fastapi-better-auth-bridge

A bridge to a TypeScript Better Auth server — not a Python port. (If you want a full Python re-implementation, this is not it.) Community-maintained; not affiliated with or endorsed by Better Auth.

Status. Two modes ship today, and every change to either is conformance-tested in CI against a real Better Auth server rather than a mock: Mode B — a JWT verified against your server's JWKS — and Mode A — the session cookie, verified against a shared session store, with its cross-site request forgery protection built in and required. Mode C (remote get-session) is not yet shipped. No dates are promised.

Modes

Better Auth is TypeScript-only: sign-in/up, OAuth and 2FA run on your Node service. This package makes the sessions that service issues first-class in FastAPI.

Which better-auth and Python versions each lane exercises: COMPATIBILITY.md.

Install

Python 3.10+. The distribution is fastapi-better-auth-bridge — the shorter spelling collides with an unrelated package under PyPI's name-similarity rules — and the import is fastapi_better_auth. JwtVerifier fetches your key set through a pluggable Transport: pick the adapter for the client your project already has, or install neither extra and pass your own Transport.

pip install "fastapi-better-auth-bridge[httpx]"   # or: uv add "fastapi-better-auth-bridge[httpx]"

Quickstart (Mode B)

Upstream prerequisite: the JWT plugin has to be mounted on your Better Auth server (plugins: [jwt()]). It serves /api/auth/jwks, the key set verified against, and /api/auth/token, where a signed-in client fetches its token; without it there is nothing here to verify. Then, in FastAPI:

from typing import Annotated

from fastapi import Depends, FastAPI

from fastapi_better_auth import BetterAuth, JwtVerifier, Session, User

auth = BetterAuth(verifiers=[JwtVerifier(base_url="https://auth.example.com")])
CurrentSession = Annotated[Session[User], Depends(auth.current_session())]

app = FastAPI()


@app.get("/me")
async def me(session: CurrentSession) -> User:
    return session.user

base_url is the whole of the trust configuration: canonicalized once, it is the required iss, the required aud, and the origin the key set is fetched from — and nothing is derived from the incoming request. If your deployment already sets BETTER_AUTH_URL for the Node side, BetterAuth.from_env() reads exactly that one variable, raises if missing, and builds the same:

from fastapi_better_auth import BetterAuth

auth = BetterAuth.from_env()

Call the factory — Depends(auth.current_session()), with the parentheses. Passed bare it would make the factory itself the dependency, a silent bypass of every route beneath a router, so every Depends and Security planting of a bare factory is refused with a ConfigurationError while the route is registered and the application never starts. The one exception is a bare factory assigned into app.dependency_overrides, a plain dict this library has no hook into: there the application does start, and the same refusal fires on the first request touching that dependency — still verifying nothing, and still serving nobody.

/docs needs no wiring: the security scheme is derived from each verifier's own credential source, so the Authorize button works out of the box.

Your own user model, and optional authentication

from typing import Annotated

from fastapi import Depends, FastAPI

from fastapi_better_auth import BetterAuth, JwtVerifier, Session, User


class Member(User):
    role: str | None = None


auth = BetterAuth(verifiers=[JwtVerifier(base_url="https://auth.example.com")])
CurrentMember = Annotated[Session[Member], Depends(auth.current_session(user_model=Member))]
MaybeMember = Annotated[Session[Member] | None, Depends(auth.optional_session(user_model=Member))]

app = FastAPI()


@app.get("/role")
async def role(session: CurrentMember) -> str:
    return session.user.role or "member"


@app.get("/greeting")
async def greeting(session: MaybeMember) -> str:
    return "hello" if session is None else f"hello, {session.user.id}"

optional_session returns None for one situation only: no credential was presented at all. One that was presented and did not verify still fails — a forged or expired token is never downgraded to "anonymous".

Quickstart (Mode A — session cookie)

The mode to reach for when the browser talks to FastAPI directly, carrying the cookie Better Auth set on sign-in. It verifies that cookie's signature against your shared secret, looks the session up in the store Better Auth writes to, and enforces expiry and revocation the moment the store says the session is gone — the instant revocation a JWT cannot give you.

