Cannot set headers after they are sent to the client

I have used login system from this repo https://github.com/itsrakeshhq/finance-tracker/tree/main/backend/src/modules/auth but reworked it to tRPC v11 (11.0.0-rc.824). I changed nothing regarding logic, only stuff regarding new version of tRPC and also i am using tRPC standalone not express. Now I have issue when i try to set cookies in authRouter.login

const authRouter = router({
    login: publicProcedure
        .input(UsersInsertSchema)
        .mutation(({ input, ctx }) => new AuthController().loginHandler(input, ctx)),
})

const authRouter = router({
    login: publicProcedure
        .input(UsersInsertSchema)
        .mutation(({ input, ctx }) => new AuthController().loginHandler(input, ctx)),
})

Which leads to:

async loginHandler(data: typeof users.$inferInsert, ctx: Context) {
        const { accessToken, refreshToken } = await super.login(data)

        const cookies = new Cookies(ctx.req, ctx.res, {
            secure: process.env.NODE_ENV === 'production',
        })
        cookies.set('accessToken', accessToken, { ...accessTokenCookieOptions })
        cookies.set('refreshToken', refreshToken, {
            ...refreshTokenCookieOptions,
        })
        cookies.set('logged_in', 'true', { ...accessTokenCookieOptions })

        return { success: true }
    }

async loginHandler(data: typeof users.$inferInsert, ctx: Context) {
        const { accessToken, refreshToken } = await super.login(data)

        const cookies = new Cookies(ctx.req, ctx.res, {
            secure: process.env.NODE_ENV === 'production',
        })
        cookies.set('accessToken', accessToken, { ...accessTokenCookieOptions })
        cookies.set('refreshToken', refreshToken, {
            ...refreshTokenCookieOptions,
        })
        cookies.set('logged_in', 'true', { ...accessTokenCookieOptions })

        return { success: true }
    }

Setting cookies there throws this error.

{
    "message": "Cannot set headers after they are sent to the client",
    "code": "ERR_HTTP_HEADERS_SENT"
}

{
    "message": "Cannot set headers after they are sent to the client",
    "code": "ERR_HTTP_HEADERS_SENT"
}

How is that possible? Nothing is sent to client before return from this function right?
If I remove cookies code, it works fine. But it is quite important to set cookies for login endpoint

I am stuck on this for hours and I tried everything! Please help

Node v22.12.0
npm v11.0.0

GitHub

finance-tracker/backend/src/modules/auth at main · itsrakeshhq/fina...

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Cannot set headers after they are sent to the client

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