moonbridge 0.12.1

MCP server for spawning AI coding agents (Kimi, Codex, and more)

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

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.

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)
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 the configured CLI is installed and authenticated
list_adapters	Show available adapters and their status

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 (default: kimi)
model	string	No	Model override (e.g., gpt-5.2-codex). For codex, default is gpt-5.3-codex.
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)
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

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_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

1. 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

2. 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.

3. 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.

4. Process Isolation

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 arbitrary host files. For strong isolation, use containers/VMs.

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

"auth_error" responses

Authenticate with your chosen CLI:

# Kimi
kimi login

# Codex
export OPENAI_API_KEY=sk-...

Timeout errors

Adapters have sensible defaults: Codex=1800s (30min), Kimi=600s (10min).

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

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.