A cookie-authenticated mode has to answer cross-site request forgery, so csrf= is required and has no default. OriginCheck is what most deployments want; SignedDoubleSubmit and — deliberately, spelled out in your own source — CsrfDisabled(reason="…") are the alternatives.

Bring your own store. Any object with these two async reads is a SessionStore, and the shipped adapters get no privilege yours does not. A dict-backed one is enough to run:

from typing import Annotated

from fastapi import Depends, FastAPI

from fastapi_better_auth import (
    BetterAuth,
    CookieVerifier,
    OriginCheck,
    Session,
    SessionStore,
    SharedSecret,
    StoredSession,
    StoredUser,
    User,
)


class DictSessionStore:
    """A SessionStore backed by two dicts: the whole Protocol, and nothing more.

    A real deployment reads the rows or keys Better Auth already writes (see below); this is
    the shape of the two questions a store answers — a read, never a write.
    """

    def __init__(self, sessions: dict[str, StoredSession], users: dict[str, StoredUser]) -> None:
        self._sessions = sessions
        self._users = users

    async def fetch_session_by_token(self, token: str) -> StoredSession | None:
        return self._sessions.get(token)

    async def fetch_user_by_id(self, user_id: str) -> StoredUser | None:
        return self._users.get(user_id)


store: SessionStore = DictSessionStore(sessions={}, users={})

auth = BetterAuth(
    verifiers=[
        CookieVerifier(
            # A literal only so this page runs; in production read it from the environment —
            # SharedSecret(os.environ["BETTER_AUTH_SECRET"]) — the value the Node side signs with.
            secret=SharedSecret("replace-this-with-your-own-32-plus-character-secret"),
            store=store,
            csrf=OriginCheck(allowed_origins=["https://app.example.com"]),
        )
    ]
)
CurrentSession = Annotated[Session[User], Depends(auth.current_session())]
MaybeSession = Annotated[Session[User] | None, Depends(auth.optional_session())]

# withCredentials lets the /docs "Try it out" button send the browser's own cookie to a same-site API.
app = FastAPI(swagger_ui_parameters={"withCredentials": True})


@app.post("/posts")
async def create_post(session: CurrentSession) -> dict[str, str]:
    return {"author": session.user.id}


@app.post("/reactions")
async def add_reaction(session: MaybeSession) -> dict[str, str | None]:
    return {"author": None if session is None else session.user.id}

The protected routes are POSTs on purpose: cross-site request forgery is a threat to unsafe methods, so a cookie-authenticated write is exactly where OriginCheck earns its place. A forged or absent cookie is a terminal 401; a cross-site write carrying a real cookie is a 403, decided before the signature is even checked.

In production, point the store at the rows Better Auth already writes. That needs an extra — fastapi-better-auth-bridge[sqlalchemy] or [redis] — and a live backend, so it is shown here rather than executed on this page:

import os

from sqlalchemy.ext.asyncio import create_async_engine

from fastapi_better_auth import (
    BetterAuth, CookieVerifier, OriginCheck, SharedSecret, SqlAlchemySessionStore,
)

engine = create_async_engine(os.environ["DATABASE_URL"])   # the database Better Auth writes sessions to
store = SqlAlchemySessionStore(engine=engine)

auth = BetterAuth(
    verifiers=[
        CookieVerifier(
            secret=SharedSecret(os.environ["BETTER_AUTH_SECRET"]),
            store=store,
            csrf=OriginCheck(allowed_origins=["https://app.example.com"]),
        )
    ]
)

# Redis secondary-storage instead of SQL:
#   from fastapi_better_auth import RedisSessionStore
#   store = RedisSessionStore(url=os.environ["REDIS_URL"])   # a miss is a 401, never a DB fall-back

SqlAlchemySessionStore reads the session and user tables directly; RedisSessionStore reads the raw-token key Better Auth's secondaryStorage writes. A store reads; it never writes — no touch, no EXPIRE-on-read — because a write here would extend or resurrect a session this side was only asked to verify.

What Mode A refuses, and when

A Redis-authoritative deployment is a hard rule: a store miss is a 401, never a fall-back to a database. When Better Auth runs with secondaryStorage, sign-out deletes the Redis key while a stale row can still sit in Postgres, so a fall-back would resurrect exactly the sessions a sign-out revoked.

