I have the following problem:

I have super simple SQL code from a tutorial, saved into a file test.sql.

CREATE OR REPLACE FUNCTION add(integer, integer) RETURNS integer
    LANGUAGE SQL
    AS 'select $1 + $2;'
    RETURNS NULL ON NULL INPUT
    COST 100;

This code works fine when I am pasting it into the psql console manually,
starting psql as:

PGPASSWORD=... psql -v ON_ERROR_STOP=1 -U postgres -S postgres

However, when passing this file as input to the command via a redirection fails:

PGPASSWORD=... psql -v ON_ERROR_STOP=1 -U postgres -S postgres < test.sql
psql:test.sql:1: ERROR:  no language specified

This also fails:

PGPASSWORD=... psql -v ON_ERROR_STOP=1 -U postgres -S postgres -f test.sql
psql:test.sql:1: ERROR:  no language specified

What is going on?