emit the deprecated flat ui/resourceUri key alongside ui.resourceUri; the example does the same until hosts drop it.

Injected parameters

Two annotations are filled server-side and excluded from the input schema, so the model never sees them and cannot supply them:

from micromcp import Principal, Context

@mcp.tool
def whoami(who: Principal) -> dict:
    return {"sub": who["sub"] if who else None}

@mcp.tool
async def segment(volume: str, ctx: Context) -> dict:
    for i, step in enumerate(steps, 1):
        await ctx.report_progress(i, len(steps), message=step.name)
    await ctx.info("done")
    return {"labels": 117}

Principal receives whatever the authenticate(headers) callback returned. Both annotations work on tools, resources, and prompts, on plain functions, bound methods, callable instances, and functools.partials, and through Optional, Union, Annotated, and PEP 695 type aliases. Detection survives an otherwise-unresolvable signature (PEP 563 / 649) as long as micromcp.Principal itself is importable at runtime; an annotation that merely reads Principal but does not resolve to this class — your own ACL type under a TYPE_CHECKING import, say — or a subclass of it, is refused at registration rather than guessed at, because guessing either way is an auth bug.

Declaring ctx: Context is what makes a tool stream: on ASGIServer the call is answered with a request-scoped SSE stream carrying notifications/progress and notifications/message frames, then the final JSON-RPC response. Client hang-up cancels the handler task.

Behaviors worth knowing:

• Streaming needs the client's Accept to admit text/event-stream; otherwise the call is answered with plain JSON and notifications are dropped. Validation, Origin, and authentication all run before the stream is committed, so a rejection keeps its real HTTP status.

• The frame queue is bounded, so a chatty handler paces itself against the socket instead of growing memory, and always reaches a cancellation point. Cancellation is cooperative: an async handler that swallows CancelledError keeps running and is logged, and a def handler cannot be interrupted at all — its thread runs until it returns (logged), so long sync loops should poll ctx.cancelled. A : keepalive comment goes out after 15 s of silence so proxies keep a long-running call open. Notification payloads must be JSON-serializable; a bad one raises inside the handler rather than tearing the stream down.

• Progress requires the client to send a progressToken; without one the notifications are dropped (there is nothing to correlate them to) while the result is still delivered normally. Logging carries no token and is unaffected.

• Under WSGI, notifications are dropped entirely and the call returns ordinary JSON. WSGI has no portable client-disconnect signal, so it cannot honor the spec's cancellation rule; shipping a stream that keeps computing after the client is gone would be worse than not streaming.

Auth

def authenticate(headers):                 # lowercase header mapping
    return verify(headers.get("authorization", ""))

app = ASGIServer(mcp, authenticate=authenticate,
                 allowed_origins={"https://app.example.com"},
                 allowed_hosts={"mcp.example.com"},     # DNS-rebinding guard
                 path="/mcp",                           # 404 elsewhere
                 max_body=1_000_000,                    # bytes; default 4 MiB
                 workers=64,                            # handler threads; default 32
                 timeout=120,                           # WSGI request cap, seconds
                 stream_budget=8_000_000,               # notification bytes per call
                 keepalive=15, cancel_grace=2,          # SSE tunables, seconds
                 queue_size=64, offload_bytes=65536)    # frames buffered; parse-off-loop size

authenticate may be async def; a sync one runs on a separate small pool, so it may block on a token store without competing with handlers. Guards run inline and must be cheap. Handler threads are daemons and the pool knows how busy it is: when all workers are occupied a sync call is refused with 503 rather than queued behind threads that may never return (a def handler cannot be interrupted, so poll ctx.cancelled in long loops). Both pools rebuild themselves after fork(). On WSGI a request that outlives timeout fails with 500 instead of hanging a worker; timeout=None or 0 disables the cap. stream_budget caps the notification bytes one streaming call may emit — the backstop for servers such as daphne whose send() never blocks, where the bounded queue alone cannot apply backpressure.

allowed_origins also drives CORS: preflights from a listed Origin get 204 with Access-Control-Allow-*, other Origins (including an empty one) get 403, and no Origin header means a non-browser client. allowed_hosts is off when empty. Content-Type must be application/json when present.

Guards are predicates over the principal only — they cannot see call arguments, so per-record authorization belongs inside the handler. They apply to tools, resources, and prompts alike: a guarded tool, resource, or prompt the caller may not use is indistinguishable from one that does not exist (404 / -32601), and listings show only what the caller may use. A guard that raises denies; a guard that raises Unauthorized (or another Error) chooses the answer when the entry is invoked, and hides it in listings.