/docs

The cookie route publishes an APIKeyCookie scheme, so /docs shows an Authorize field for it and the security requirement appears on the operation. Two honest limits:

• Swagger UI cannot set a cookie from the Authorize modal (swagger-api/swagger-ui#9710); the field is declarative. What makes "Try it out" work is FastAPI(swagger_ui_parameters={"withCredentials": True}), which tells Swagger to send the browser's own cookie — the one Better Auth already set — so a developer signed in on a same-site /docs can exercise the route.
• The scheme is documentation only. What it would read is never read; every credential comes from the verifier that owns it.

Deploying across two origins

The common shape is a front end on app.example.com and this API on api.example.com. For the browser to send the cookie cross-origin you configure Better Auth to set it SameSite=None; Secure in its __Secure- prefixed form — which CookieVerifier reads by default.

SameSite=None is exactly where CSRF stops being optional. OriginCheck is the floor; but a bare double-submit cookie proves only that the sender could set a cookie, and a sibling subdomain can set one on the shared parent domain — so on a shared parent domain reach for SignedDoubleSubmit, whose token is HMAC(secret, session_token), bound to the session and useless to a sibling. A non-browser client (mobile, server-to-server) has no Origin for OriginCheck to trust and belongs on Mode B (bearer) instead: compose both verifiers and each request picks its own by which credential it carries.

Errors

Every request-time failure is an HTTPException subclass, so FastAPI answers it with no handler of yours: 401, the body {"detail": "Not authenticated"}, and a WWW-Authenticate: Bearer header. Missing, malformed, expired, revoked and "the key set could not be fetched" are byte-identical on the wire, deliberately — a client must not be able to tell them apart and use the difference to probe. Two credentials on one request are a 400 ({"detail": "Ambiguous request"}), decided before anything is verified. A cookie-mode request that fails its CSRF check is a 403 ({"detail": "Forbidden"}) with no challenge — it carried a credential, so there is nothing to re-authenticate.

Why a request was refused lives on the exception, as .reason, and nowhere else. This library does not log ordinary refusals — a forged, expired or malformed token, an unknown key id, a missing or ambiguous credential — and that is deliberate rather than an omission: what to record about a failed authentication, and where, is the deployment's decision. If you want them, register a FastAPI exception handler for SessionError and log exc.reason explicitly; note that logging.exception() renders str(exc), which does not carry it. A reason holds identifiers and fingerprints — a key id, a truncated hash — never a raw credential.

Do I need to run a Node service?

Better Auth itself always runs in a Node/TypeScript process — sign-up, sign-in, OAuth, 2FA, and session issuance stay there; this library makes FastAPI a first-class consumer of the sessions it issues. Two topologies:

• You have a JS frontend server (Next.js, Nuxt, SvelteKit, …): no extra service — Better Auth is already mounted at /api/auth/* inside the frontend you deploy, and FastAPI verifies what it issues: nothing shared at all for Mode B, a shared Postgres or Redis for Mode A.
• No JS server (static SPA, mobile app, pure API): deploy one tiny Node service whose only job is mounting Better Auth, and keep 100% of the business logic in FastAPI. This repository's conformance harness (harness/) is exactly that service, in Hono.

Either way the browser or app performs its login flows against Better Auth, then presents the resulting credential to FastAPI, where this library verifies it — the session cookie for Mode A, a JWT for Mode B.

Why a library instead of the snippet

The hand-rolled verifiers circulating in Better Auth issues split the signed cookie on the wrong dot, miss the __Secure- name, compare HMACs non-constant-time, and never enforce expiresAt (upstream's findSession doesn't either — the route layer does, so a bare DB join honours expired sessions forever). Those are Mode A's details, and Mode A handles every one of them: the last-dot split, the __Secure- name, a constant-time keyring compare, and expiresAt enforced by the verifier itself. The wire facts behind them are pinned as golden vectors captured from a running Better Auth server in tests/vectors/, and re-verified against a live server in the conformance lane — not guessed.

Mode B is held to the same standard: JwtVerifier refuses an algorithm the token's own header chose, refuses an unknown kid rather than trying every published key, spells out the five required claims because PyJWT requires none by default, and refuses a token whose lifetime upstream would never have minted. A present-but-invalid credential is terminal — no falling through to a second verifier — and no failure reason ever reaches the client.

License

MIT © Mulugeta Solomon