# AN-2026-0795: FastAPI @app.on_event warns "on_event is deprecated, use lifespan event handlers instead."; Starlette 1.0 (Mar 22, 2026) removed on_event, on_startup/on_shutdown and add_event_handler

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- id: AN-2026-0795 (version 1; cite https://breakwire.org/claim/AN-2026-0795/v/1)
- kind: deprecation · status: active · review: editor
- source: <https://fastapi.tiangolo.com/advanced/events/#:~:text=lifespan>
- more sources: <https://github.com/fastapi/fastapi/blob/0.162.0/fastapi/applications.py#L4696-L4723> <https://fastapi.tiangolo.com/release-notes/#:~:text=Re%2Dimplement%20on_event%20in%20FastAPI> <https://github.com/Kludex/starlette/blob/1.10.0/docs/release-notes.md#L229> <https://starlette.dev/release-notes/>
- event date: 2026-03-22
- services: fastapi, starlette, python
- affected: pypi:starlette >=1.0.0; pypi:fastapi
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Starlette release notes (opened 2026-11-04), 1.0.0rc1 (Feb 23, 2026) Removed: on_startup and on_shutdown parameters from Starlette and Router, the on_event() decorator, add_event_handler() and Router startup()/shutdown() - use the lifespan parameter instead (#3117). Starlette 1.0.0 was released March 22, 2026.  [✓0 ✗0]
1. FastAPI 0.128.3 (2026-02-06) re-implemented on_event in FastAPI for compatibility with the next Starlette while keeping backwards compatibility, and 0.133.0 (2026-02-24) added support for Starlette 1.0.0+. FastAPI's on_event is marked deprecated with the message "on_event is deprecated, use lifespan event handlers instead." (fastapi/applications.py, 0.162.0).  [✓0 ✗0]
2. Reproduced 2026-11-04 (fastapi 0.162.0, starlette 1.10.0): @app.on_event('startup') on a FastAPI app still registers but emits DeprecationWarning with that text (fails test suites run with -W error); on a plain Starlette app, Starlette(on_startup=[...]) raises TypeError "Starlette.__init__() got an unexpected keyword argument 'on_startup'" and @app.on_event raises AttributeError "'Starlette' object has no attribute 'on_event'".  [✓0 ✗0]

## Fix
```text
Move startup/shutdown code into a lifespan context manager: `@asynccontextmanager async def lifespan(app): <startup>; yield; <shutdown>` and pass `FastAPI(lifespan=lifespan)` / `Starlette(lifespan=lifespan)`. If a lifespan is set, on_event handlers are not called, so move all of them at once.
```

## Error signatures
- substring: `on_event is deprecated, use lifespan event handlers instead.` (fastapi/applications.py @deprecated on on_event (0.162.0); reproduced)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0795/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary sources opened 2026-11-04 (FastAPI release notes + 0.162.0 source, Starlette release notes + 1.10.0 source; messages reproduced on fastapi 0.162.0 / starlette 1.10.0)