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MCPJam supports two harness hosts that run a real agent runtime inside your project’s Computer (a cloud Linux sandbox) instead of MCPJam’s emulated chat loop. You are observing the actual runtime — its native tools, its own agent loop, its real execution behavior — not a simulation of it.
  • Claude Code — runs the real Claude Code agent with Anthropic models. Supports selected MCP servers and native tool approval.
  • Codex — runs the real Codex CLI agent with OpenAI gpt-5 family models. Supports skills; does not support selected MCP servers.
The Claude Code and Codex hosts are available to organizations where the respective feature has been enabled. Contact your account team if you don’t see them in the host template catalog.

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What happens when you run a turn

  1. Pre-flight check. Before the stream opens, MCPJam verifies that the host can run: the model is supported by the selected harness, the Computer data plane is configured, and the host settings are ones the runtime can honor. Any failure returns a clear error before the turn starts — a turn never silently falls back to the emulated engine.
  2. Computer wake. The host’s project Computer is reserved or woken (provisioned on first use). Both harness hosts require a Computer — there is no local fallback.
  3. Credential delivery. MCPJam installs a short-lived model credential into the sandbox’s egress layer outside the VM. Neither the sandbox nor your browser ever holds a real model key. The agent runs pointed at MCPJam’s model proxy, which verifies the credential and meters every generation. There is no raw-key fallback.
  4. MCP delivery. For Claude Code hosts, selected MCP servers are written into the session’s MCP config, each pointed at MCPJam’s per-server proxy tunnel — no upstream credentials enter the sandbox. Codex hosts do not support selected MCP servers.
  5. The turn runs. The agent’s own loop executes; native tools run inside the sandbox; file changes land on the Computer’s disk; the transcript and trace persist like any other chat.

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Host settings

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Claude Code

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Codex

Host-page tool-level toggles (such as tool visibility) are not re-enforced for harness runs. Controls the runtime cannot honor are disabled in the UI rather than silently ignored.

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Requirements

  • A project Computer (the E2B data plane must be configured for your deployment).
  • For Claude Code: an MCPJam-provided Anthropic model selected on the host.
  • For Codex: an MCPJam-provided OpenAI gpt-5 family model selected on the host.
  • A signed-in project member — guest sessions cannot run harness hosts.
No credential configuration is needed. Broker delivery is on by default and the model proxy is always-on.

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Billing

  • Computer time — the harness runtime keeps the Computer awake, so it meters into your org’s monthly computer-time allowance exactly like terminal use.
  • Model tokens — every generation is priced and settled by the model proxy against your org, the same accounting as regular chat. Spend caps and empty-wallet rejections apply before the stream starts.

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Failure modes

None of these fall back to the emulated engine — a turn that says it ran the real runtime did. All fail closed; a failed start spends nothing.
  • Computer — Manage your project’s Computer, sandbox images, and lifecycle.
  • Playground — The workspace where you select hosts and run turns.