How-to: API versioning

Separate endpoints into /v1, /v2 using FastAPI's APIRouter + prefix.

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Basic structure

from fastapi import APIRouter, FastAPI

app = FastAPI()

v1_router = APIRouter(prefix="/v1", tags=["v1"])
v2_router = APIRouter(prefix="/v2", tags=["v2"])

@v1_router.get("/users")
def list_users_v1() -> list[UserResponseV1]:
    ...

@v2_router.get("/users")
def list_users_v2() -> list[UserResponseV2]:
    ...

app.include_router(v1_router)
app.include_router(v2_router)

An APIRouter's prefix is prepended automatically to the paths inside the router. You do not write /v1 on the in-router paths (it would be duplicated).

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Share the domain layer; absorb version differences in the HTTP layer

Keep logic shared across versions in the domain layer, and express the per-version differences in the Pydantic models.

# Domain layer (version-independent)
@dataclass(frozen=True, slots=True)
class User:
    user_id: int
    first_name: str
    last_name: str
    email: str
    age: int

# v1: joined into full_name
class UserResponseV1(BaseModel):
    user_id: int
    full_name: str
    email: str

    @classmethod
    def from_domain(cls, user: User) -> "UserResponseV1":
        return cls(
            user_id=user.user_id,
            full_name=f"{user.first_name} {user.last_name}",
            email=user.email,
        )

# v2: split first_name/last_name + add age + email → contact_email
class UserResponseV2(BaseModel):
    user_id: int
    first_name: str
    last_name: str
    contact_email: str
    age: int

    @classmethod
    def from_domain(cls, user: User) -> "UserResponseV2":
        return cls(
            user_id=user.user_id,
            first_name=user.first_name,
            last_name=user.last_name,
            contact_email=user.email,
            age=user.age,
        )

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OpenAPI schema

Setting tags=["v1"] / tags=["v2"] on each APIRouter groups them per version in the Swagger UI. UserResponseV1 / UserResponseV2 are defined separately in the schema.

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See also

• FT109: docs/field-trials/2026-05-field-trial-109.md