mcp-test-driver

Interactive REPL for testing any MCP server, over stdio or HTTP. No AI wiring, just debugging.

Point it at a command (stdio) or a URL (HTTP) and get a prompt with tab-completion for tools, argument keys, and enum values.

Usage

# Stdio transport — pass the server command and its arguments:
mcp-test-driver aifr mcp
mcp-test-driver character agent mcp
mcp-test-driver nats-lsp mcp-stdio

# HTTP transport — pass a URL:
mcp-test-driver https://unicode.mcp.pennock.tech/mcp

# Enable roots capability (exposes cwd to the server):
mcp-test-driver --roots aifr mcp
mcp-test-driver --roots=/path/to/project https://example.com/mcp

At the prompt, type a tool name and its arguments:

mcp> unicode_search query=snowman
mcp> unicode_lookup_char char=✓

CLI Options

Option	Description
-h, --help	Show help message
--trace	Enable protocol tracing (default)
--no-trace	Disable protocol tracing at startup
--roots	Advertise cwd as filesystem root to the server
--roots=<path>	Advertise a specific path as filesystem root

Built-in Commands

Built-in commands use the / prefix. Tool names are stripped of leading / during sanitization, for namespace separation.

Tools

Command	Alias	Description
/list	/l	List available tools
/describe <tool>	/d	Show full schema for a tool

Resources

Command	Alias	Description
/resources	/lr	List available resources
/templates	/lt	List resource templates
/read <uri>	/r	Read a resource by URI
/subscribe <uri>	/sub	Subscribe to resource updates
/unsubscribe <uri>	/unsub	Unsubscribe from resource updates

Prompts

Command	Alias	Description
/prompts	/lp	List available prompts
/prompt <name> [args]	/p	Get a prompt (with optional key=val arguments)

Session & Diagnostics

Command	Alias	Description
/ping		Ping the server
/loglevel <level>	/ll	Set server log level
/roots		Show roots status
/roots on [path]		Enable roots capability (takes effect on /reconnect)
/roots off		Disable roots capability
/reconnect	/rc	Reconnect to the server
/cache-flush	/cf	Clear cached tools/resources/prompts, re-fetch
/trace	/t	Toggle JSON-RPC protocol tracing
/help	/h	Show help (or /help <tool> for tool help)
/quit	/q	Exit

Press Tab to complete commands, tool names, argument keys (key=), and enum values. Press F1 or Esc-H for context-sensitive help, at any point in a command.

Client Capabilities

Roots

The --roots flag (or /roots on command) advertises the roots capability to the server, telling it which filesystem directories are relevant.

When enabled, the client responds to roots/list requests from the server with the configured base path. This is read-only: it hints to the server where to look, but the server accesses files using its own mechanisms.

Security constraints:

• All root paths are resolved to absolute canonical paths (symlinks followed)
• Additional roots added via RootsHandler.add_root() must be descendants of the base path (Path.relative_to() check prevents traversal)
• The base path must exist at the time roots are enabled

Sampling

The MCP sampling capability (server asks the client to generate LLM completions) is not implemented. This is currently considered out of scope.

Protocol Coverage

mcp-test-driver implements the following MCP protocol methods:

Method	Direction	Notes
initialize	client → server	Advertises capabilities from handler registry
notifications/initialized	client → server
tools/list	client → server	Paginated
tools/call	client → server
resources/list	client → server	Paginated; fetched if server advertises resources
resources/templates/list	client → server	Paginated
resources/read	client → server
resources/subscribe	client → server
resources/unsubscribe	client → server
prompts/list	client → server	Paginated; fetched if server advertises prompts
prompts/get	client → server
ping	both directions	Client handles server-initiated pings
logging/setLevel	client → server
completion/complete	client → server
roots/list	server → client	When --roots is enabled

Known Limitations

• HTTP server-to-client requests: Server-initiated requests (like roots/list) over the HTTP Streamable transport require a long-lived GET SSE stream, which is not yet implemented. Server requests are fully supported on the stdio transport. The --roots flag still works over HTTP for the initial handshake capabilities, but the server cannot dynamically query roots mid-session.

• Partial-line stdio timeout: The select()-based read timeout (120s) catches complete server silence, but if a server sends bytes without a newline, readline() may still block.

• No OAuth/authentication: The MCP spec defines OAuth 2.1 flows, but as a test driver we don't implement auth. Use Authorization headers or environment-based credentials if needed.

• No DNS discovery: SRV/TXT/DANE-based MCP server discovery (SEP-1959) is proposed but not standardized.

• No .well-known metadata: Neither OAuth resource metadata (RFC 9728) nor MCP server cards (SEP-1649/1960) are fetched.

Installation

# Install as a tool (recommended for CLI use):
uv tool install .

# Or install into a project:
uv pip install .

# Or run directly from the source tree:
uv run mcp-test-driver <server-command...>

Development

uv sync --group dev
uv run ruff check src/
uv run ty check
uv run pytest tests/

AI Disclosure

A LLM development tool was used extensively in the creation of this tool.

(It's a tool for exploring services designed for LLMs to use, what did you expect?)

License

ISC — see LICENSE.