OAuth handoff. Being an authorization server is out of scope, but the handoff to one is not. Raise Unauthorized from authenticate (or from a handler) and the answer is 401 with a WWW-Authenticate: Bearer challenge, which is what makes an OAuth-capable client (Claude.ai, the Inspector, the SDKs) start its flow. Pass resource_metadata= to serve the RFC 9728 document at /.well-known/oauth-protected-resource (and at the path-suffixed form when path= is set); the challenge then names that document's URL by default, derived at construction from the document's own resource (its origin plus the well-known path plus its path), never from Host or a proxy header, and the document is served at exactly that path (server.well_known_path), so the challenge always names something this server answers. If an outer router mounts the server under a prefix, requests for the well-known path never reach it: give the outer app a route for it, or run the server at the root with path=. A challenge with no URL at all is logged once as a warning: MCP clients cannot start OAuth from a bare Bearer. Guards may raise Unauthorized too, and do so before any byte is committed. A tool that raises it after its SSE stream has opened cannot change the status any more; the error travels in-band, which is why authentication and guards are the place for it.

from micromcp import Unauthorized

def authenticate(headers):
    token = headers.get("authorization", "").removeprefix("Bearer ")
    if not token:
        raise Unauthorized(scope="read")                 # start the OAuth flow
    principal = store.lookup(token)
    if principal is None:
        raise Unauthorized.invalid()                     # error="invalid_token"
    return principal

app = ASGIServer(mcp, authenticate=authenticate, path="/mcp",
                 resource_metadata={"resource": "https://mcp.example.com/mcp",
                                    "authorization_servers": ["https://auth.example.com"],
                                    "scopes_supported": ["read"],
                                    "bearer_methods_supported": ["header"]})

Mounting

# Flask, alongside ordinary routes
from werkzeug.middleware.dispatcher import DispatcherMiddleware
combined = DispatcherMiddleware(flask_app, {"/mcp": Server(mcp)})

# Starlette / FastAPI, native
from starlette.routing import Mount
routes = [Mount("/mcp", app=ASGIServer(mcp))]

# Django (adapters live in micromcp.contrib.django; also re-exported from micromcp)
from micromcp.contrib.django import django_view, django_async_view
urlpatterns = [path("mcp", django_async_view(ASGIServer(mcp)))]

Mount at the exact path clients will use: a framework Mount("/mcp") typically 307-redirects /mcp to /mcp/, and some clients (urllib, not the mcp SDK) will not replay a POST across a redirect. django_async_view streams Context tools as a StreamingHttpResponse of SSE frames; django_view (sync) returns plain JSON.

Scope

Implemented: server/discover, tools/list, tools/call, resources/list, resources/read, resources/templates/list, prompts/list, prompts/get, plus progress and logging notifications.

Deliberately not implemented: sessions, resumable streams, change notifications (subscriptions/listen is accepted and closed gracefully), completion/complete, MRTR/elicitation, pagination cursors, and being an OAuth authorization server. The initialize-handshake era is refused by default and served per request, without sessions, with legacy="stateless".

Requests without an id (JSON-RPC notifications) pass the Origin, Host, and Content-Type checks, are acknowledged with 202, and are never dispatched. Bodies that are not a single JSON-RPC object, including batch arrays, are 400 / -32600; a repeated Mcp-* routing header is -32020 (Mcp-Method is compared verbatim; only Mcp-Name carries the base64 sentinel, decoded exactly as the SDK does), and on WSGI, which folds duplicates into one comma-joined value, a comma in MCP-Protocol-Version, Mcp-Method, Authorization, Host, or Origin is refused for the same reason; any pagination cursor is -32602 because this server never issues one. Server identity travels in result._meta on every result, as the 2026 revision expects. Handler exceptions outside tools/call are logged server-side and answered with a constant -32603 message.

Client compatibility. This server speaks 2026-07-28, and refuses everything else unless told otherwise. Verified against the Python mcp SDK 2.x, FastMCP 4.x, and Claude Code 2.1.258, whose first request is a server/discover probe at 2026-07-28; it then lists prompts, resources, and tools, calls tools (a Context tool is answered with an SSE stream carrying its progress and log notifications), and reads resources. A client that only speaks the initialize-handshake era is refused with -32022 and a data.supported / data.requested pair, which is what a dual-era client parses as "modern peer, renegotiate".

subscriptions/listen is accepted and closed gracefully: the server never emits change notifications (every listChanged it advertises is false), so it acknowledges with an empty honored set, answers the request with a completion result carrying the subscription id, and closes the stream. That matters for Go SDK 1.7 clients such as Crush, which open a listen stream whenever the host registers a list-changed handler and treat any error as a failed connection; Go SDK 1.8 gates the request on the advertised capability.

