The backend actually POST's a form

main.py

@@ -69,15 +69,10 @@ def ok(**kwargs):
 
 
 @app.post("/revbank_plugin_backend")
-async def rb_backend(request: Request):
-    body = await request.json()
-
-    action = body.get("action", None)
-    payment_id = body.get("id", None)
-    if not payment_id:
-        raise HTTPException(status_code=400, detail="missing id")
-
-    payment = mollie_client.payments.get(payment_id)
+async def rb_backend(
+    id: Annotated[str, Form()], action: Annotated[str | None, Form()] = None
+):
+    payment = mollie_client.payments.get(id)
 
     if not payment.is_paid():
         return not_ok(f"payment {payment.status}")