FastAPI will generate schemas for models that are used either as a Request Body or Response Model. When declaring query_args: FaultQueryParams = Depends() (using Depends), your endpoint would not expect a request body, but rather query parameters; hence, FaultQueryParams would not be included in the schemas of the OpenAPI docs.

To add additional schemas, you could extend/modify the OpenAPI schema. Example is given below (make sure to add the code for modifying the schema after all routes have been defined, i.e., at the end of your code).

class FaultQueryParams(BaseModel):
    f_id: Optional[int] = Field(None, description="id for the host", example=12345, title="Fault ID")
    hostname: Optional[str]
    status: Literal["open", "closed", "all"] = Field("open")
    ...
    
@app.post('/predict')
def predict(query_args: FaultQueryParams = Depends()):
    return query_args

def get_extra_schemas():
    return {
              "FaultQueryParams": {
                "title": "FaultQueryParams",
                "type": "object",
                "properties": {
                  "f_id": {
                    "title": "Fault ID",
                    "type": "integer",
                    "description": "id for the host",
                    "example": 12345
                  },
                  "hostname": {
                    "title": "Hostname",
                    "type": "string"
                  },
                  "status": {
                    "title": "Status",
                    "enum": [
                      "open",
                      "closed",
                      "all"
                    ],
                    "type": "string",
                    "default": "open"
                  },
                   ...
                }
              }
            }

from fastapi.openapi.utils import get_openapi

def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="FastAPI",
        version="1.0.0",
        description="This is a custom OpenAPI schema",
        routes=app.routes,
    )
    new_schemas = openapi_schema["components"]["schemas"]
    new_schemas.update(get_extra_schemas())
    openapi_schema["components"]["schemas"] = new_schemas
    
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Some Helpful Notes

Note 1

Instead of manually typing the schema for the extra models that you would like to add to the docs, you can have FastAPI do that for you by adding to your code an endpoint (which you would subsequently remove, after getting the schema) using that model as a request body or response model, for example:

@app.post('/predict') 
def predict(query_args: FaultQueryParams):
    return query_args

Then, you can get the generated JSON schema at http://127.0.0.1:8000/openapi.json, as described in the documentation. From there, you can either copy and paste the schema of the model to your code and use it directly (as shown in the get_extra_schema() method above) or save it to a file and load the JSON data from the file, as demonstrated below:

import json
...

new_schemas = openapi_schema["components"]["schemas"]

with open('extra_schemas.json') as f:    
    extra_schemas = json.load(f)
    
new_schemas.update(extra_schemas)   
openapi_schema["components"]["schemas"] = new_schemas

...

Note 2

To declare metadata, such as description, example, etc, for your query parameter, you should define your parameter with Query instead of Field, and since you can't do that with Pydantic models, you either need to define the Query parameter(s) directly in the endpoint or use a custom dependency class, as shown below:

from fastapi import FastAPI, Query, Depends
from typing import Optional

class FaultQueryParams:
    def __init__(
        self,
        f_id: Optional[int] = Query(None, description="id for the host", example=12345)

    ):
        self.f_id = f_id

app = FastAPI()

@app.post('/predict')
def predict(query_args: FaultQueryParams = Depends()):
    return query_args

The above could be re-written using the @dataclass decorator as follows:

from fastapi import FastAPI, Query, Depends
from typing import Optional
from dataclasses import dataclass

@dataclass
class FaultQueryParams:
    f_id: Optional[int] = Query(None, description="id for the host", example=12345)

app = FastAPI()

@app.post('/predict')
def predict(query_args: FaultQueryParams = Depends()):
    return query_args

Update

There is no need for using a custom dependency class anymore, as FastAPI now allows using a Pydantic BaseModel to define query parameters by wrapping the Query() in a Field(); hence, one should be able to use a Pydantic model to define multiple query parameters and declare metadata for them, i.e., description, example, etc. Related answers could be found here and here.