mcptee

A simple tool that proxies input from MCP client to MCP server and returns the output back to the client, while also logging both to the file line by line indicating the direction of the data with a prefixes "in:" and "out:", resulting in a file that is normally valid YAML.

Before every "in:", tool would print --- and endline that would make file multi-document YAML, so normally every combination of request/respons would be a separate document.

This could be pretty handy for testing and troubleshooting MCP clients and servers.

Installation

npm

npm install -g @strowk/mcptee

Github Releases

Download prebulit binaries from the releases page and put in your PATH

Build from source

go install github.com/strowk/mcptee@latest

Usage

mcptee <out-file> <command> [args...]

Some examples:

mcptee ./server_log.yaml mcp-k8s-go

mcptee ./server_log.yaml npx @strowk/mcp-k8s

You would probably configure it with corresponding client. For example, with Claude:

{
    "mcpServers": {
        "my_server": {
            "command": "mcptee",
            "args": ["/path/to/log_file.yaml", "npx", "my_mcp_server"]
        }
    }
}

Example

If you have for example a postgres server running in docker:

docker run --rm --name postgres-mcp-test -e POSTGRES_PASSWORD=thesecret -p 7777:5432 postgres:latest

, and you want to test it with mcp-server-postgres (run it in another terminal):

mcptee log.yaml npx @modelcontextprotocol/server-postgres postgres://postgres:thesecret@localhost:7777

Now send these one by one:

{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"testing","version":"0.0.1"}}}

{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}

{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"query", "arguments": {"sql": "SELECT datname FROM pg_database"} }}

You should see the output in the log.yaml file like this:

---
in: {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"testing","version":"0.0.1"}}}
out: {"result":{"protocolVersion":"2024-11-05","capabilities":{"resources":{},"tools":{}},"serverInfo":{"name":"example-servers/postgres","version":"0.1.0"}},"jsonrpc":"2.0","id":0}
---
in: {"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}
out: {"result":{"tools":[{"name":"query","description":"Run a read-only SQL query","inputSchema":{"type":"object","properties":{"sql":{"type":"string"}}}}]},"jsonrpc":"2.0","id":1}
---
in: {"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"query", "arguments": {"sql": "SELECT datname FROM pg_database"} }}
out: {"result":{"content":[{"type":"text","text":"[\n  {\n    \"datname\": \"postgres\"\n  },\n  {\n    \"datname\": \"template1\"\n  },\n  {\n    \"datname\": \"template0\"\n  }\n]"}],"isError":false},"jsonrpc":"2.0","id":1}