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Issue with client generation for FastApi version > 118 #1340
Description
Describe the bug
Pydantic V1 and V2 models co-existing is creating an issue with backwards compatibility. Please check the latest changes from fast api viz fastapi/fastapi#14168 which create the models as xxxx-Input and xxxx-Output which is breaking the existing code.
Reacted by Rodrigo SN Mancini
Activity
dbanty commented on Oct 28, 2025
Can you give me an example OpenAPI document that shows the issue? And what expected vs actual code is generated?
estyrke commented on Oct 28, 2025
I have the same issue. This is a condensed example:
{ "openapi": "3.1.0", "info": { "title": "Test", "version": "0.0.1" }, "paths": { "/endpoint": { "post": { "tags": [ "test" ], "summary": "Test Endpoint", "operationId": "test_endpoint", "requestBody": { "content": { "application/json": { "schema": { "$ref": "#/components/schemas/MySchema-Input" } } }, "required": true }, "responses": { "200": { "description": "Successful Response", "content": { "application/json": { "schema": { "$ref": "#/components/schemas/MySchema-Output" } } } } } } } }, "components": { "schemas": { "MySchema-Input": { "properties": { "id": { "type": "string", "format": "uuid", "title": "Id" }, "name": { "type": "string", "title": "Name" } }, "type": "object", "required": [ "id" ], "title": "MySchema" }, "MySchema-Output": { "properties": { "id": { "type": "string", "format": "uuid", "title": "Id" }, "name": { "type": "string", "title": "Name" } }, "type": "object", "required": [ "id", "name" ], "title": "MySchema" } } } }Yields this output:
python -m openapi_python_client generate --meta=uv --overwrite --path failing.json chore ✭ ✖ ✱ ◼ Generating /Users/emil/Code/arbo-api-client/test-client Warning(s) encountered while generating. Client was generated, but some pieces may be missing WARNING parsing POST /endpoint within test. Cannot parse response for status code 200 (Could not find reference in parsed models or enums), response will be omitted from generated client Reference(ref='#/components/schemas/MySchema-Output') Unable to parse schema /components/schemas/MySchema-Output Unable to parse this part of your OpenAPI document: : Attempted to generate duplicate models with name "MySchema" Schema(title='MySchema', multipleOf=None, maximum=None, exclusiveMaximum=None, minimum=None, exclusiveMinimum=None, maxLength=None, minLength=None, pattern=None, maxItems=None, minItems=None, uniqueItems=None, maxProperties=None, minProperties=None, required=['id', 'name'], enum=None, const=None, type=<DataType.OBJECT: 'object'>, allOf=[], oneOf=[], anyOf=[], schema_not=None, items=None, prefixItems=[], properties={'id': Schema(title='Id', multipleOf=None, maximum=None, exclusiveMaximum=None, minimum=None, exclusiveMinimum=None, maxLength=None, minLength=None, pattern=None, maxItems=None, minItems=None, uniqueItems=None, maxProperties=None, minProperties=None, required=None, enum=None, const=None, type=<DataType.STRING: 'string'>, allOf=[], oneOf=[], anyOf=[], schema_not=None, items=None, prefixItems=[], properties=None, additionalProperties=None, description=None, schema_format='uuid', default=None, nullable=False, discriminator=None, readOnly=None, writeOnly=None, xml=None, externalDocs=None, example=None, deprecated=None), 'name': Schema(title='Name', multipleOf=None, maximum=None, exclusiveMaximum=None, minimum=None, exclusiveMinimum=None, maxLength=None, minLength=None, pattern=None, maxItems=None, minItems=None, uniqueItems=None, maxProperties=None, minProperties=None, required=None, enum=None, const=None, type=<DataType.STRING: 'string'>, allOf=[], oneOf=[], anyOf=[], schema_not=None, items=None, prefixItems=[], properties=None, additionalProperties=None, description=None, schema_format=None, default=None, nullable=False, discriminator=None, readOnly=None, writeOnly=None, xml=None, externalDocs=None, example=None, deprecated=None)}, additionalProperties=None, description=None, schema_format=None, default=None, nullable=False, discriminator=None, readOnly=None, writeOnly=None, xml=None, externalDocs=None, example=None, deprecated=None)The problem is that there are multiple schemas with the same title. There is a flag in FastAPI, separate_input_output_schemas, which is supposed to prevent this, but it doesn't seem to work anymore. It seems it doesn't correctly detect if it needs to split the schemas or not anymore - as far as I can see, our -Input and -Output variants are identical in actual usage under FastAPI 0.120. But then again, it evidently splits the schemas even when I tell it not to, so...
In any case, it would be nice if the api generator worked even in the presence of these split models, maybe there could be a config to take the model name from the schema id instead of the title or something?
One final note is that it doesn't seem to help to specify class_overrides in the config (gives the exact same output), I guess those aren't applied until later or something?
estyrke commented on Oct 28, 2025
Actually, I missed that we had customized our openapi routes. the separate_input_output_schemas=False flag does seem to work if applied properly. 😅
Mulugruntz commented on Oct 31, 2025
It does not seem to work for us. Even with separate_input_output_schemas=False.
I'm trying to have a minimal reproducible one, but I haven't managed yet (would need to spend more time).
openapi-python-client will still need to fix this, as it seems it's there to last :-/
rdgsnm commented on Nov 6, 2025
I was having the same issues with FastAPI > 118;
I tried:
With openapi-python-client 0.25.3
With openapi-python-client 0.27.1
Hope that helps