polyagent-mcp

MCP server that lets any agent or MCP host delegate to headless Codex, Grok, Claude Code, and OpenCode CLIs, with Cursor available as an opt-in fallback. Use the fleet for implementation, planning, and project exploration without burning the caller's context on raw worker output.

Worker tools take optional model and effort overrides, return a session_id, and support follow_up. Difficulty levels select a distinct default model across the three active subscriptions; delegate also accepts an explicit engine, including pay-per-token OpenCode.

Tools

The server exposes ten tools:

Worker tools accept cwd, model, and effort where applicable. delegate requires a level (1-5); fast_delegate has none and picks the fastest healthy engine. Explicit model/effort values override the selected tier.

When an engine runs out of quota

A call that fails because the engine's plan quota is exhausted does not silently retry on another engine — spending the next subscription is your decision. The call fails with an actionable error naming the engines still available (installed, enabled, and capable of what that tool needs) and how to switch: engine:"<x>" on the four auxiliary tools and delegate, or the lowest still-usable level:<n> when a tier engine remains. Tools that pick the engine themselves (fast_delegate, fan_out) and follow_up (pinned to the resumed session's engine) report the quota without suggesting a parameter, and generate_image reports it against the two engines that have an image tool at all (codex, grok). A transient rate limit is reported separately and asks you to wait, since switching engines would not help. Anything the classifier does not recognize — an expired login, for one — propagates as the raw CLI failure instead of being guessed at.

Related MCP server: subagents

Requirements

• Node ≥ 18

• bubblewrap (bwrap) installed — required, not recommended: the sandbox is mandatory and the server refuses to start without it (sudo apt install bubblewrap). Only POLYAGENT_SANDBOX=off waives it, as an explicit operator choice.

• Codex installed and authenticated for read tools and levels 1/3; Grok for levels 2/4; Claude Code for level 5.

• Optional Cursor fallback: install cursor-agent and set POLYAGENT_ENABLE_CURSOR=1.

Install

Installing via an AI agent? Point it at INSTALL.md — an agent-facing, copy-paste guide that detects the host and registers the bridge in Claude Code, Cursor, Codex, Grok, or any generic MCP host.

git clone https://github.com/JaimeJunr/polyagent-mcp.git
cd polyagent-mcp
npm install
npm run build

Register in an MCP host

Claude Code:

claude mcp add polyagent -s user -- node /abs/path/to/polyagent-mcp/dist/index.js

Any host — add to its mcp.json:

{
  "mcpServers": {
    "polyagent": {
      "command": "node",
      "args": ["/abs/path/to/polyagent-mcp/dist/index.js"]
    }
  }
}

Permissions (Claude Code): claude mcp add registers the server but does not grant tool permission — without an allowlist every bridge call prompts for approval. After registering, add either "mcp__polyagent__*" (full; also auto-approves mutating tools delegate/fast_delegate/run_filtered/follow_up) or a read-only subset (explore/read_slice/web_lookup/bridge_stats) under permissions.allow in settings.json. Full options and trade-offs: INSTALL.md §3. Cursor/Codex/other hosts have their own approval settings — consult the host.

Configuration (env)

Breaking change: env var rename

Every CURSOR_BRIDGE_* variable was renamed to POLYAGENT_* (same suffix), and CURSOR_BIN became POLYAGENT_CURSOR_BIN. This is a clean cut: the old names are no longer read at all — setting one has zero effect (no fallback, no warning). Update your host config (mcp.json / settings.json "env" blocks) and any shell profile before upgrading.

"cursor" survives only where it names the actual Cursor engine (POLYAGENT_CURSOR_BIN, POLYAGENT_ENABLE_CURSOR).

Security: delegate, fast_delegate, and run_filtered have full access and auto-approve their work. explore, read_slice, and web_lookup use Codex's read-only sandbox. Named agents are resolved on the host, reject path traversal, and are injected without mounting agent directories.

Make the agent actually use it

Registering the tools is not enough. Two structural forces push the agent back to native tools: (1) the host rule "prefer the dedicated file/search tools", and (2) MCP tools used to be deferred — the agent had to run a tool-search to load their schemas, so always-loaded Read/Grep/WebSearch won by default. The server now publishes startup instructions (routing boundary) and marks the five core tools with _meta: { "anthropic/alwaysLoad": true } (Claude Code ≥2.1.121) so their schemas load eagerly. fast_delegate joined them for the same reason: deferred, it was never picked. The remaining secondary tools stay deferred. Four fixes, strongest first:

1. Call-time hook (recommended). A PreToolUse hook that steers the agent toward the bridge at the moment it reaches for a native tool — text in a config file loses under pressure, a call-time reminder does not. This repo ships one at hooks/prefer-polyagent.mjs: it runs on node (already required) and only fires where it pays. Default mode is redirect (POLYAGENT_HOOK_MODE=redirect): for the two safe-to-block cases it returns permissionDecision: "deny" once and names the bridge tool; other cases stay non-blocking nudges. Wire it into your host's settings (Claude Code settings.json):

Breaking change (US-007): The hook file was renamed from hooks/prefer-cursor-bridge.mjs to hooks/prefer-polyagent.mjs. Update any host settings.json entry that points to the old path.

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Read|Grep|Glob|WebSearch|WebFetch|Bash|Edit|Write|MultiEdit",
        "hooks": [
          { "type": "command", "command": "node /abs/path/to/polyagent-mcp/hooks/prefer-polyagent.mjs", "timeout": 5 }
        ]
      }
    ]
  }
}

What it emits, and when — each fires at most once per session (deduplicated in a tmp file keyed by session_id), because a repeated fire is worse than none: the agent learns to ignore it and every fire costs tokens. Dedup keys are saved before emitting so redirect is one-shot and fail-open (a second identical call is allowed through).

