Common FastAPI Errors
FastAPI errors including Pydantic validation, dependency injection, async issues, and deployment configuration.
75 errors documented with step-by-step fixes
422 validation error FastAPI
422 Unprocessable EntityThe request body or parameters failed Pydantic validation.
Request validation error FastAPI Pydantic
RequestValidationErrorPydantic couldn't validate the incoming request data.
Value not valid integer FastAPI
value is not a valid integerA path or query parameter couldn't be converted to the expected type.
Field required Pydantic FastAPI
field requiredA required field is missing from the request body.
Not valid Pydantic field type FastAPI
TypeError: is not a valid Pydantic field typeA model field has an unsupported type annotation.
Depends FastAPI error
RuntimeError: Use params for dependsDependency injection was used incorrectly.
No response returned FastAPI
No response returnedThe endpoint function didn't return anything.
CORS error FastAPI
CORS blockedCross-origin requests are blocked by the browser.
Lifespan event error FastAPI
lifespan event errorAn error occurred during startup or shutdown events.
WebSocket closed FastAPI
WebSocket connection closedThe WebSocket connection was unexpectedly closed.
SQLAlchemy connection error FastAPI
sqlalchemy.exc.OperationalErrorThe database connection failed.
Detached instance error SQLAlchemy FastAPI
DetachedInstanceErrorAccessing a lazy-loaded attribute after the session was closed.
Address already in use Uvicorn FastAPI
Uvicorn bind errorAnother process is using the port.
Not an iterator async FastAPI
TypeError: object is not an iterator asyncA sync generator was used where async is expected.
Response model error FastAPI
Response model validation errorThe response doesn't match the declared response_model.
422: value is not a valid list in FastAPI
422: value is not a valid listThis FastAPI error occurs when value is not a valid list. Check the stack trace for the exact location and fix the root cause.
422: value is not a valid dict in FastAPI
422: value is not a valid dictThis FastAPI error occurs when value is not a valid dict. Check the stack trace for the exact location and fix the root cause.
422: value is not a valid email address in FastAPI
422: value is not a valid email addressThis FastAPI error occurs when value is not a valid email address. Check the stack trace for the exact location and fix the root cause.
422: value is not a valid URL in FastAPI
422: value is not a valid URLThis FastAPI error occurs when value is not a valid url. Check the stack trace for the exact location and fix the root cause.
422: ensure this value has at least N characters in FastAPI
422: ensure this value has at least N charactersThis FastAPI error occurs when ensure this value has at least n characters. Check the stack trace for the exact location and fix the root cause.
422: ensure this value has at most N characters in FastAPI
422: ensure this value has at most N charactersThis FastAPI error occurs when ensure this value has at most n characters. Check the stack trace for the exact location and fix the root cause.
422: value is not a valid enumeration member in FastAPI
422: value is not a valid enumeration memberThis FastAPI error occurs when value is not a valid enumeration member. Check the stack trace for the exact location and fix the root cause.
422: none is not an allowed value in FastAPI
422: none is not an allowed valueThis FastAPI error occurs when none is not an allowed value. Check the stack trace for the exact location and fix the root cause.
422: extra fields not permitted in FastAPI
422: extra fields not permittedThis FastAPI error occurs when extra fields not permitted. Check the stack trace for the exact location and fix the root cause.
422: str type expected in FastAPI
422: str type expectedThis FastAPI error occurs when str type expected. Check the stack trace for the exact location and fix the root cause.
Pydantic V2 migration: validator deprecated in FastAPI
Pydantic V2 migration: validator deprecatedThis FastAPI error occurs when validator deprecated. Check the stack trace for the exact location and fix the root cause.
Pydantic V2: model_validator vs root_validator in FastAPI
Pydantic V2: model_validator vs root_validatorThis FastAPI error occurs when model_validator vs root_validator. Check the stack trace for the exact location and fix the root cause.
Pydantic V2: field_validator vs validator in FastAPI
Pydantic V2: field_validator vs validatorThis FastAPI error occurs when field_validator vs validator. Check the stack trace for the exact location and fix the root cause.
Pydantic V2: ConfigDict vs class Config in FastAPI
Pydantic V2: ConfigDict vs class ConfigThis FastAPI error occurs when configdict vs class config. Check the stack trace for the exact location and fix the root cause.
Pydantic V2: model_dump vs dict() in FastAPI
Pydantic V2: model_dump vs dict()This FastAPI error occurs when model_dump vs dict(). Check the stack trace for the exact location and fix the root cause.
FastAPI: Depends() circular dependency in FastAPI
FastAPI: Depends() circular dependencyThis FastAPI error occurs when depends() circular dependency. Check the stack trace for the exact location and fix the root cause.
