Content
# mcp-pool
**MCP Key Pool Proxy** — pool multiple API keys across upstream MCP servers to distribute rate limits.
## How it works
mcp-pool is itself an MCP server. You configure one or more "pools" — each pool represents one upstream MCP server type (e.g. Brave Search) with N API keys.
- **1 subprocess per pool at a time**, not per key. No warm standby processes.
- On rate-limit or transport error, the current upstream is closed and a new one spawns with the next key (~1-2s failover).
- Tools are exposed with `{poolName}__` prefix to avoid name collisions.
- Route calls are serialized via a promise-chain mutex to prevent state races.
## Config
```yaml
pools:
brave-search:
command: bunx
args: ["-y", "@brave/brave-search-mcp-server"]
keys:
- {BRAVE_API_KEY: "sk-first"}
- {BRAVE_API_KEY: "sk-second"}
rateLimitPatterns:
- "rate_limit_exceeded"
- "too many requests"
```
| Field | Description |
|---|---|
| `command` | Executable to spawn (required) |
| `args` | CLI arguments |
| `keys` | Array of environment-variable objects, one per key (at least 1); values may be literal strings or `${NAME}` references |
| `rateLimitPatterns` | Regex patterns matched against tool error text (required, at least 1 — safety gate) |
| `cooldownSeconds` | Seconds a rate-limited key is skipped; defaults to 300 |
| `cwd` | Working directory for the upstream process |
Config paths (first found wins): `--config <path>`, `./mcp-pool.yaml`, `./.mcp-pool.yaml`, `~/.config/mcp-pool/mcp-pool.yaml`.
Values support `${VAR}` expansion from environment variables. An undefined reference is a configuration error.
## Usage
```json
{
"mcpServers": {
"mcp-pool": {
"command": "bunx",
"args": ["mcp-pool@0.1.4", "--config", "/path/to/mcp-pool.yaml"]
}
}
}
```
Tool names are prefixed: `brave-search__brave_web_search`. The client calls the prefixed name; mcp-pool strips the prefix and routes to the correct upstream.
Pass `--verbose` for per-request trace logs.
## Logging
All logs are JSON lines on stderr (stdout is reserved for MCP protocol):
```json
{"ts":"...","level":"info","pool":"brave-search","event":"upstream_start","keyIndex":0}
{"ts":"...","level":"warn","pool":"brave-search","event":"rate_limited","upstream":0}
{"ts":"...","level":"error","pool":"brave-search","event":"upstream_error","upstream":0,"error":"..."}
```
## Design
- **Lazy failover**: 1 process per pool. Rate-limited/crashed keys are closed and replaced with the next key on demand; rate-limited keys are skipped for `cooldownSeconds` (300 seconds by default).
- **Serialized calls**: promise-chain mutex prevents races on shared state across `await` points.
- **Tool validation**: each spawned upstream's tools are validated against the cached set (by name→schema map, order-independent).
- **`rateLimitPatterns` is required**: without explicit patterns, all `isError` results are terminal — no retries. Use `[".*"]` for blanket retry on read-only pools.
- **Stdio upstreams only**: upstream MCP servers are spawned as subprocesses.
## Development
```bash
bun install --frozen-lockfile
bun run typecheck
bun run test
bun run smoke
bun run build
bun run check:package
```
