VS Code 1.133 moved AI agent sessions out of the editor's extension host and into a standalone Agent Host process that outlives the window and can be reached over SSH or a dev tunnel, per DevOps.com. For teams standardising on Copilot, Claude or Codex in a build, an agent run stops behaving like a chat tab and starts behaving like a background service they have to monitor.
The release, which DevOps.com dates to August 12, publishes the wire format between client and host as the Agent Host Protocol (AHP), released on GitHub under an MIT license. Microsoft is positioning AHP next to the Language Server Protocol and the Debug Adapter Protocol: a spec other IDEs and tools can implement, not a VS Code internal. The protocol uses JSON-RPC over immutable state, with sequence-numbered updates, so multiple clients can attach to the same session and stay in sync.
What actually moved
First-party adapters ship for Copilot, Claude and Codex. Inside a Claude session the model picker groups Anthropic and Copilot models under separate headings, and per turn you choose which one runs; billing follows the choice, with Anthropic entries hitting the API key and Copilot entries drawing from the Copilot subscription. Switching providers used to require reconfiguring the agent host.
Two more knobs matter for platform teams. Typing code agent host in a terminal starts a local server, protected by a connection token by default; adding a --tunnel flag exposes it through a dev tunnel. An experimental setting, chat.agentHost.allowSignedOutWhenUsable, drops the mandatory GitHub sign-in prompt on the Agents window, so a machine that cannot reach github.com but has an API key configured is no longer gated. For now the setting only works with Claude; per DevOps.com, Copilot with custom model keys and Codex support is coming later.
The catch
The open spec is on paper, but the load-bearing part is that the first-party adapters keep parity. The Futurum Group's Mitch Ashley told DevOps.com that "open beats proprietary only if the adapters for Claude and Codex stay first-class." Old sessions still run under the extension-host model, and flipping chat.agentHost.enabled to false reverts to that behaviour entirely, so a bad rollout is a settings-toggle away from the previous default. The rollout is going through VS Code's built-in updater; teams that manage the editor centrally will see it land on whatever cadence their update policy allows, not on August 12.