FastAPI: Depends() with yield cleanup error in FastAPI
FastAPI: Depends() with yield cleanup errorThis FastAPI error occurs when depends() with yield cleanup error. Check the stack trace for the exact location and fix the root cause.
FastAPI: Background task exception not caught in FastAPI
FastAPI: Background task exception not caughtThis FastAPI error occurs when background task exception not caught. Check the stack trace for the exact location and fix the root cause.
FastAPI: BackgroundTasks vs background_tasks in FastAPI
FastAPI: BackgroundTasks vs background_tasksThis FastAPI error occurs when backgroundtasks vs background_tasks. Check the stack trace for the exact location and fix the root cause.
FastAPI: Response model exclude_unset behavior in FastAPI
FastAPI: Response model exclude_unset behaviorThis FastAPI error occurs when response model exclude_unset behavior. Check the stack trace for the exact location and fix the root cause.
FastAPI: Query parameter validation error in FastAPI
FastAPI: Query parameter validation errorThis FastAPI error occurs when query parameter validation error. Check the stack trace for the exact location and fix the root cause.
FastAPI: Path parameter type mismatch in FastAPI
FastAPI: Path parameter type mismatchThis FastAPI error occurs when path parameter type mismatch. Check the stack trace for the exact location and fix the root cause.
FastAPI: Header parameter case sensitivity in FastAPI
FastAPI: Header parameter case sensitivityThis FastAPI error occurs when header parameter case sensitivity. Check the stack trace for the exact location and fix the root cause.
FastAPI: Cookie parameter not found in FastAPI
FastAPI: Cookie parameter not foundThis FastAPI error occurs when cookie parameter not found. Check the stack trace for the exact location and fix the root cause.
FastAPI: File upload too large in FastAPI
FastAPI: File upload too largeThis FastAPI error occurs when file upload too large. Check the stack trace for the exact location and fix the root cause.
FastAPI: UploadFile read() returns bytes not str in FastAPI
FastAPI: UploadFile read() returns bytes not strThis FastAPI error occurs when uploadfile read() returns bytes not str. Check the stack trace for the exact location and fix the root cause.
FastAPI: StreamingResponse generator error in FastAPI
FastAPI: StreamingResponse generator errorThis FastAPI error occurs when streamingresponse generator error. Check the stack trace for the exact location and fix the root cause.
FastAPI: JSONResponse vs Response in FastAPI
FastAPI: JSONResponse vs ResponseThis FastAPI error occurs when jsonresponse vs response. Check the stack trace for the exact location and fix the root cause.
FastAPI: Custom exception handler not working in FastAPI
FastAPI: Custom exception handler not workingThis FastAPI error occurs when custom exception handler not working. Check the stack trace for the exact location and fix the root cause.
FastAPI: HTTPException vs RequestValidationError in FastAPI
FastAPI: HTTPException vs RequestValidationErrorThis FastAPI error occurs when httpexception vs requestvalidationerror. Check the stack trace for the exact location and fix the root cause.
FastAPI: OAuth2PasswordBearer token not found in FastAPI
FastAPI: OAuth2PasswordBearer token not foundThis FastAPI error occurs when oauth2passwordbearer token not found. Check the stack trace for the exact location and fix the root cause.
FastAPI: JWT decode error in FastAPI
FastAPI: JWT decode errorThis FastAPI error occurs when jwt decode error. Check the stack trace for the exact location and fix the root cause.
FastAPI: CORS preflight request blocked in FastAPI
FastAPI: CORS preflight request blockedThis FastAPI error occurs when cors preflight request blocked. Check the stack trace for the exact location and fix the root cause.
FastAPI: TrustedHostMiddleware blocking requests in FastAPI
FastAPI: TrustedHostMiddleware blocking requestsThis FastAPI error occurs when trustedhostmiddleware blocking requests. Check the stack trace for the exact location and fix the root cause.
FastAPI: GZipMiddleware error in FastAPI
FastAPI: GZipMiddleware errorThis FastAPI error occurs when gzipmiddleware error. Check the stack trace for the exact location and fix the root cause.
SQLAlchemy: DetachedInstanceError in response in FastAPI
SQLAlchemy: DetachedInstanceError in responseThis FastAPI error occurs when detachedinstanceerror in response. Check the stack trace for the exact location and fix the root cause.
SQLAlchemy: greenlet_spawn has not been called in FastAPI
SQLAlchemy: greenlet_spawn has not been calledThis FastAPI error occurs when greenlet_spawn has not been called. Check the stack trace for the exact location and fix the root cause.
