Hi, sorry to resurrect an old topic, but I have the same exact question, and can provide redacted logs. I've installed Core Helper.
Could it be a path issue? It doesn't find the aws command without the full path, unlike my usual shells.
Equivalent Command: ssh -4 -i "/Users/aleksikallio/Library/Group Containers/E78WKS7W4U.io.coressh.ssh/.ssh/privatekey/REDACTED" -vvv -L 0.0.0.0:3306:REDACTEDDBHOST:3306 -o ExitOnForwardFailure=yes -o ProxyCommand="sh -c "/usr/local/bin/aws ssm start-session --target %h --document-name AWS-StartSSHSession --parameters \"portNumber=%p\""" -o ServerAliveInterval=300 -o ServerAliveCountMax=2 ec2-user@i-REDACTED
07:36:14 Connecting…
07:36:14 Using Core Helper 5.0 (r50)
07:36:14 OpenSSH_8.4p1, OpenSSL 1.1.1g 21 Apr 2020
07:36:14 debug1: Reading configuration data /Users/aleksikallio/.ssh/config
07:36:14 debug1: /Users/aleksikallio/.ssh/config line 54: Applying options for i-*
07:36:14 debug1: Reading configuration data /etc/ssh/ssh_config
07:36:14 debug1: /etc/ssh/ssh_config line 47: Applying options for *
07:36:14 debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' -> '/Users/aleksikallio/.ssh/known_hosts'
07:36:14 debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' -> '/Users/aleksikallio/.ssh/known_hosts2'
07:36:14 debug1: Executing proxy command: exec sh -c "/usr/local/bin/aws ssm start-session --target i-REDACTED --document-name AWS-StartSSHSession --parameters \\"portNumber=22\\""
07:36:14 Jumping…
07:36:14 debug1: identity file /Users/aleksikallio/Library/Group Containers/E78WKS7W4U.io.coressh.ssh/.ssh/privatekey/REDACTED type 0
07:36:14 debug1: identity file /Users/aleksikallio/Library/Group Containers/E78WKS7W4U.io.coressh.ssh/.ssh/privatekey/REDACTED-cert type -1
07:36:14 debug1: identity file /Users/aleksikallio/Google Drive File Stream/My Drive/REDACTED/REDACTED type 0
07:36:14 debug1: identity file /Users/aleksikallio/Google Drive File Stream/My Drive/REDACTED/REDACTED-cert type -1
07:36:14 debug1: Local version string SSH-2.0-OpenSSH_8.4
SessionManagerPlugin is not found. Please refer to SessionManager Documentation here: http://docs.aws.amazon.com/console/systems-manager/session-manager-plugin-not-found
07:36:17 kex_exchange_identification: Connection closed by remote host
07:36:17 Connection closed by UNKNOWN port 65535
07:36:17 Abnormal Disconnect
07:36:17 Connection failed, retry after 3s…
07:36:17 Disconnected