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Moonbridge

Your MCP client just got a team.

Spawn AI coding agents from Claude Code, Cursor, or any MCP client. Run 10 approaches in parallel for a fraction of the cost.

uvx moonbridge

Quick Start

  1. Install at least one supported CLI:

    Adapter Install Authenticate
    Kimi (default) uv tool install --python 3.13 kimi-cli kimi login
    Codex npm install -g @openai/codex Set OPENAI_API_KEY
    OpenCode curl -fsSL https://opencode.ai/install | bash opencode auth login
    Gemini CLI npm install -g @google/gemini-cli Run gemini login flow or set GEMINI_API_KEY

  2. Add to MCP config (~/.mcp.json):

    {
  "mcpServers": {
    "moonbridge": {
      "type": "stdio",
      "command": "uvx",
      "args": ["moonbridge"]
    }
  }
}

  3. Use it. Your MCP client now has spawn_agent and spawn_agents_parallel tools.

Security Warning (Read First)

Moonbridge executes agentic CLIs (Kimi/Codex/OpenCode/Gemini). A malicious or careless prompt can cause an agent to run shell commands, read accessible files, or exfiltrate data via network calls.

Moonbridge adds guardrails (MOONBRIDGE_ALLOWED_DIRS, environment allowlists, optional MOONBRIDGE_SANDBOX=1), but these are not equivalent to OS-level containment.

For untrusted prompts or shared environments, run Moonbridge inside a container or VM with least-privilege filesystem and network access.

Updating

Moonbridge checks for updates on startup (cached for 24h). To update manually:

# If using uvx (recommended)
uvx moonbridge --refresh

# If installed as a tool
uv tool upgrade moonbridge

Disable update checks for CI/automation:

export MOONBRIDGE_SKIP_UPDATE_CHECK=1

When to Use Moonbridge

Task Why Moonbridge
Parallel exploration Run 10 approaches simultaneously, pick the best
Frontend/UI work Kimi excels at visual coding and component design
Tests and documentation Cost-effective for high-volume tasks
Refactoring Try multiple strategies in one request

Best for: Tasks that benefit from parallel execution or volume.

How it Works

Connection Flow

  1. MCP client (Claude Code, Cursor, etc.) connects to Moonbridge over stdio
  2. Client discovers available tools via list_tools
  3. Client calls spawn_agent or spawn_agents_parallel

Spawn Process

  1. Moonbridge validates the prompt and working directory
  2. Resolves which adapter to use (Kimi, Codex, OpenCode, Gemini)
  3. Adapter builds the CLI command with appropriate flags
  4. Spawns subprocess in a separate process group
  5. Captures stdout/stderr, enforces timeout
  6. Returns structured JSON result

Parallel Execution

  • spawn_agents_parallel runs up to 10 agents concurrently via asyncio.gather
  • Each agent is independent (separate process, separate output)
  • All results returned together when the last agent finishes (or times out)
MCP Client → stdio → Moonbridge → adapter → CLI subprocess
                                          → CLI subprocess (parallel)
                                          → CLI subprocess (parallel)

Tools

Tool Use case
spawn_agent Single task: "Write tests for auth.ts"
spawn_agents_parallel Go wide: 10 agents, 10 approaches, pick the best
check_status Verify an adapter CLI is installed and authenticated
list_adapters Show available adapters and their status
list_models Show known/dynamic model options for an adapter

Example: Parallel Exploration

{
  "agents": [
    {"prompt": "Refactor to React hooks"},
    {"prompt": "Refactor to Zustand"},
    {"prompt": "Refactor to Redux Toolkit"}
  ]
}

Three approaches. One request. You choose the winner.

