ValidationError fail OpenAPI Schema validation

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Example Code

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "Hello World"}

Description

• Running openapi-generator against the schema generated by FastAPI fails.
• ValidationError schema cannot be processed.
• Generation fails with FastAPI v0.75.0 or higher
• Generated successfully with FastAPI v0.74.1

$ openapi-generator-cli generate -g typescript-axios -i http://localhost:8000/openapi.json

[main] INFO  o.o.codegen.DefaultGenerator - Generating with dryRun=false
[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: typescript-axios (client)
[main] INFO  o.o.codegen.DefaultGenerator - Generator 'typescript-axios' is considered stable.
[main] INFO  o.o.c.l.AbstractTypeScriptClientCodegen - Hint: Environment variable 'TS_POST_PROCESS_FILE' (optional) not defined. E.g. to format the source code, please try 'export TS_POST_PROCESS_FILE="/usr/local/bin/prettier --write"' (Linux/Mac)
[main] INFO  o.o.c.l.AbstractTypeScriptClientCodegen - Note: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
[main] INFO  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
[main] INFO  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
[main] WARN  o.o.c.l.AbstractTypeScriptClientCodegen - The import is a union type. Consider using the toModelImportMap method.
Exception in thread "main" java.lang.RuntimeException: Could not process model 'ValidationError'.Please make sure that your schema is correct!
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:492)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:888)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:441)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.StringIndexOutOfBoundsException: begin 1, end 0, length 0
        at java.base/java.lang.String.checkBoundsBeginEnd(String.java:3751)
        at java.base/java.lang.String.substring(String.java:1907)
        at java.base/java.lang.String.substring(String.java:1880)
        at org.openapitools.codegen.languages.TypeScriptAxiosClientCodegen.postProcessModels(TypeScriptAxiosClientCodegen.java:233)
        at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1302)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:487)
        ... 4 more

Process finished with exit code 1

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.75.2

Python Version

3.10.4

Additional Context

No response

[flxdot]

Well, it is due to this change: https://github.com/tiangolo/fastapi/pull/4806

But I would rather argue this is an issue of openapi-generator, as the schema is valid according to the specification of OpenAPI. There is even an open issue for that: https://github.com/OpenAPITools/openapi-generator/issues/4576

[gaganpreet]

This issue with typescript-axios generator is fixed in the main branch of openapi generator, but it’s not packaged yet. Using git bisect I pinned the fix down to this commit

Edit: Downgrading to 5.3.x also works.