leonardsellem/codex-subagents-mcp

codex-subagents-mcp

File‑based sub‑agents for Codex CLI. One MCP tool. Zero fluff.

• Auditable: agents are files reviewed in PRs
• CI‑friendly: validate_agents, list_agents
• Safer ops: temp workdirs, quiet stdout, git worktree isolation

Claude‑style sub‑agents for Codex CLI via a tiny MCP server. Each call spins up a clean context in a temp workdir, injects a persona via AGENTS.md, and runs codex exec --profile <agent> to preserve isolated state.

Quickstart

• Prereqs: Node.js >= 18, npm, Codex CLI installed and on PATH.
• Install deps and build:

npm install
npm run build

• Start server (manual run):

npm start

Tools exposed by this server:

• Primary: delegate
• Support: list_agents, validate_agents

Agents directory discovery (in order): --agents-dir arg, CODEX_SUBAGENTS_DIR env, then defaults ./agents, ./.codex-subagents/agents, dist/../agents.

60‑second Quickstart (copy‑paste)

# 1) Install + build
npm i && npm run build

# 2) Point Codex at the built server (absolute paths recommended)
# ~/.codex/config.toml
[mcp_servers.subagents]
command = "/usr/bin/env"
args    = ["node", "/ABS/PATH/TO/dist/codex-subagents.mcp.js", "--agents-dir", "/ABS/PATH/TO/agents"]

# Example profiles you can reference from agent frontmatter
[profiles.review]
model = "gpt-5"
approval_policy = "on-request"
sandbox_mode    = "read-only"

[profiles.debugger]
model = "o3"
approval_policy = "on-request"
sandbox_mode    = "workspace-write"

# 3) In Codex, verify tools and agents
tools.call name=list_agents
tools.call name=validate_agents

# 4) Delegate one task to an agent
subagents.delegate(agent="review", task="Summarize and review the last commit")

Tip: See docs/SECURITY.md for trust boundaries and docs/OPERATIONS.md for E2E and logging.

Wiring with Codex CLI

Build the server and point Codex at the absolute path to the compiled entrypoint. Pass the agents directory explicitly so the server doesn't scan until after the handshake. The server also falls back to an agents/ folder adjacent to the installed binary (e.g. dist/../agents) if --agents-dir and CODEX_SUBAGENTS_DIR are not provided:

# ~/.codex/config.toml
[mcp_servers.subagents]
command = "/absolute/path/to/node"
args    = ["/absolute/path/to/dist/codex-subagents.mcp.js", "--agents-dir", "/absolute/path/to/agents"]

[profiles.review]
model = "gpt-5"
approval_policy = "on-request"
sandbox_mode    = "read-only"

[profiles.debugger]
model = "o3"
approval_policy = "on-request"
sandbox_mode    = "workspace-write"

[profiles.security]
model = "gpt-5"
approval_policy = "never"
sandbox_mode    = "workspace-write"

Usage (in Codex):

• “Review my last commit. Use the review sub-agent.”
• “Reproduce and fix the failing tests in api/ using the debugger sub-agent.”
• “Audit for secrets and unsafe shell calls; propose fixes with rationale using the security sub-agent.”

AGENTS hint to drop into your repo’s AGENTS.md

Route all work through the orchestrator:

subagents.delegate(agent="orchestrator", task="<goal>")

Example security review routed via orchestrator:

subagents.delegate(agent="orchestrator", task="Audit the repo for secrets and unsafe shell calls")

Prefer tool calls over in-thread analysis to keep the main context clean.

Note: MCP servers run outside Codex’s sandbox. Keep surfaces narrow and audited. This server exposes a single tool: delegate.

Who This Is For / Not For

• For: teams already on Codex CLI who want auditable specialist agents and CI gating.
• Not for: folks seeking a general agent framework or multi‑tool orchestrator.

Terminology: Agents vs Profiles

• agent (what you pass to subagents.delegate): the name of an agent loaded from your registry directory. The name is the file basename, e.g., agents/review.md → agent review.
• profile (Codex CLI): an execution profile you define in ~/.codex/config.toml under [profiles.<name>]. Agents typically specify which profile to use via frontmatter (profile: <name>), but agent names and profile names don’t have to match.

There are no hardcoded built‑in agent names. The server loads agents from disk (agents/*.md|*.json) or accepts an ad‑hoc agent when both persona and profile are provided inline.

Tool: delegate

• Parameters:
  • agent: string — the agent name from your registry (basename of agents/<name>.md|json)
  • task: string (required)
  • cwd?: string (defaults to current working directory)
  • mirror_repo?: boolean (default false). If true and cwd provided, mirrors the repo into the temp workdir for maximal isolation.
  • profile? and persona?: optional ad-hoc definition when agent is not found in registry. Provide both.
• Behavior:
  1. Creates a temp workdir; writes AGENTS.md with the agent persona.
  2. Optionally mirrors the repo into the temp dir via cp -R fast path.
     • Safer alternative (recommended for large repos): git worktree add <tempdir> <branch-or-HEAD> (documented in docs/INTEGRATION.md).
  3. Spawns codex exec --profile <agent-profile> "<task>" with cwd set to the temp dir if mirrored, else your provided cwd.
  4. Returns JSON: { ok, code, stdout, stderr, working_dir }.