Tool Parameters

spawn_agent

Parameter Type Required Description
prompt string Yes Task description for the agent
adapter string No Backend to use: kimi, codex, opencode, gemini (default from MOONBRIDGE_ADAPTER, fallback kimi)
model string No Model override (e.g., gpt-5.2-codex, openrouter/minimax/minimax-m2.5, gemini-2.5-pro). For opencode, models use provider/model.
thinking boolean No Enable reasoning mode (Kimi only)
reasoning_effort string No Reasoning budget: low, medium, high, xhigh (Codex only, default xhigh)
timeout_seconds integer No Override default timeout (30-3600)

spawn_agents_parallel

Parameter Type Required Description
agents array Yes List of agent configs (max 10)
agents[].prompt string Yes Task for this agent
agents[].adapter string No Backend for this agent
agents[].model string No Model override for this agent (codex default: gpt-5.3-codex; opencode uses provider/model; gemini default: gemini-2.5-pro)
agents[].thinking boolean No Enable reasoning (Kimi only)
agents[].reasoning_effort string No Reasoning budget (Codex only, default xhigh)
agents[].timeout_seconds integer No Timeout for this agent

check_status

Parameter Type Required Description
adapter string No Adapter to check explicitly. Defaults to MOONBRIDGE_ADAPTER when omitted.

list_models

Parameter Type Required Description
adapter string No Adapter to inspect. Defaults to MOONBRIDGE_ADAPTER when omitted.
provider string No Provider filter for OpenCode model catalogs (e.g., openrouter).
refresh boolean No Refresh model catalog for adapters that support dynamic discovery.

Response Format

All tools return JSON with these fields:

Field Type Description
status string success, error, timeout, auth_error, or cancelled
output string stdout from the agent
stderr string|null stderr if any
returncode int Process exit code (-1 for timeout/error)
duration_ms int Execution time in milliseconds
agent_index int Agent index (0 for single, 0-N for parallel)
message string? Human-readable error context (when applicable)
raw object? Optional structured metadata (e.g., sandbox diff)

When output is too large, Moonbridge truncates it and adds raw.output_limit metadata with original sizes.

Configuration

Environment Variables

Variable Description
MOONBRIDGE_ADAPTER Default adapter (default: kimi)
MOONBRIDGE_TIMEOUT Default timeout in seconds (30-3600)
MOONBRIDGE_KIMI_TIMEOUT Kimi-specific default timeout
MOONBRIDGE_CODEX_TIMEOUT Codex-specific default timeout
MOONBRIDGE_OPENCODE_TIMEOUT OpenCode-specific default timeout
MOONBRIDGE_GEMINI_TIMEOUT Gemini-specific default timeout
MOONBRIDGE_MODEL Global default model override
MOONBRIDGE_KIMI_MODEL Kimi-specific model override
MOONBRIDGE_CODEX_MODEL Codex-specific model override
MOONBRIDGE_OPENCODE_MODEL OpenCode-specific model override
MOONBRIDGE_GEMINI_MODEL Gemini-specific model override
MOONBRIDGE_MAX_AGENTS Maximum parallel agents
MOONBRIDGE_MAX_OUTPUT_CHARS Max chars returned per agent across stdout+stderr (default 120000; timeout tails are per stream)
MOONBRIDGE_ALLOWED_DIRS Colon-separated allowlist of working directories
MOONBRIDGE_STRICT Set to 1 to require ALLOWED_DIRS (exits if unset)
MOONBRIDGE_SANDBOX Set to 1 to run agents in a temp copy of cwd
MOONBRIDGE_SANDBOX_KEEP Set to 1 to keep sandbox dir for inspection
MOONBRIDGE_SANDBOX_MAX_DIFF Max diff size in bytes (default 500000)
MOONBRIDGE_SANDBOX_MAX_COPY Max sandbox copy size in bytes (default 500MB)
MOONBRIDGE_LOG_LEVEL Set to DEBUG for verbose logging

Security

Moonbridge inherits the security model of the selected adapter CLI. Kimi, Codex, OpenCode, and Gemini are agentic CLIs; prompts can trigger command execution, file access, and network activity within the process permissions.

1. Threat Model (Prompt Injection Included)

If an attacker can influence prompt input sent through MCP, they can attempt to make the agent:

  • read sensitive files (for example ~/.ssh or .env),
  • run destructive shell commands,
  • exfiltrate data over the network.

Moonbridge does not inspect prompt intent. Treat prompt input as potentially untrusted.

2. Directory Restrictions (MOONBRIDGE_ALLOWED_DIRS)

Default: agents can operate in any directory. Set MOONBRIDGE_ALLOWED_DIRS to restrict: colon-separated allowed paths. Symlinks resolved via os.path.realpath before checking. Strict mode (MOONBRIDGE_STRICT=1) exits on startup if no valid allowed directories are configured.

export MOONBRIDGE_ALLOWED_DIRS="/home/user/projects:/home/user/work"
export MOONBRIDGE_STRICT=1  # require restrictions

3. Environment Sanitization

Only whitelisted env vars are passed to spawned agents. Each adapter defines its own allowlist (PATH, HOME, plus adapter-specific like OPENAI_API_KEY for Codex). Your shell environment (secrets, tokens, SSH keys) is not inherited by default.

