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Outside agents

Agents that T3 Code did not start can work with an environment through its MCP server. They can read projects and threads, start and message threads, and check which providers and models are available. This covers Claude Code or Codex in your own terminal, ChatGPT, and bots that support MCP. Each agent signs in once, and you choose what it may do.

Get the MCP URL

In Settings → Connections, open a saved environment's menu and choose Copy MCP URL. The URL is the environment's address followed by /mcp, for example:

https://<environment-address>/mcp

The copied URL uses the route this device is connected over, so an agent on the same device can reach it.

  • An agent on your own computers can use localhost on the host, T3 Connect, or any https address, such as Tailscale HTTPS. Signing in sends the agent's credentials to the environment, and agents only do that over https or to localhost. A plain-http LAN or Tailscale address, like http://100.x.y.z:3773, lets an agent find the environment but not finish signing in.
  • A hosted agent, such as ChatGPT or a bot running in the cloud, must reach the environment from the internet. Use the T3 Connect address. A Tailscale address only works from your own tailnet.

Approve a sign-in

The first time an agent connects, it opens a sign-in page on the environment. The page names the agent and where its access goes:

  • For an agent on your computer, approval returns to a localhost address on the computer that opened the page.
  • For a hosted agent, approval goes to that service, such as chatgpt.com. Anyone who runs that service gets the access.

Only approve a sign-in you just started. To approve, enter a pairing code from Settings → Connections or from t3 auth pairing create on the host. A browser already signed in to the environment with permission to manage access can approve without a code.

Then choose what the agent may do:

  • Read only is the default. The agent can read projects and threads and see the available providers and models. It cannot change anything.
  • Supervised through Full access also let it start, message and stop threads in every project. A thread it starts or steers cannot run with more permissions than the mode you chose.

Claude Code

claude mcp add --transport http t3 https://<environment-address>/mcp
claude mcp login t3

claude mcp login opens the sign-in page in your browser. On a machine without a browser, add --no-browser, open the printed URL elsewhere, and paste the final URL back when asked. claude mcp list shows t3 as connected once you approve.

Codex

codex mcp add t3 --url https://<environment-address>/mcp
codex mcp login t3

codex mcp login --no-browser prints the sign-in URL instead of opening it. codex mcp list shows t3 with OAuth once you approve.

ChatGPT

Add the environment as a custom MCP app in ChatGPT's apps settings:

  1. Create a new app or plugin with a custom MCP server.
  2. Set the server URL to https://<environment-address>/mcp, using the T3 Connect address.
  3. Choose OAuth for authentication. ChatGPT discovers the rest; you do not need a client ID or secret.
  4. Create the app, then approve the sign-in page that opens.

Other agents and bots

Any agent that supports remote MCP servers over HTTP with OAuth can connect. Give it the MCP URL and choose OAuth, and the agent finds the sign-in settings itself. Agents without OAuth support cannot connect.

Scripts on the host

On the machine that runs the environment, t3 thread uses the same tools without a sign-in:

id=$(t3 thread start . "Find why the tests are slow")
t3 thread send "$id" "Now fix it" --wait
t3 thread show "$id"
t3 thread list

. means the project that contains the current folder; a project id also works. A new thread uses the project's default model and permission mode, and runs in the project folder unless you pass --worktree <base-branch>. send never interrupts: a busy thread gets the message as its next turn. Pass - as the message to read it from stdin.

--wait prints the agent's reply. It exits 1 when the run fails or stops, and 2 when the agent waits for an answer you give in the app. Add --json for a result that always includes the thread id.

Remove an agent

Approved agents appear under Settings → Connections like other clients. Revoke one there to cut off its access immediately. A sign-in lasts 30 days; after that the agent asks you to approve it again.