[md]CLI client (mcp-client)

modules/example-client · Stdio + HTTP · JVM

A tiny interactive REPL over any MCP server — handy for sanity-checking a server you're writing, and a worked example of using StdioMcpClient and StreamableHttpMcpClient together behind a single decline CLI.

• Subcommands: mcp-client stdio <command> [args…] spawns a subprocess server; mcp-client http <url> [-H 'k: v']… connects to a streamable HTTP server.
• REPL keys: t lists tools, r lists resources, p lists prompts, q quits. Each lists the entries with numbers, lets you pick one, and invokes it (tool calls prompt for JSON arguments; prompts ask for each defined argument).
• Capabilities banner: prints the server's name, version, protocol version, and which capabilities it advertised during initialize.

Build and run

# Stdio: spawn a subprocess MCP server
sbt 'exampleClient/run stdio sbt exampleDiceJVM/run'

# HTTP: connect to a networked MCP server
sbt 'exampleClient/run http http://localhost:25000/mcp'

# HTTP with a bearer token
sbt 'exampleClient/run http https://my-server/mcp -H "Authorization: Bearer …"'

--no-sse disables the long-poll SSE GET if you only want plain request/response over HTTP POST.

What it shows

The whole client is ~280 lines in modules/example-client/src/main/scala/net/andimiller/mcp/examples/client/McpCliClient.scala. Main shapes worth lifting into your own code:

• One CLI binary covering both transports — decline subcommands feed into a single Resource[IO, McpClient[IO]] (see stdioResource / httpResource in the source).
• StdioMcpClient.builder and StreamableHttpMcpClient.builder produce the same McpClient[IO] shape, so the REPL itself is transport-agnostic.
• client.serverCapabilities.{tools,resources,prompts,logging} — used to print a capability summary on connect.

For a runnable agent example that drives multiple servers from a Claude-style .mcp.json, see the LLM harness.