addon.sh

#!/usr/bin/env bash

# Debug output
for i in $(seq 10); do echo  >> /tmp/debug.log; done
echo ====================================================================== >> /tmp/debug.log
echo args: "$@" >> /tmp/debug.log
cat $ITEM_DATA_JSON >> /tmp/debug.log
echo >> /tmp/debug.log
env >> /tmp/debug.log

# Create the output JSON
jq '
	{
		"commands": [
			{
				"itemId": .id,
				"actions": [
					{
						"tags": [
							"tag1",
							"tag2"
						],
						"action" : "add-tags"
					}
				]
			}
		]
	}
' $ITEM_DATA_JSON |
	tee -a /tmp/debug.log	# also log it

docspell-addon.yaml

meta:
  name: "minimal-addon"
  version: "$VERSION"
triggers:
  - existing-item
  - scheduled
runner:
  trivial:
    enable: true
    exec: addon.sh

options:
  # If true, the stdout of the program is parsed into a JSON structure
  # that is interpreted as actions executed by the task that runs the
  # addon. If the addon runs side effects only, set this to `false`
  # and the output is ignored.
  #
  # It is recommended to use this approach, if possible. It allows
  # docspell itself to apply any changes and the addon can run
  # completely isolated.
  #
  # Default is true.
  collectOutput: true
  networking: false

Makefile

target/addon.zip: docspell-addon.yaml addon.sh
	# prepare
	mkdir -p target
	rm -rf target/unpacked
	mkdir target/unpacked
	# addon.sh
	cp addon.sh target/unpacked
	chmod +x target/unpacked/addon.sh
	# docspell-addon.yaml
	cat docspell-addon.yaml | sed 's#$$VERSION#0.$(shell date +%s)#' > target/unpacked/docspell-addon.yaml
	# zip
	env -C target/unpacked zip -r ../addon.zip.new .
	# commit
	mv target/addon.zip.new target/addon.zip
server:
	mkdir -p target
	env -C target python3 -m http.server

Can you add

exec >>&2 /tmp/debug.log

at the top, change all your >> /tmp/debug.log to >&2 and also set -x at the top, then send me the debug.log?