mcpstead

MCP Gateway

One downstream /mcp endpoint that fronts many upstream MCP servers. Persistent upstream connections with reconnect, tool registry with qualified names, JSON or SSE responses based on client Accept, per-upstream auth, Prometheus metrics.

Install

# npm (macOS, Linux, WSL)
npm i -g @ahkohd/mcpstead

# homebrew (macOS, Linux)
brew install ahkohd/tap/mcpstead

# cargo
cargo install mcpstead --locked --force

# verify
mcpstead --version

Quick start

# 1. write a config
mkdir -p ~/.config/mcpstead
cat > ~/.config/mcpstead/config.yaml <<'EOF'
host: 0.0.0.0
port: 8766

mcp:
  auth:
    mode: none

servers:
  - name: example
    url: http://127.0.0.1:3000/mcp
    protocol: streamable
    auth: none
EOF

# 2. run
mcpstead --config ~/.config/mcpstead/config.yaml

Then point any MCP client at http://127.0.0.1:8766/mcp.

Docker

docker build -t mcpstead .
docker run --rm \
  -p 8766:8766 \
  -v "$PWD/config:/etc/mcpstead:ro" \
  mcpstead

The Dockerfile installs from crates.io.

HTTP API

| Method | Path | Purpose | |---|---|---| | POST | /mcp | MCP over HTTP JSON-RPC | | GET | /mcp | returns 405 | | DELETE | /mcp | terminate downstream session | | GET | /health | upstream status, tool counts, last-seen, reconnects | | GET | /metrics | Prometheus text format | | POST | /-/reload | reload config without restart |

MCP client setup

Local, no-auth:

mcpstead:
  url: http://127.0.0.1:8766/mcp
  tools:
    resources: false
    prompts: false

Bearer auth:

mcpstead:
  url: http://127.0.0.1:8766/mcp
  headers:
    Authorization: Bearer ${MCPSTEAD_BEARER_TOKEN}
  tools:
    resources: false
    prompts: false

Tools appear with qualified names - <server>__<tool> - so multiple upstreams can ship overlapping tool names without collision.

Auth

Downstream (clients to mcpstead)

Default is no auth:

mcp:
  auth:
    mode: none

Bearer auth:

mcp:
  auth:
    mode: bearer

Token comes from env:

export MCPSTEAD_BEARER_TOKEN='replace-with-strong-secret'
mcpstead --config ~/.config/mcpstead/config.yaml

Clients send Authorization: Bearer <token>. Missing or wrong token returns 401. mcp.auth.bearer_token in config is rejected at startup.

/health and /metrics stay open regardless of auth mode - for monitoring without exposing the token.

Upstream (mcpstead to MCP servers)

Per-server in the servers list. Three modes:

servers:
  - name: local
    url: http://127.0.0.1:3000/mcp
    auth: none

  - name: workflow
    url: https://workflow.example/mcp-server/http
    auth:
      type: bearer
      token_env: WORKFLOW_TOKEN

  - name: custom
    url: https://api.example/mcp
    headers:
      X-API-Key: '${EXAMPLE_KEY}'

token_env resolves the named env var at startup and on config reload.

Config

Set the config path with --config <path> or the MCPSTEAD_CONFIG env var.

host: 0.0.0.0
port: 8766

mcp:
  auth:
    mode: none           # none | bearer
  session:
    idle_ttl_seconds: 3600
    gc_interval_seconds: 60
    shutdown_grace_seconds: 5

servers:
  - name: local
    url: http://127.0.0.1:3000/mcp
    protocol: streamable # streamable | sse | auto
    required: false      # if true, gateway won't start without this upstream
    auth: none
    reconnect:
      max_attempts: 0    # 0 = infinite
      backoff_base_ms: 1000
      backoff_max_ms: 30000
    tools:
      ttl_seconds: 300
    tls_skip_verify: false
    quirks:
      normalize_sse_events: true
      inject_accept_header: 'application/json, text/event-stream'

metrics:
  enabled: true

logging:
  level: info

Hot reload

mcpstead reloads its config without restart on:

• SIGHUP (systemctl reload mcpstead or kill -HUP <pid>)
• POST /-/reload (gated by bearer auth in bearer mode)

Hot-reloadable:

• upstream list
• per-upstream auth, headers, quirks, reconnect, tools, URL, protocol, and TLS settings
• mcp.auth.mode and MCPSTEAD_BEARER_TOKEN
• metrics.enabled

Restart required:

• host
• port
• logging.level
• mcp.session.*

Reload is best-effort. Bad config is rejected and ignored; the running config stays in place. Check logs and mcpstead_config_reloads_total{result="error"} for failures.

