baton-proxy

Transparent MCP proxy. Wraps a stdio MCP server as a subprocess, or bridges to a remote Streamable-HTTP MCP server (--url); injects an annotation tool and three intent parameters into the handshake, and emits friction events to one or more sinks (stderr, a JSONL file, or a Baton Console).

Zero changes to the underlying MCP server. The proxy is the MCP server from Claude's perspective; the real server is either its child process (stdio) or the endpoint it forwards to (--url).

┌──────────┐      ┌───────────────┐      ┌────────────────────┐
│  Claude  │ ◀──▶ │  baton-proxy  │ ◀──▶ │ your MCP server    │
└──────────┘      └───────┬───────┘      └────────────────────┘
                          │
                          │ async fan-out, pick any subset
                          ▼
   ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐
   │  stderr:        │  │  file://        │  │  Baton Console  │
   │  JSONL stream   │  │  JSONL file     │  │  (HTTPS POST)   │
   └─────────────────┘  └─────────────────┘  └─────────────────┘

The docs are at goodtiming.ai/docs.html#proxy: the full configuration reference, what gets emitted, the intent parameters and the sink ladder. This page is the short version.

Quick start

pipx install baton-proxy  # or: pip install baton-proxy

pipx installs the CLI into its own isolated venv and puts baton-proxy on your PATH, so Claude's config can invoke it directly without env activation. Plain pip install works if you already manage your own Python env. Python 3.11+, pure stdlib, no third-party runtime dependencies.

Replace your MCP server entry in Claude's config:

// Before
{ "command": "npx", "args": ["@vendor/mcp-server"] }

// After. Zero-config: events go to stderr + /tmp/baton-proxy.jsonl
{ "command": "baton-proxy", "args": ["--", "npx", "@vendor/mcp-server"] }

That's the entire install. Start a new Claude session and drive the wrapped server.

For a remote server, name it with --url instead of a command after --. The two forms are mutually exclusive, and BATON_UPSTREAM_AUTH_TOKEN is sent upstream as a bearer token:

{
  "command": "baton-proxy",
  "args": ["--url", "https://mcp.example.com/mcp"],
  "env": { "BATON_UPSTREAM_AUTH_TOKEN": "..." }
}

The token goes in the entry's env, not your shell: the MCP client starts the proxy with a fixed six-name environment allowlist, so an exported value never arrives. The upstream then rejects the call and the proxy degrades to a synthetic empty tool list: a server that looks connected and offers nothing.

Either form is started by your MCP client, not by you: the proxy speaks JSON-RPC on stdin, so running it straight from a shell just waits for input.

Where events go

BATON_EVENT_SINK takes a comma-separated list, and the URL scheme picks the sink: stderr: writes JSON Lines to stderr, file:///tmp/events.jsonl appends one JSON object per event, and https://console.example.com POSTs to {url}/v0/events. The default is stderr:,file:///tmp/baton-proxy.jsonl, so a bare install writes only to your own machine.

A misconfigured sink fails loudly at startup rather than silently dropping events. The full variable list, covering timeouts, tenant shape, the upstream token and the intent-parameter mode, is in the configuration reference.

Payload scrubbing

Always on. Nothing turns it off, not an environment variable and not a flag: the proxy constructs its scrubber unconditionally. Tool params, results and error bodies run through the same ruleset the Baton SDK ships: email, Bearer values, sk-* and AKIA* keys, JWTs, North-American-format phone numbers, Luhn-checked card numbers, plus force-redaction on sensitive field names.

It is pattern matching, not a guarantee. A name and a street address pass through untouched. Decide what your server puts in tool params and results on that basis. What it does and does not catch.

Trust properties

• Open source, Apache 2.0. Auditable end-to-end.
• Fail-open. A Console outage, a network issue or an instrumentation bug never breaks the MCP pipe. If injection or stripping raises, the message is forwarded unmodified.
• Outbound-only. The proxy never accepts inbound connections. Events go to the configured sink, either an HTTPS POST out or a local file write, and that is the only egress surface.
• Source-side scrubbing, on by default, with the limits stated above.
• Emission off the hot path. Events are enqueued onto a background thread; the I/O pump does not wait for the POST.

Trust model. baton-proxy and the wrapped MCP server run in the same trust domain (same user, vendor's own MCP server). The proxy filters BATON_* out of the upstream subprocess env as a least-privilege measure: the upstream has no need for Baton credentials, and accidental leakage paths such as debug logging or crash-report env dumps should not see them. This is not a cross-process trust boundary: do not use baton-proxy to instrument an MCP server you do not trust.

More

Apache-2.0.