# 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 ``` `/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____`, 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. ```json { "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](https://modelcontextprotocol.io) 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. ```json { "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____`: 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____`, 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____` 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.