Subagent MCP

Independent harnesses. One Codex orchestrator.

When one model plans a change, implements it, and reviews it, the reviewer shares the author's context and blind spots. It can end up confirming its own plan instead of testing it.

Subagent MCP keeps Codex as the main agent and final decision-maker while delegating bounded work to external agent runtimes. Each runtime is a model paired with its native harness, so Codex can get implementation or review from an independent model with different context and assumptions.

That expands Codex's effective sub-agent pool and can use provider quota you already have. Subagent MCP never enables, purchases, auto-reloads, or silently opts into usage credits or paid overage.

Adapters translate every native harness into the same lifecycle: delegate, observe, steer, and close. The core hard-codes no provider role or model name.

Stable: 1.0.29 targets Windows. The MCP, package, localhost UI, and Claude Code and DeepSeek native-harness integrations are ready.

Runtime status

• Claude Code — Ready. Uses the native Claude Code harness, provider-native model and reasoning settings, subscription OAuth identity, and live no-overage evidence before accepting its output. Project/local context and exact auto-compaction-trigger attestation remain explicit capability gaps.
• DeepSeek Harness — Ready. Uses its native ACP transport and harness-published model catalog for bounded tasks. Resume after an MCP restart, exact provider quota evidence, interactive input, and declared MCP remain explicit capability gaps. A provider-retired model route fails terminally and is never replaced without the user's explicit selection.
• Grok Build — Ready for read-only review. The Windows adapter uses cached native login only and allows no credits, paid overage, or model fallback. Version 1.0.29 advertises and accepts repo_read only. Bounded writing remains In development and workspace_write requests fail before a native process starts. Other explicit gaps are terminal/test/Git execution, network/web/browser, MCP/plugins/hooks, nested agents, native worktrees, restart recovery, macOS/Linux support, and exact pre-request quota.

Runtime behavior comes from adapters rather than provider-specific branches in the core.

Quick start

1. Install

Install uv if needed, then register the exact isolated release and start its background UI:

winget install --id=astral-sh.uv -e
codex mcp add subagent-mcp -- uvx --isolated --from subagent-harness-mcp==1.0.29 subagent-harness-mcp serve
uvx --isolated --from subagent-harness-mcp==1.0.29 subagent-harness-mcp ui --background

Start a new Codex task after registration.

2. Configure runtimes

Open http://127.0.0.1:8765 in a browser. If the background UI was stopped, start the same exact release again:

uvx --isolated --from subagent-harness-mcp==1.0.29 subagent-harness-mcp ui --background

The settings and read-only activity UI stays on the fixed loopback port and does not depend on an active MCP connection.

If a runtime shows Sign in required, click Sign in. Its native harness opens the operating system's default browser; Subagent MCP never asks for or stores credentials. Complete login there, then click Refresh. A status check never opens a browser by itself.

Current and recent activity is listed by external agent. Select a row to inspect its model, native harness, workspace, permissions, write set, current stage, lifecycle, elapsed time, and redacted terminal result. Prompts, transcripts, hidden thinking, and raw provider events are never displayed.

3. Delegate

Ask Codex in plain language:

Use Subagent MCP to ask an external agent to review this change, then evaluate its findings independently.

Codex chooses what to delegate, observes the result, and keeps the final judgment. Lifecycle responses are compact by default; full redacted reports remain in local product state and can be read or relayed later by hash-bound reference.

For an approval tied to an exact file, pass inputs on agent_spawn (inside task) or agent_send:

[{"path":"docs/specs/review.md","expected_sha256":"<lowercase SHA-256>"}]

The MCP hashes each repository-relative file read-only immediately before the native turn. Status returns the verified hash and configured reasoning attestation; a changed file fails before the external agent runs.

Models and fallback order

Each native harness publishes its own model choices. The UI shows friendly names and an ordered priority stack; exact provider IDs remain available for advanced routes.

When a provider explicitly reports exhausted quota or credit (QUOTA_PAUSED), Subagent MCP moves that exact model to the bottom for future tasks. It does not retry the failed task. Ambiguous failures, crashes, and timeouts do not reorder models or trigger another paid request.