4. Input Validation

Model parameters are validated to prevent flag injection (values starting with - are rejected). Prompts are capped at 100,000 characters and cannot be empty.

5. Process Isolation and Sandbox Mode

Agents run in separate process groups (start_new_session=True). Orphan cleanup on exit. Sandbox mode available (MOONBRIDGE_SANDBOX=1) for copy-on-run isolation.

Not OS-level sandboxing. Agents can still read or write arbitrary host files they can access.

6. Hardened Deployment Checklist

  • Set MOONBRIDGE_ALLOWED_DIRS to the smallest possible set.
  • Enable MOONBRIDGE_STRICT=1 so missing restrictions fail closed.
  • Enable MOONBRIDGE_SANDBOX=1 to avoid direct workspace mutation.
  • Run Moonbridge in a container/VM for strong isolation.
  • Do not expose Moonbridge to untrusted clients without additional auth controls.

Troubleshooting

"CLI not found"

Install the CLI for your chosen adapter:

# Kimi
uv tool install --python 3.13 kimi-cli
which kimi

# Codex
npm install -g @openai/codex
which codex

# OpenCode
curl -fsSL https://opencode.ai/install | bash
which opencode

# Gemini
npm install -g @google/gemini-cli
which gemini

"auth_error" responses

Authenticate with your chosen CLI:

# Kimi
kimi login

# Codex
export OPENAI_API_KEY=sk-...

# OpenCode
opencode auth login

# Gemini
gemini  # complete login flow, or set GEMINI_API_KEY

Timeout errors

Adapters have sensible defaults: Codex=1800s, Kimi=600s, OpenCode=1200s, Gemini=1200s.

For exceptionally long tasks, override explicitly:

{"prompt": "...", "timeout_seconds": 3600}

Or set per-adapter defaults via environment:

export MOONBRIDGE_CODEX_TIMEOUT=2400  # 40 minutes
export MOONBRIDGE_KIMI_TIMEOUT=900    # 15 minutes
export MOONBRIDGE_OPENCODE_TIMEOUT=1200  # 20 minutes
export MOONBRIDGE_GEMINI_TIMEOUT=1200  # 20 minutes

Timeout Best Practices

Task Type Recommended
Quick query, status 60-180s
Simple edits 300-600s
Feature implementation 1200-1800s
Large refactor 1800-3600s

Priority resolution: explicit param > adapter env > adapter default > global env > 600s fallback

"MOONBRIDGE_ALLOWED_DIRS is not set" warning

By default, Moonbridge warns at startup if no directory restrictions are configured. This is expected for local development. For shared/production environments, set allowed directories:

export MOONBRIDGE_ALLOWED_DIRS="/path/to/project:/another/path"

Sandbox Mode (Copy-on-Run)

Enable sandbox mode to run agents in a temporary copy of the working directory:

export MOONBRIDGE_SANDBOX=1

When enabled:

  • Agents run in a temp copy of cwd.
  • Host files stay unchanged by default.
  • A unified diff + summary is included in raw.sandbox.

Optional:

export MOONBRIDGE_SANDBOX_KEEP=1       # keep temp dir
export MOONBRIDGE_SANDBOX_MAX_DIFF=200000
export MOONBRIDGE_SANDBOX_MAX_COPY=300000000

Limitations: this is not OS-level isolation. Agents can still read/write arbitrary host paths if they choose to. Use containers/VMs for strong isolation.

To enforce restrictions (exit instead of warn):

export MOONBRIDGE_STRICT=1

Permission denied on working directory

Verify the directory is in your allowlist:

export MOONBRIDGE_ALLOWED_DIRS="/path/to/project:/another/path"

Debug logging

Enable verbose logging:

export MOONBRIDGE_LOG_LEVEL=DEBUG

Platform Support

macOS and Linux only. Windows is not supported.

License

MIT. See LICENSE.

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MCP server for spawning Kimi K2.5 agents

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