Custom agents

Add agents without code changes:

1. File-based registry (recommended)

• Create an agents directory and point the server to it via either:

  • Config args: add "--agents-dir", "/path/to/agents" in ~/.codex/config.toml under the MCP server args
  • Env var: CODEX_SUBAGENTS_DIR=/path/to/agents
  • Defaults (auto-detected): ./agents or ./.codex-subagents/agents

• Define agents as files using the basename as the agent name:

Example agents/perf.md:

---
profile: debugger
approval_policy: on-request   # one of: never | on-request | on-failure | untrusted
sandbox_mode: workspace-write # one of: read-only | workspace-write | danger-full-access
---
You are a pragmatic performance analyst. Identify hotspots, measure, propose minimal fixes with benchmarks.

Or JSON agents/migrations.json:

{
  "profile": "debugger",
  "approval_policy": "on-request",
  "sandbox_mode": "workspace-write",
  "persona": "You plan and validate safe DB migrations with rollbacks.",
  "personaFile": null
}

2. Ad-hoc agent via tool params

Call delegate with a new name and supply both profile and persona:

subagents.delegate(
  agent="perf",
  task="Analyze render jank",
  profile="debugger",
  approval_policy="on-request",
  sandbox_mode="workspace-write",
  persona="You are a perf specialist..."
)

List available agents:

tools.call name=list_agents

Validation: approval_policy and sandbox_mode are validated against the allowed values above. They are advisory metadata and should match the Codex profile you run under. Enforce actual behavior via profiles in ~/.codex/config.toml.

Validate agent files:

tools.call name=validate_agents
# or
tools.call name=validate_agents arguments={"dir":"/abs/path/to/agents"}

Returns per-file errors/warnings and a summary. Invalid values are flagged; missing profile in Markdown is a warning (loader defaults to default).

Build, Lint, Test

npm install
npm run build
npm run lint
npm test

Note: Do not edit dist/ manually; it is build output.

E2E Demo

Automated end-to-end check using the real Codex CLI:

npm run e2e

It will:

1. Build the project.
2. Write a temporary ~/.codex/config.toml pointing to the built server.
3. Run /mcp to verify the server is connected.
4. Pick the first agent on disk and call subagents.delegate.

The script requires OPENAI_API_KEY and a working Codex CLI binary.

Safety & Operations

Troubleshooting MCP timeouts

Warning: Stdout must be newline‑delimited JSON only. Any logs on stdout will break the MCP handshake. Use DEBUG_MCP=1 to emit diagnostics to stderr.

• “codex not found”: Install Codex CLI and ensure it is on PATH. Re-run npm run e2e.
• Timeout on startup: confirm the config points at the absolute dist/codex-subagents.mcp.js path and passes --agents-dir.
• Logs on stdout break the handshake. Set DEBUG_MCP=1 to log timing to stderr only.
• The server speaks newline-delimited JSON; older configs expecting HTTP headers will stall.
• Large repos: prefer git worktree over mirror_repo=true (see docs/INTEGRATION.md).
• Slow start: agent files are loaded lazily after initialization.

Agent Recipes (starter personas)

These come as file‑based agents under agents/. Each link includes a one‑line goal and suggested metadata (frontmatter).

• agents/review.md: Code review and refactor checklist. Frontmatter: profile: review (suggested: approval_policy: on-request, sandbox_mode: read-only).
• agents/debugger.md: Reproduce failures and propose minimal fixes. Frontmatter: profile: debugger (suggested: approval_policy: on-request, sandbox_mode: workspace-write).
• agents/security.md: Threat modeling and concrete mitigations. Frontmatter: profile: security (suggested: approval_policy: never, sandbox_mode: workspace-write).
• agents/perf.md: Identify hotspots; measure and optimize. Frontmatter: profile: default (suggested: approval_policy: on-request, sandbox_mode: workspace-write).
• agents/docs.md: Improve and restructure docs. Frontmatter: profile: default (suggested: approval_policy: on-request, sandbox_mode: read-only).
• agents/a11y.md: Accessibility reviews and fixes. Frontmatter: profile: default (suggested: approval_policy: on-request, sandbox_mode: read-only).

Invite PRs that add new agents—see “Contribute an agent”.

Contribute an agent (fast path)

1. Fork and create agents/<name>.md with frontmatter:

   profile: debugger
   approval_policy: on-request
   sandbox_mode: workspace-write
   

   Then describe the persona in free text.

2. Run tools.call name=validate_agents.

3. Open a PR using the “Agent request / contribution” template.

We track requested personas in GitHub Discussions (open a new topic under Show & Tell if none exists yet).

Comparisons

• vs. building your own MCP server: this repo keeps a single tool and file‑based agents to minimize attack surface.
• vs. complex orchestrators: this intentionally avoids graphs/routers—Codex CLI profiles + file personas keep it simple.

If this project helps, a star helps others discover it.

Docs

• docs/INTEGRATION.md: deeper wiring, profiles, AGENTS.md guidance.
• docs/SECURITY.md: isolation, trust boundaries, sandbox guidance.
• docs/OPERATIONS.md: logs, env vars, upgrades.

License

MIT