How do I pass JSON to a Postgres COPY tmp FROM PROGRAM command

I'm trying to use Postgres (PL/pgSQL) to communicate with a shell script that expects JSON data. A simplified example of what I'm trying to do is

CREATE OR REPLACE FUNCTION json_func(IN json_in JSONB, OUT json_out JSONB)
LANGUAGE plpgsql
AS $code_block$
BEGIN
CREATE TEMPORARY TABLE tmp (json_tmp JSONB);
EXECUTE format($bash$ COPY tmp FROM PROGRAM 'echo ''%s'' | jq . ' $bash$, json_in);
SELECT json_tmp FROM tmp INTO json_out;
DROP TABLE tmp;
END;
$code_block$;

When I run

SELECT json_func('{"key1": "val1", "key2": "val2"}'::JSONB);

I get

ERROR:  invalid input syntax for type json
DETAIL:  The input string ended unexpectedly.
CONTEXT:  JSON data, line 1: {
COPY tmp, line 1, column json_tmp: "{"
SQL statement " COPY tmp FROM PROGRAM 'echo ''{"key1": "val1", "key2": "val2"}'' | jq . ' "
PL/pgSQL function json_func(jsonb) line 4 at EXECUTE

I've tried various ways of escaping the single quotes needed to surround the JSON input, but no luck.

You're getting the error because COPY expects one value per line, while jq pretty-prints by default, splitting the single json structure into multiple lines. As already pointed out by @Francisco Puga, you can disable that:

--compact-output / -c:

By default, jq pretty-prints JSON output. Using this option will result in more compact output by instead putting each JSON object on a single line.

Otherwise, this

'{"key1": "val1", "key2": "val2"}'

Gets turned by jq into this

{
    "key1": "val1",
    "key2": "val2"
}

And due to the line-separation thing, copy thinks you're trying to do something this:

insert into tmp(json_tmp)values 
 ('{')
,('"key1": "val1",')
,('"key2": "val2"')
,('}');

Hence, the error complaining about the single opening curly bracket. It seems to work fine if you enable compaction:

CREATE OR REPLACE FUNCTION json_func(IN json_in JSONB, OUT json_out JSONB)
LANGUAGE plpgsql
AS $code_block$
BEGIN
CREATE TEMPORARY TABLE tmp (json_tmp JSONB);
EXECUTE format($dynsql$ COPY tmp FROM PROGRAM $bash$echo %1$L |jq -c . $bash$ 
               $dynsql$, json_in);
SELECT json_tmp FROM tmp INTO json_out;
DROP TABLE tmp;
END;
$code_block$;

---

If that's an attempt to pretty-print, Postgres offers a built-in jsonb_pretty():
_{demo at db<>fiddle}

select jsonb_pretty('{"abc":123}');

If it's for validation, In version 16+ you can check that with 'abc' IS JSON or pg_input_is_valid('abc','jsonb'):

select 'abc' is json;                          --false
select pg_input_is_valid('abc','json');        --false
select '{"abc":123}' is json;                  --true
select pg_input_is_valid('{"abc":123}','json');--true