Hi Tanguy,

happy to hear you've finally fixed it.

Again, %ZSTART is not at all mandatory ; as soon as your ENV variables are available in your container you should be able to get it either from $system.Util.GetEnviron(env) or ##class(%SYS.Python).Import("os").getenv(env)

With these variables : 

{"Env": [
			"API_CLIENT_SECRET=ea5663885513e5b00df120fa4b4da8e1150398cde9d41ee27b5a8c6f1898dfa63ae711b82bf06b36475b646453a9092f5653895ddd2c3bb067d9a4f562a6b625",
			"TZ=Europe/Paris",
			"ISC_DATA_DIRECTORY=/irisdata",
			"ISC_CPF_MERGE_FILE=/app/merge.cpf",
			"API_CLIENT_ID=3e726f42daaa06a8",
			"ISC_PACKAGE_IRISGROUP=irisowner",
			"ISC_PACKAGE_IRISUSER=irisowner",
			"ISC_PACKAGE_MGRGROUP=irisowner",
			"ISC_PACKAGE_MGRUSER=irisowner",
			"IRISSYS=/home/irisowner/irissys",
			"TINI_VERSION=v0.19.0",
			"ISC_PACKAGE_INSTANCENAME=IRIS",
			"ISC_PACKAGE_INSTALLDIR=/usr/irissys",
			"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/home/irisowner/bin",
			"PYTHONPATH=/usr/irissys/mgr/python",
			"LANG=en_US.UTF-8",
			"LANGUAGE=en_US.UTF-8",
			"LC_ALL=en_US.UTF-8"
		]}

You should get :
 

write $SYSTEM.Util.GetEnviron("API_CLIENT_ID")
3e726f42daaa06a8
write $SYSTEM.Util.GetEnviron("API_CLIENT_SECRET")
ea5663885513e5b00df120fa4b4da8e1150398cde9d41ee27b5a8c6f1898dfa63ae711b82bf06b36475b646453a9092f5653895ddd2c3bb067d9a4f562a6b625
write $SYSTEM.Util.GetEnviron("TZ")
Europe/Paris
write ##class(%SYS.Python).Import("os").getenv("API_CLIENT_ID")
3e726f42daaa06a8
write ##class(%SYS.Python).Import("os").getenv("API_CLIENT_SECRET")
ea5663885513e5b00df120fa4b4da8e1150398cde9d41ee27b5a8c6f1898dfa63ae711b82bf06b36475b646453a9092f5653895ddd2c3bb067d9a4f562a6b625
write ##class(%SYS.Python).Import("os").getenv("TZ")
Europe/Paris

Or, with %ZSTART containing :
 

ROUTINE %ZSTART
  QUIT ; Prevents direct execution without a label

SYSTEM ; Subroutine called at system startup
  ; Initialize the IRIS application secrets
  Set ^Secrets("API_CLIENT_ID") = $System.Util.GetEnviron("API_CLIENT_ID")
  Set ^Secrets("API_CLIENT_ID","LastUpdated") = $ZDATETIME($HOROLOG,3)
  Set ^Secrets("API_CLIENT_SECRET") = $System.Util.GetEnviron("API_CLIENT_SECRET")
  Set ^Secrets("API_CLIENT_SECRET","LastUpdated") = $ZDATETIME($HOROLOG,3)
  QUIT

you should obtain :

IRISAPP>zw ^Secrets
^Secrets("API_CLIENT_ID")="3e726f42daaa06a8"
^Secrets("API_CLIENT_ID","LastUpdated")="2025-08-07 17:16:19"
^Secrets("API_CLIENT_SECRET")="ea5663885513e5b00df120fa4b4da8e1150398cde9d41ee27b5a8c6f1898dfa63ae711b82bf06b36475b646453a9092f5653895ddd2c3bb067d9a4f562a6b625"
^Secrets("API_CLIENT_SECRET","LastUpdated")="2025-08-07 17:16:19"

IRISAPP>d SYSTEM^%ZSTART

IRISAPP>zw ^Secrets
^Secrets("API_CLIENT_ID")="3e726f42daaa06a8"
^Secrets("API_CLIENT_ID","LastUpdated")="2025-08-07 17:24:02"
^Secrets("API_CLIENT_SECRET")="ea5663885513e5b00df120fa4b4da8e1150398cde9d41ee27b5a8c6f1898dfa63ae711b82bf06b36475b646453a9092f5653895ddd2c3bb067d9a4f562a6b625"
^Secrets("API_CLIENT_SECRET","LastUpdated")="2025-08-07 17:24:02"

Tanguy,

using my sample repo, you should be able to see in the ^Secrets global :

Tanguy,

you can find a full example of retrieving environment variables from a Business Operation either from a global or from $system.Util.GetEnviron(env) in this repo:
https://github.com/SylvainGuilbaud/iris-dev