shiro-neko/docs/mcp.md

MCP

Model Context Protocol servers contribute tools to the agent. Two transports: a local command over stdio, and a remote http or sse endpoint.

Adding one from the prompt

/mcp              list what is configured, with the tool count (or "lazy") each contributed
/mcp add          wizard: local or remote, then the fields that kind needs
/mcp remove <name>

/mcp add asks for the kind first, because the two need different fields — a command and its arguments against a URL and its headers — and a single form with half of it inapplicable is worse than two short ones.

Add an MCP server
none configured yet
> local    a command on this machine, over stdio
  remote   an http or sse endpoint

The name is validated as it is typed. Tools register as mcp__<server>__<tool>, so a name with a space or a double underscore produces a tool the model cannot address and two servers whose namespaces can collide — both are refused in place rather than at connect time. A name already in the config is refused too.

For a local server the wizard then asks for the command and its arguments; arguments split on spaces and keep quoted runs together, so --root "/home/my folder" arrives as one argument. For a remote one it asks for the URL — http or https only — and optional headers as KEY: value, OTHER: value.

Both write straight to config.json and merge with whatever is already there. A new server connects on the next start, not mid-session: connecting during a turn would change the tool list under a request that is already running.

/mcp shows the state of each configured server, which is what makes a typo visible:

mcp servers
/mcp add to add one

- `filesystem` (local) - connected (lazy)
  npx -y @modelcontextprotocol/server-filesystem .
- `api` (remote) - failed: fetch failed
  https://example.com/mcp

configured in /home/you/.shiro-neko/config.json

Under eager the same list shows a tool count instead of (lazy), because every server's tools are registered up front.

The config file

The wizard writes this; it is equally editable by hand.

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "."]
    },
    "api": {
      "url": "https://example.com/mcp",
      "headers": { "Authorization": "Bearer sk-..." }
    }
  }
}

Field	Kind	Meaning
command	local	the executable to spawn
args	local	its arguments
env	local	extra environment variables
cwd	local	working directory
url	remote	the MCP endpoint
type	remote	http (default) or sse
headers	remote	sent with every request, for auth

--no-mcp skips every server for one run, which is the first thing to try when the agent is behaving oddly and a server is in play.

Model Context Protocol servers contribute tools. Configure them in ~/.shiro-neko/config.json and they appear alongside the builtins.

Configuration

Everything the wizard writes is equally editable by hand, and a hand-written entry that /mcp add would have rejected still connects — the validation is on the input path, not a schema check at load.

{
  "mcpMode": "eager",
  "mcpServers": {
    "fs": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "."]
    },
    "db": {
      "command": "python",
      "args": ["-m", "my_mcp_server"],
      "env": { "DATABASE_URL": "postgres://localhost/dev" },
      "cwd": "/home/you/tools"
    },
    "api": {
      "url": "http://localhost:3000/mcp",
      "type": "http",
      "headers": { "Authorization": "Bearer local-dev-token" }
    }
  }
}

stdio servers take command, and optionally args, env, cwd. The process is spawned at startup and closed on exit. env is merged over the inherited environment, so a server inherits your PATH unless you replace it.

Remote servers take url, and optionally type (http or sse, default http) and headers.

A sibling key, "mcpMode", picks how the configured servers' tools reach the model: lazy (the default) or eager. It is hand-edited — the /mcp add wizard does not set it — and applies to every server, so it lives beside mcpServers, not inside one.

A token in headers sits in config.json in plain text, same as apiKey. For anything beyond a local dev token, prefer a stdio server that reads its own credential from the environment.

Startup cost

Servers connect in parallel, so the slowest one sets how long startup takes rather than the sum of them. npx -y some-server re-resolves the package on each launch; installing it and calling the binary directly is usually the difference between a noticeable wait and none.

--no-mcp skips them all, which is also the quickest way to tell whether a slow start is MCP or something else.

How a server's tools reach the model

Two modes, switched with "mcpMode" in config. lazy is the default; eager is the opt-in.

