name: fastapi-endpoint description: Guide for creating FastAPI endpoints following this project's conventions including routers, dependency injection, error handling, and OpenAPI documentation.

FastAPI Endpoint Creation

Use this skill when creating new API endpoints for this FastAPI application. Follow these patterns to ensure consistency with the existing codebase.

For comprehensive coding guidelines, see AGENTS.md in the repository root.

Router Setup

Create routers in app/api/ with proper configuration:

"""
Resource router for resource management.
"""

import logging

from fastapi import APIRouter, HTTPException, Request, Response, status

from app.core.cbor import CBORRoute
from app.dependencies import CurrentUser, ResourceServiceDep
from app.exceptions import ResourceAlreadyExistsError, ResourceNotFoundError
from app.models.error import ProblemResponse, ValidationProblemResponse
from app.models.resource import Resource, ResourceCreate, ResourceUpdate

logger = logging.getLogger(__name__)

router = APIRouter(
    prefix="/resource",
    tags=["Resource"],
    route_class=CBORRoute,
    responses={
        401: {"model": ProblemResponse, "description": "Unauthorized"},
        422: {"model": ValidationProblemResponse, "description": "Validation error"},
        500: {"model": ProblemResponse, "description": "Server error"},
    },
)

Endpoint Pattern

Always include:

status_code for non-200 responses
Return type annotation (serves as implicit response_model)
summary and description for OpenAPI docs
operation_id with pattern <resource>_<action>
responses dict for all possible status codes

POST with 201 Created

Return resources directly with Location header:

@router.post(
    "",
    status_code=status.HTTP_201_CREATED,
    summary="Create resource",
    description="Create a new resource for the authenticated user.",
    operation_id="resource_create",
    responses={
        201: {"model": Resource, "description": "Resource created successfully"},
        403: {"model": ProblemResponse, "description": "Forbidden"},
        409: {"model": ProblemResponse, "description": "Resource already exists"},
    },
)
async def create_resource(
    request: Request,
    resource_data: ResourceCreate,
    current_user: CurrentUser,
    service: ResourceServiceDep,
    response: Response,
) -> Resource:
    """
    Create a new resource for the authenticated user.

    Stores the resource data in Firestore under the user's UID.
    Returns 409 Conflict if a resource already exists.
    """
    try:
        resource = await service.create_resource(current_user.uid, resource_data)
        response.headers["Location"] = str(request.url.path)
        response.headers["Link"] = '</schemas/ResourceData.json>; rel="describedBy"'
        return Resource(
            schema_url=str(request.base_url) + "schemas/ResourceData.json",
            id=resource.id,
            name=resource.name,
            created_at=resource.created_at,
            updated_at=resource.updated_at,
        )
    except (HTTPException, ResourceAlreadyExistsError):
        raise
    except Exception:
        logger.exception("Error creating resource", extra={"user_id": current_user.uid})
        raise HTTPException(
            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail="Failed to create resource"
        ) from None

GET Endpoint

@router.get(
    "",
    summary="Get resource",
    description="Get the resource of the authenticated user.",
    operation_id="resource_get",
    responses={
        200: {"model": Resource, "description": "Resource retrieved successfully"},
        404: {"model": ProblemResponse, "description": "Resource not found"},
    },
)
async def get_resource(
    request: Request,
    response: Response,
    current_user: CurrentUser,
    service: ResourceServiceDep,
) -> Resource:
    """
    Retrieve the resource of the authenticated user.

    Returns 404 Not Found if no resource exists for the user.
    """
    try:
        resource = await service.get_resource(current_user.uid)
        response.headers["Link"] = '</schemas/ResourceData.json>; rel="describedBy"'
        return Resource(
            schema_url=str(request.base_url) + "schemas/ResourceData.json",
            id=resource.id,
            name=resource.name,
            created_at=resource.created_at,
            updated_at=resource.updated_at,
        )
    except (HTTPException, ResourceNotFoundError):
        raise
    except Exception:
        logger.exception("Error getting resource", extra={"user_id": current_user.uid})
        raise HTTPException(
            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail="Failed to retrieve resource"
        ) from None

DELETE with 204 No Content

@router.delete(
    "",
    status_code=status.HTTP_204_NO_CONTENT,
    summary="Delete resource",
    description="Delete the resource of the authenticated user.",
    operation_id="resource_delete",
    responses={
        204: {"description": "Resource deleted successfully"},
        404: {"model": ProblemResponse, "description": "Resource not found"},
    },
)
async def delete_resource(
    current_user: CurrentUser,
    service: ResourceServiceDep,
) -> None:
    """
    Delete the resource of the authenticated user.

    Returns 404 Not Found if no resource exists.
    """
    try:
        await service.delete_resource(current_user.uid)
        return None
    except (HTTPException, ResourceNotFoundError):
        raise
    except Exception:
        logger.exception("Error deleting resource", extra={"user_id": current_user.uid})
        raise HTTPException(
            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail="Failed to delete resource"
        ) from None

Dependencies

Use typed dependency aliases from app/dependencies.py:

CurrentUser for authenticated user context
Service dependencies like ResourceServiceDep

Create new service dependencies in app/dependencies.py:

from typing import Annotated
from fastapi import Depends
from app.services.resource import ResourceService


def get_resource_service() -> ResourceService:
    """
    Dependency provider for ResourceService.
    """
    return ResourceService()


ResourceServiceDep = Annotated[ResourceService, Depends(get_resource_service)]

Error Handling

Re-raise domain exceptions and HTTPException to let handlers convert them
Use logger.exception() with structured extra={} for unexpected errors
Use from None to suppress exception chaining in generic 500 responses
Never expose internal error details to clients

PATCH Endpoints

For partial updates, use response_model_exclude_unset=True:

@router.patch(
    "",
    response_model=Resource,
    response_model_exclude_unset=True,
    summary="Update resource",
    description="Partially update the resource of the authenticated user.",
    operation_id="resource_update",
    responses={
        200: {"model": Resource, "description": "Resource updated successfully"},
        404: {"model": ProblemResponse, "description": "Resource not found"},
    },
)
async def update_resource(
    request: Request,
    resource_data: ResourceUpdate,
    current_user: CurrentUser,
    service: ResourceServiceDep,
    response: Response,
) -> Resource:
    ...

Router Registration

# In app/api/__init__.py - add to v1_router for versioned endpoints
from app.api import resource

v1_router.include_router(resource.router)

# In app/main.py - for unversioned endpoints
from app.api import resource

app.include_router(resource.router)

URL Conventions

Always use empty string "" for root resource paths (e.g., @router.post(""))
Use plural nouns for collection endpoints
Keep routes RESTful: POST for create, GET for read, PATCH for update, DELETE for delete

Error Response Models

Use ProblemResponse for standard errors and ValidationProblemResponse for 422 validation errors:

from app.models.error import ProblemResponse, ValidationProblemResponse

These models follow RFC 9457 Problem Details format.

fastapi-endpoint

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