MCP session config keys

• mcp.session.idle_ttl_seconds - evict inactive sessions after this many seconds (default 3600)
• mcp.session.gc_interval_seconds - idle-session GC wake interval (default 60)
• mcp.session.shutdown_grace_seconds - max shutdown accounting sweep time (default 5)

Per-upstream config keys

• name - required, used as tool prefix
• url - required, MCP endpoint
• protocol - streamable | sse | auto (default auto)
• required - block startup if upstream fails to initialize (default false)
• auth - none, bearer (with token_env), or headers map
• reconnect.max_attempts - 0 = infinite (default)
• reconnect.backoff_base_ms / backoff_max_ms - exponential backoff bounds
• tools.ttl_seconds - refresh cached tools/list after this interval
• tls_skip_verify - disable TLS cert checks for this upstream (default false, use only for trusted local nets)
• quirks.normalize_sse_events - strip event: lines from upstream SSE responses
• quirks.inject_accept_header - override the Accept header sent upstream

Observability

Metrics

/metrics exposes Prometheus-format counters, gauges, and histograms. Label cardinality assumes a small bounded set of upstreams and tools; tool-call series are keyed by (server, tool).

mcpstead_build_info{version="...",rust_version="...",git_sha="..."}
mcpstead_start_time_seconds
mcpstead_uptime_seconds
mcpstead_process_resident_memory_bytes
mcpstead_process_virtual_memory_bytes
mcpstead_process_cpu_seconds_total
mcpstead_process_open_fds
mcpstead_process_max_fds
mcpstead_process_threads
mcpstead_upstream_connected{server="..."}
mcpstead_upstream_tools_count{server="..."}
mcpstead_upstream_reconnects_total{server="..."}
mcpstead_upstream_last_seen_seconds{server="..."}
mcpstead_upstream_initialize_total{server="...",result="success|error"}
mcpstead_upstream_initialize_duration_seconds_bucket{server="...",le="..."}
mcpstead_upstream_health_checks_total{server="...",result="success|failure"}
mcpstead_upstream_reconnect_attempts_total{server="...",result="success|error"}
mcpstead_upstream_backoff_seconds_total{server="..."}
mcpstead_upstream_in_backoff{server="..."}
mcpstead_upstream_current_backoff_seconds{server="..."}
mcpstead_upstream_session_resets_total{server="...",reason="unknown_session|expired|terminated"}
mcpstead_upstream_tools_refresh_total{server="...",result="success|error"}
mcpstead_upstream_tools_refresh_duration_seconds_bucket{server="...",le="..."}
mcpstead_upstream_tools_last_refresh_timestamp_seconds{server="..."}
mcpstead_upstream_bytes_total{server="...",direction="sent|received"}
mcpstead_downstream_sessions_active
mcpstead_downstream_sessions_total
mcpstead_downstream_session_duration_seconds_bucket{le="..."}
mcpstead_downstream_session_terminations_total{reason="..."}
mcpstead_mcp_requests_total{method="...",result="success|error"}
mcpstead_mcp_request_duration_seconds_bucket{method="...",le="..."}
mcpstead_mcp_auth_attempts_total{result="success|failure"}
mcpstead_mcp_auth_failures_total{reason="..."}
mcpstead_config_reloads_total{result="success|error"}
mcpstead_config_last_reload_timestamp_seconds
mcpstead_tool_calls_total{server="...",tool="..."}
mcpstead_tool_call_errors_total{server="...",tool="...",reason="..."}
mcpstead_tool_call_duration_seconds_bucket{server="...",tool="...",le="..."}

Scrape config

- job_name: mcpstead
  metrics_path: /metrics
  static_configs:
    - targets: ['mcpstead:8766']

Health check

curl http://127.0.0.1:8766/health

Returns JSON: per-upstream connection state, tool counts, last successful contact, reconnect counts, last error.

Troubleshooting

• All tools list empty - at least one upstream failed to initialize. Check /health for per-server status; check upstream URL, auth, and reachability.
• Sporadic SSE parse failed - upstream sends an SSE dialect mcpstead doesn't recognize. Try quirks.normalize_sse_events: true for that server, or set quirks.inject_accept_header: 'application/json' to force JSON.
• tools/call returns auth error - upstream rejected the bearer token. Confirm token_env resolves to the right value at startup; check upstream's expected header name.
• TLS handshake errors against a self-signed upstream - set tls_skip_verify: true on that server. Only safe on trusted local networks.
• 401 from /mcp with mode bearer - client missing or sending wrong Authorization: Bearer <token>. Verify MCPSTEAD_BEARER_TOKEN matches what the client sends.
• Upstream goes red in /health repeatedly - check mcpstead_upstream_reconnects_total and mcpstead_upstream_last_seen_seconds. Tune reconnect.backoff_max_ms if the upstream needs longer recovery.