# AN-2026-0800: FastAPI 0.147-0.162 (Oct 19-23, 2026): form, default and JSON-null handling changed: empty form strings and unselected files are missing, defaults are not validated, explicit null is not the default

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- id: AN-2026-0800 (version 1; cite https://breakwire.org/claim/AN-2026-0800/v/1)
- kind: breaking · status: active · review: editor
- source: <https://fastapi.tiangolo.com/release-notes/#:~:text=Restore%20default%20handling%20for%20omitted%20boolean%20form%20fields>
- more sources: <https://fastapi.tiangolo.com/release-notes/#:~:text=Distinguish%20JSON%20null%20from%20omitted%20body%20values> <https://fastapi.tiangolo.com/tutorial/request-forms/#boolean-form-fields> <https://pypi.org/project/fastapi/0.162.0/>
- event date: 2026-10-19
- services: fastapi, python
- affected: pypi:fastapi >=0.147.0
- 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. FastAPI release notes (opened 2026-11-04), Breaking Changes: 0.148.0 (2026-10-19) defaults for omitted Query/Header/Cookie/Form parameters are no longer treated as user input and are not validated unless validate_default=True; applied defaults no longer count as set in model_dump(exclude_unset=True). 0.149.0 empty scalar form values are missing in form models. 0.150.0 (2026-10-20) unselected browser file inputs are missing. 0.151.0 (2026-10-20) JSON bodies distinguish explicit null from omitted:  [✓0 ✗0]
1. 0.147.0 (2026-10-19) briefly made omitted bool form fields False; 0.162.0 (2026-10-23) reverted that: omitted bool and bool | None form fields use their declared defaults or produce missing-field errors, so HTML checkboxes need a default of False.  [✓0 ✗0]

## Fix
```text
After upgrading to >=0.148, re-run API tests that post forms or null JSON. Give checkbox booleans `= False`; add `validate_default=True` where validators must run on defaults; make fields `X | None` if clients send `null`; with `extra="forbid"` parameter models, declare every sibling query/header field in the model. Pin `fastapi<0.147` if you cannot test yet.
```

## Outcome reports (0)
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## 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)