OpenAPI generator crashes with AttributeError on SSE async-generator handlers (AsyncIterable has no __parameters__)

[RobertoPrevato]

---

name: "🐛 Bug Report"
about: "If something isn't working as expected."
title: ''

Description

When a controller method is defined as an async generator with the return type
-> AsyncIterable[ServerSentEvent] (as shown in the SSE documentation), the
OpenAPI documentation builder raises an AttributeError at startup and the
application fails to start.

Traceback

File ".../blacksheep/server/openapi/v3.py", line 1024, in _try_get_schema_for_generic
    parameters = origin.__parameters__
                 ^^^^^^^^^^^^^^^^^^^^^
AttributeError: type object 'AsyncIterable' has no attribute '__parameters__'

Minimal reproduction

from collections.abc import AsyncIterable
from blacksheep import Application, get
from blacksheep.server.sse import ServerSentEvent
from blacksheep.server.openapi.v3 import OpenApiHandler
from blacksheep.server.controllers import Controller

docs = OpenApiHandler(info=Info("Test", "0.1.0"))

app = Application()
docs.bind_app(app)

class Home(Controller):
    @get("/events")
    async def events(self) -> AsyncIterable[ServerSentEvent]:
        yield ServerSentEvent({"msg": "hello"})

Running this causes startup failure when the OpenAPI docs are generated.

Workaround

Use ServerSentEventsResponse with a plain (non-annotated) handler instead:

from blacksheep.server.sse import ServerSentEvent, ServerSentEventsResponse

class Home(Controller):
    @get("/events")
    def events(self):
        async def generate():
            yield ServerSentEvent({"msg": "hello"})
        return ServerSentEventsResponse(generate)

Expected behaviour

The OpenAPI generator should either handle AsyncIterable[ServerSentEvent] gracefully
(e.g. treat it as a streaming/SSE response and skip schema generation for it), or raise
a clear, actionable error message.

Environment

• BlackSheep version: 2.x
• Python version: 3.14