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Connect T3 Code to a sandbox

Table of contents
Availability: GA
Requires: Docker Sandboxes 0.37.0 or later

T3 Code's SSH integration lets the desktop app drive coding agents inside a sandbox. T3 Code has no dedicated Docker Sandboxes integration — it treats the sandbox as an ordinary SSH host, connects to it, and starts a T3 server inside that tunnels back to the app.

Prerequisites

Connect

Confirm that you can connect to the sandbox from a terminal:

$ ssh demo.sbx

In T3 Code, add an SSH environment and enter the sandbox hostname, such as demo.sbx, as the host. The first connection installs the T3 server inside the sandbox, so it can take a moment. Later connections are faster.

Then add a new project, select the SSH environment from the list, and choose the mounted workspace as the project directory inside the sandbox.

Troubleshoot a server that never becomes ready

T3 Code can fail to connect with an error like the following, wrapped here for readability. It concatenates the connection failure with npm's install output from inside the sandbox into a single error dialog:

Could not prepare the SSH environment: ... SshCommandError: Connecting to
sandbox "sandboxes"… Remote T3 server did not become ready on
127.0.0.1:3773. npm WARN EBADENGINE Unsupported engine { package:
'ini@7.0.0', required: { node: '^22.22.2 || ^24.15.0 || >=26.0.0' },
current: { node: 'v22.22.1', npm: '9.2.0' } }

The npm WARN EBADENGINE lines warn about the transitive ini dependency. They aren't the cause of the failure — npm doesn't enforce engine requirements by default, so this warning alone doesn't stop the install. Check free disk space in the sandbox. A full disk can fail the T3 server install partway through, and T3 Code reports this the same way as a connection timeout:

$ sbx exec <sandbox> -- df -h /

Free up space and reconnect if the sandbox is close to full.

Troubleshoot turn/setPermissionMode failed

If your organization manages Claude Code with a policy file, a local T3 Code thread can fail to start with turn/setPermissionMode failed. T3 Code's default runtime mode is Full access, which maps to the Claude Agent SDK's bypassPermissions mode. A managed policy that disables that mode rejects the request.

On macOS, check whether this applies to you:

$ cat "/Library/Application Support/ClaudeCode/managed-settings.json"

If permissions.disableBypassPermissionsMode is set to disable, switch T3 Code to a different runtime mode, such as Supervised, Auto-accept edits, or Auto, then start a new thread. The permission mode is captured once when a thread starts, so switching modes in an already-failing thread doesn't recover it.

This restriction applies to the host running Claude Code, not to a sandbox. A thread connected to a sandbox isn't subject to the host's managed policy, so Full access works normally there.