That failed task stays terminal, but the next explicit delegation checks the provider again live; no cached reset clock or wait-until checkpoint substitutes for that check. When the native harness exposes no safe pre-request quota endpoint, status stays unknown rather than inferred exhausted.

Operational recovery is capped at three actions. Local state work and explicit pre-provider failures may be retried. DeepSeek also retries an explicit temporary upstream HTTP 429 at most three total attempts. A generic provider failure is never replayed automatically: a read-only task returns bounded guidance for a new explicit attempt, while a write task remains non-retryable until its write set and possible effects are reconciled. Quota, credit, billing, timeout, and ambiguous lifecycle failures never trigger another provider call. Native ACP failures include bounded redacted RPC/provider details when the harness exposes them; unknown quota remains a provider error, not exhaustion.

DeepSeek routes may use an existing subscription, unlimited offer, or funded balance that the user authorizes. Subagent MCP never purchases, reloads, or increases that balance.

Long-running work

agent_wait is a bounded local observation, not a model deadline. If it returns running with wait_policy=continue_while_running, the external agent may still be working or thinking. Keep observing the same conversation; elapsed time alone never triggers retry, fallback, interruption, or another provider request.

Concurrent writers

A write task can declare up to 32 repository-relative file or directory roots in write_set. External writers may run concurrently when their canonical absolute sets are disjoint. Equal paths and parent/child paths conflict; task and lane names do not affect locking.

Omitting write_set gives the execution the whole workspace for backwards compatibility. Each adapter also enforces the normalized paths at its native harness boundary. Inspect runtime_list before creating a write request: write_root_mode=path-prefix supports exact file or directory prefixes, while existing-directory requires an existing directory. The current DeepSeek Harness native session advertises existing-directory and one writable root. Subagent MCP never widens an exact-file scope to its parent directory; use that broader directory only when it is explicitly acceptable, otherwise choose a runtime that enforces path-prefix. Multiple valid directory roots are split into disjoint writer calls. These leases coordinate Subagent MCP executions; they are not an operating-system sandbox for unrelated local processes.

On the current Windows public slice, set cwd to the checkout root and use workspace="current". For a DeepSeek write, pass workspace_write and exactly one existing directory relative to cwd, for example write_set=["Assets/_Project/Core/Scripts/GameSettings"]. An absolute path, missing path, or exact file is not a valid existing-directory root.

On Windows, the DeepSeek native sandbox must materialize a restricted-token ACE, so the current user also needs WRITE_DAC on the selected directory. Subagent MCP checks that capability read-only before ACP or provider work. It does not change ownership or ACLs. A failed check stays CAPABILITY_MISSING; it never falls back to danger-full-access.

How it fits together

flowchart LR
    C["Codex<br/>Main agent & orchestrator"]
    M["Subagent MCP<br/>Gateway"]
    UI["Localhost UI<br/>Settings & activity"]

    C -->|"delegate · steer · observe"| M
    UI --> M

    subgraph E["External agent runtimes — adapter-driven"]
        R1["Model<br/>+<br/>native harness"]
        R2["Model<br/>+<br/>native harness"]
        RN["Future runtimes<br/>via adapters"]
    end

    M -->|"normalized lifecycle"| R1
    M -->|"normalized lifecycle"| R2
    M -->|"normalized lifecycle"| RN

Subagent MCP owns lifecycle normalization, status, redaction, leases, and circuits. Each adapter translates that contract to its native harness. See Architecture for the full contract.

Update or roll back on Windows

Switch versions without reinstalling an environment that may still be running. The first command uses the source version; the add/start commands use the target version. This example upgrades 1.0.28 to 1.0.29:

uvx --isolated --from subagent-harness-mcp==1.0.28 subagent-harness-mcp ui --stop
codex mcp remove subagent-mcp
codex mcp add subagent-mcp -- uvx --isolated --from subagent-harness-mcp==1.0.29 subagent-harness-mcp serve
uvx --isolated --from subagent-harness-mcp==1.0.29 subagent-harness-mcp ui --background

Start a fresh Codex task after changing the entry. Existing tasks keep their old runtime until they end. Use the same sequence with the exact versions reversed to roll back.