Which clients can reach a server that speaks only this revision, as of 2026-09-14: Claude.ai and Claude desktop connectors, Claude Code 2.1.232+, GitHub Copilot CLI 1.0.81+, Goose 1.50+, Crush 0.88+, and anything built on the Python mcp 2.x, Go 1.7+, C# 2.x, or Ruby 1.2+ SDK clients, whose default is to probe server/discover first. TypeScript SDK 2.x and Rust rmcp 3.x can, but only when the host opts in (Codex CLI needs its mcp_2026_07_28 feature flag; Mastra needs protocolVersion: 'auto'). Hosts still on TypeScript SDK 1.x (VS Code, Cursor as far as is known, Gemini CLI, Cline, LibreChat, Open WebUI), Zed, ChatGPT connectors, and the Java, Kotlin, and Swift SDKs cannot, unless the server opts in below.

Legacy clients, opt-in. Server(mcp, legacy="stateless") (and the same on ASGIServer) serves 2025-era clients the way the official SDKs' stateless legacy mode does: per request, with no sessions. A request without the per-request envelope is answered on the 2025 ladder: initialize returns the capabilities and server identity from the registry every time (echoing a requested 2025-11-25 or 2025-06-18; anything else, including 2025-03-26, whose batch arrays this server refuses, gets 2025-11-25), the initialized notification is 202, ping is {}, results carry no resultType/ttlMs/cacheScope, every response (results, errors, and SSE streams) names the era served in MCP-Protocol-Version, a -32601 travels under 200 because the 2025-era SDK clients read 404 as a terminated session, and GET and DELETE stay 405. The envelope and the Mcp-Method/Mcp-Name routing headers are not required of such requests, but when a legacy request does send them they must agree with the body. Note for gateways that authorize on Mcp-Name: a claim-less request carries no such header at all, so a gateway must deny claim-less POSTs if it relies on the header rather than the body. A Context tool still streams SSE with its progress notifications, but only to a client whose Accept names text/event-stream explicitly. A request that names 2026-07-28 in its header or envelope (even with a null value) always takes the modern ladder, and server/discover then advertises both eras. Verified with the Python mcp 2.x client in both mode="legacy" and mode="auto" (which still picks the modern era), the TypeScript SDK 2.0.0 client in its default legacy mode, and the Go SDK 1.7.0 client. The default stays refuse: turning the fallback on is a one-word decision, and the README should not make it for you.

OAuth is the boundary where rolling your own stops being sensible. Bearer tokens against your own store, the 401 challenge, and the RFC 9728 document are here; being an OAuth 2.1 authorization server (RFC 8414 / 7591, PKCE) is not 600 lines and is not code to hand-roll.

Tests

pip install -e . --group test            # or: uv pip install -e . --group test
pytest                                   # all ten suites, in their own processes
pytest -m "unit or conform"              # no servers, no optional clients
pytest -m "server or interop"            # uvicorn/waitress/gunicorn + the mcp client

The suites under tests/ are self-contained scripts that print PASS/FAIL lines and can be run directly (cd tests && python test_micromcp.py); test_scripts.py is the pytest entry point that runs each in its own process on ephemeral ports. Ports and servers are never shared between suites.

test_micromcp.py    29 unit assertions, stdlib only
conform.py          11 checks against the strict mcp-types wire schema
interop.py          end-to-end with the official mcp client, 9 assertions
test_asgi.py        native ASGI under uvicorn and mounted in Starlette
test_progress.py    40 SSE checks: ordering, cancellation, subscriptions/listen, WSGI degradation
test_legacy.py      legacy="stateless" serving, OAuth handoff, two review rounds, both transports
harnesses.py        wsgiref, Flask, Starlette, waitress, gunicorn
ergonomics.py       API tour, 12 assertions
test_hardening.py   247 regression checks from four adversarial reviews + UI apps
test_defender.py    60 checks from the defensive audit (mutation-derived)

conform.py validates every result and notification against the official per-version wire schema from mcp-types (MIT, 6 packages). It is the cheapest and most precise check: it names the failing field, and it is versioned against the spec, so bumping mcp-types tells you exactly what a future revision breaks. Run it on every commit.