• Lazy registers three meta-tools — mcp_list, mcp_inspect, mcp_call — instead of one schema per server tool. The server's real tools are fetched only when mcp_call actually invokes one, so a server exposing twenty tools costs almost nothing until one is used. This is why the affordability paragraph in the README says a configured server no longer taxes every request.
• Eager registers every server tool up front as in the old 1.0 behaviour. If a server exposes only two tools, eager is cheaper because there is no list-then-inspect round trip.

The model is told the connected server names through the meta-tool descriptions and a prompt line, then discovers each tool's schema on demand:

- mcp_list, mcp_inspect, mcp_call: MCP tools are fetched on demand. mcp_list names a
  server's tools, mcp_inspect reads one tool's schema, mcp_call runs it. Never guess a
  server or tool name: list first.

A mcp_call still routes through the same permission rules and guard as a built-in, so the lazy path is not a way around approval.

Naming

In lazy mode (the default) tools are addressed as mcp_call(server, toolName, args); the server names are zero-ambiguity identifiers you list first. In eager mode tools arrive as mcp__<server>__<tool>: a server named fs exposing read_file becomes mcp__fs__read_file.

The namespace is not cosmetic. Two servers both exposing search would otherwise silently shadow each other, and the model would call one believing it was the other.

Approval

Every MCP tool requires approval on every call. They are third-party code with unknown side effects, so they are treated like bash rather than like read_file. a whitelists one tool for the session.

--yolo skips these prompts, as it does for the builtins. Plugin guards still apply.

Failure handling

A server that fails to start is reported and the session continues:

shiro-neko 0.1.0-beta.5  openai/gpt-5  session 0193ab2c
mcp: 4 tools
mcp db failed: spawn python ENOENT

Nothing else is lost — the other servers still load, the builtins still work. A missing Python interpreter should not stop you from editing a file.

--no-mcp skips them all.

Inspecting

/tools lists everything offered this turn. In eager mode that includes each MCP tool, named mcp__<server>__<tool>, described with whatever the server sent. In lazy mode the three meta-tools appear and the server's real tools are surfaced by mcp_list inside the session.

Into mcp_inspect or mcp_list goes the server name, not an mcp__ path, so the prompt tells the model which servers are connected and to list first before guessing a tool name.

Cost

In lazy mode (the default) a configured server contributes three small meta-tool schemas to every request, not one schema per tool. A server exposing twenty tools therefore costs a few hundred tokens per turn rather than roughly 2,750, and it stays cheap whether the model uses the tools or not. Browsing a server's tools and reading a schema still brings that server's schema into view one tool at a time, but only when the model asks for it.

In eager mode each tool adds its name, description, and JSON schema to every request, and that cost is sent whether or not the model uses any of them. That is the right trade only for a server with one or two tools, which is why eager exists.

MCP tools are not covered by toolSets in either mode — that budget only governs the built-ins. There is no per-server switch beyond the global mcpMode, so choosing eager turns every server eager; a server exposing many tools you never use is worth finding a narrower one for.

/tools shows the count both ways:

tools
26 offered this turn of 26 registered

A gap between the two numbers means a tool set or a read-only agent variant is withholding something. MCP tools never appear in that gap.

Writing a server

Any MCP-compliant server works. A minimal stdio one needs three methods: initialize, tools/list, and tools/call. The test suite includes one at test/fixtures/mcp-stub.ts — about 50 lines, and useful as a starting point.

The suite runs it as a real subprocess rather than mocking the transport, because the parts that break in practice are the handshake and the framing, and a mock asserts neither.

Debugging a server

A server that starts but returns nothing useful is the harder case. In order of speed:

1. /tools — did the tools arrive at all? A server with no tools is a tools/list problem.
2. shiro -p "run mcp_call(server, \"api\", \"query\", {...}) with ..." --json --yolo — the exact tool-call input and tool-result output, one JSON object per line. In eager mode the address is mcp__<server>__<tool> instead.
3. Run the server by hand: echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | your-server. If that is wrong, nothing above it can be right.

For an HTTP server, curl -X POST $URL -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' answers the same question without shiro in the way.