• Read whole-file (no offset/limit) over POLYAGENT_HOOK_MIN_LINES lines → redirect (default) or nudge toward read_slice (once per file). Partial reads are left alone.

• WebSearch/WebFetch → redirect (default) or nudge toward web_lookup (once).

• Grep/Glob → emits the one-time preload reminder to run the ToolSearch for any still-deferred bridge tools (nudge only — never redirected). The dedup collapses them to a single fire.

• Bash whose command writes an artifact (git commit/push, git worktree add, gh pr create, gh issue create, bkt pr create) → suggests offloading that grunt-work to delegate (once, nudge only). Read-only Bash (status/diff/log/checkout) is left alone — the orchestrator needs that state, and a mechanical filter (e.g. rtk) already trims the noise.

• Edit/Write/MultiEdit → once per session, reminds that a self-contained task (feature, bugfix, mechanical multi-file change, build fix) can go whole to delegate(prompt, level) — the selected worker edits with full access — instead of the orchestrator implementing it on expensive tokens. It never blocks the edit (nudge only); the once-per-session dedup means the orchestrator still edits inline freely (the nudge repositions execution, it doesn't police every edit).

• The first qualifying fire of the session (whichever tool triggers it) also carries that preload reminder, so secondary schemas get loaded even in a Read-only or web-only session.

• Redirect deny reasons end with a fail-open suffix: if the bridge tool isn't loaded yet, run ToolSearch first; if the native tool is genuinely needed, call it again and it will be allowed (critical under headless -p so the agent never hard-stalls).

Set POLYAGENT_HOOK_MODE=nudge for the old non-blocking behavior, or off to disable. To reset the dedup and see the fires again, start a new session (or delete polyagent-nudged-<session_id>.json from your OS temp dir — os.tmpdir(), e.g. /tmp on Linux, not necessarily $TMPDIR).

Preloading at session start (SessionStart)

The PreToolUse preload above only fires when the agent uses the Grep/Read tool. But under pressure agents often reach for Bash grep instead, which matches no PreToolUse matcher — so the preload reminder never arrives. Wire the same hook for SessionStart to close that hole: the preload reminder then lands in context before the first tool decision, regardless of how the agent searches.

{
  "hooks": {
    "SessionStart": [
      { "hooks": [{ "type": "command", "command": "node /abs/path/to/polyagent-mcp/hooks/prefer-polyagent.mjs", "timeout": 5 }] }
    ]
  }
}

On SessionStart the hook emits the ToolSearch preload as additionalContext and pre-marks preload as seen in the session's dedup file, so the PreToolUse piggyback never repeats it.

Reaching subagents too (SubagentStart)

The nudges above only steer the main loop. Spawned subagents never see them, so wire the same hook for SubagentStart as well:

{
  "hooks": {
    "SubagentStart": [
      {
        "hooks": [
          { "type": "command", "command": "node /abs/path/to/polyagent-mcp/hooks/prefer-polyagent.mjs" }
        ]
      }
    ]
  }
}

On SubagentStart the hook injects a compact polyagent preference into every spawned subagent via additionalContext (subagentStartContext(agent_type)). When agent_type is Explore it appends an extra line: that Explore run was spawned on the orchestrator's expensive model (Explore inherits the session model, capped at Opus), so it should route all reading through explore/read_slice (which run on Codex Luna) and keep the expensive shell to orchestration only.

Coexisting with context-mode. The bridge and context-mode use separate channels (context-mode may still do its own thing; this hook only emits additionalContext), so they coexist cleanly — no updatedInput race, no delay, no import of context-mode's routing.

2. Preload any still-deferred tools. The five core tools and fast_delegate are already alwaysLoad on Claude Code ≥2.1.121. For secondary tools (or older hosts), tell the agent to load schemas once per session. Add to your CLAUDE.md/AGENTS.md:

At the start of any session involving code reading/exploration, run tool-search once for
`read_slice, explore, run_filtered, web_lookup` (and any secondary bridge tools you need) so
their schemas are loaded if the host still defers them.

3. Reconcile the conflict in CLAUDE.md. State the precedence explicitly:

The host rule "prefer dedicated file/search tools" applies to the EDIT path (Edit needs the
file content → native Read). For PURE reading/locating/web (no edit), polyagent takes
precedence over native Read/Grep/Glob/WebSearch/WebFetch. Read a large file whole with native
Read ONLY when you are about to edit it.

4. Delegate execution, not just exploration. The bridge is not only for reading — delegate runs implementation work with full read/edit/shell access, so the orchestrator shouldn't burn its own tokens on self-contained tasks. State this in CLAUDE.md so the agent routes doing, not just finding, to the cheap worker:

You are the ORCHESTRATOR. delegate(prompt, level) is the DEFAULT for BOTH execution AND judgment.
`level` picks a distinct tier: 1=GPT-5.6 Luna max (codex), 2=GPT-5.6 Sol xhigh (codex), 3=Grok 4.6
high (grok), 4=GPT-6 Astra max (codex), 5=Claude Fable 5.1 max (claude). Levels 4 and 5 are expensive
(5 the most by far) — reserve them for what cheaper levels cannot do. The worker has full read/edit/shell access
in cwd when you delegate. The constant win is context economy: the worker's raw output never enters
your context. Delegate it, then review the result; edit inline only for a quick one-off you're
already positioned for. Use fast_delegate(prompt) when the work is self-contained and you just want
the fastest healthy worker. Pass agent:"name" or agent:{prompt:"..."} when the worker
needs a specialized persona.

Develop

npm test       # vitest — unit tests for model resolution / arg building
npm run dev    # run from source via tsx

License

MIT