SQLAlchemy: MissingGreenlet error async in FastAPI
SQLAlchemy: MissingGreenlet error asyncThis FastAPI error occurs when missinggreenlet error async. Check the stack trace for the exact location and fix the root cause.
SQLAlchemy: multiple path between mappers in FastAPI
SQLAlchemy: multiple path between mappersThis FastAPI error occurs when multiple path between mappers. Check the stack trace for the exact location and fix the root cause.
SQLAlchemy: Can't reconnect until invalid transaction is rol in FastAPI
SQLAlchemy: Can't reconnect until invalid transaction is rolled backThis FastAPI error occurs when can't reconnect until invalid transaction is rolled back. Check the stack trace for the exact location and fix the root cause.
asyncpg: Interfaceconnection is closed in FastAPI
asyncpg: InterfaceError: connection is closedThis FastAPI error occurs when connection is closed. Check the stack trace for the exact location and fix the root cause.
asyncpg: TooManyConnectionsError in FastAPI
asyncpg: TooManyConnectionsErrorThis FastAPI error occurs when toomanyconnectionserror. Check the stack trace for the exact location and fix the root cause.
asyncpg: UndefinedTableError in FastAPI
asyncpg: UndefinedTableErrorThis FastAPI error occurs when undefinedtableerror. Check the stack trace for the exact location and fix the root cause.
asyncpg: Datainvalid input syntax in FastAPI
asyncpg: DataError: invalid input syntaxThis FastAPI error occurs when invalid input syntax. Check the stack trace for the exact location and fix the root cause.
Alembic: Target database is not up to date in FastAPI
Alembic: Target database is not up to dateThis FastAPI error occurs when target database is not up to date. Check the stack trace for the exact location and fix the root cause.
Alembic: Can't locate revision identified by in FastAPI
Alembic: Can't locate revision identified byThis FastAPI error occurs when can't locate revision identified by. Check the stack trace for the exact location and fix the root cause.
Alembic: Multiple head revisions in FastAPI
Alembic: Multiple head revisionsThis FastAPI error occurs when multiple head revisions. Check the stack trace for the exact location and fix the root cause.
Celery: Received unregistered task in FastAPI
Celery: Received unregistered taskThis FastAPI error occurs when received unregistered task. Check the stack trace for the exact location and fix the root cause.
Celery: Task raised unexpected exception in FastAPI
Celery: Task raised unexpected exceptionThis FastAPI error occurs when task raised unexpected exception. Check the stack trace for the exact location and fix the root cause.
httpx.ReadTimeout in FastAPI
httpx.ReadTimeoutThis FastAPI error occurs when an unexpected condition is encountered. Check the stack trace for the exact location and fix the root cause.
httpx.ConnectTimeout in FastAPI
httpx.ConnectTimeoutThis FastAPI error occurs when an unexpected condition is encountered. Check the stack trace for the exact location and fix the root cause.
httpx.HTTPStatusError in FastAPI
httpx.HTTPStatusErrorThis FastAPI error occurs when an unexpected condition is encountered. Check the stack trace for the exact location and fix the root cause.
aiofiles: File not found async in FastAPI
aiofiles: File not found asyncThis FastAPI error occurs when file not found async. Check the stack trace for the exact location and fix the root cause.
uvicorn: ERROR: Application startup failed in FastAPI
uvicorn: ERROR: Application startup failedThis FastAPI error occurs when application startup failed. Check the stack trace for the exact location and fix the root cause.
uvicorn: WARNING: Invalid HTTP request received in FastAPI
uvicorn: WARNING: Invalid HTTP request receivedThis FastAPI error occurs when invalid http request received. Check the stack trace for the exact location and fix the root cause.
gunicorn: Worker timeout in FastAPI
gunicorn: Worker timeoutThis FastAPI error occurs when worker timeout. Check the stack trace for the exact location and fix the root cause.
gunicorn: worker failed to boot in FastAPI
gunicorn: worker failed to bootThis FastAPI error occurs when worker failed to boot. Check the stack trace for the exact location and fix the root cause.
Docker: FastAPI container health check failing in FastAPI
Docker: FastAPI container health check failingThis FastAPI error occurs when fastapi container health check failing. Check the stack trace for the exact location and fix the root cause.
Nginx: 502 Bad Gateway to FastAPI in FastAPI
Nginx: 502 Bad Gateway to FastAPIThis FastAPI error occurs when 502 bad gateway to fastapi. Check the stack trace for the exact location and fix the root cause.
Traefik: routing to FastAPI error in FastAPI
Traefik: routing to FastAPI errorThis FastAPI error occurs when routing to fastapi error. Check the stack trace for the exact location and fix the root cause.
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