sidecar-mcp

An MCP server that delegates bulk file reading to a cheap worker LLM, so the main agent's context stays small.

When the main agent needs to understand 2+ files or any file over ~100 lines, it calls bulk_read. The full file content never enters the main agent's context — only the worker's summary does. Inspired by Spotify's internal shunt plugin, but generic and backend-pluggable.

Install

sidecar-mcp is a small stdio subprocess that sits next to your coding agent. Reads happen out-of-band to a cheap worker LLM, so the main agent's context stays small. Two install paths — both wired in under a minute.

A. From npm (once the package is on the npm registry)

The three blocks below use claude mcp add because it's the shortest way to register an MCP server — but claude here is Claude Code's CLI, nothing more. sidecar-mcp itself doesn't depend on Claude. The SIDECAR_BACKEND=openai env var picks which API the worker model uses; it's independent of the client. (For non-Claude clients, see Wire into any MCP client below.)

# Ollama — local, free, no API key:
claude mcp add sidecar -e SIDECAR_BACKEND=ollama -- npx -y sidecar-mcp

# OpenAI:
claude mcp add sidecar -e SIDECAR_BACKEND=openai -e SIDECAR_OPENAI_KEY="$OPENAI_API_KEY" -- npx -y sidecar-mcp

# Anthropic (or any Anthropic-compatible provider):
claude mcp add sidecar -e SIDECAR_BACKEND=anthropic -e SIDECAR_ANTHROPIC_KEY="$ANTHROPIC_API_KEY" -- npx -y sidecar-mcp

npx -y sidecar-mcp downloads and runs the published package on first call. No clone, no build, no node_modules to manage.

B. From source (works today, no publish needed)

git clone https://github.com/dEMonaRE/sidecar-mcp.git
cd sidecar-mcp
pnpm install --frozen-lockfile
pnpm build
claude mcp add sidecar -e SIDECAR_BACKEND=ollama -- node "$PWD/dist/index.js"

Pick a backend

Verify

In your MCP client (Claude Code shown), ask: "Use bulk_read to summarize README.md." You should see bulk_read fire and return a tight summary. Every reply ends with a usage footer (tokens: <prompt> in / <completion> out):

---
sidecar: model=llama3.1:8b, backend=ollama, tokens=412 in / 87 out

If bulk_read doesn't show up:

• Claude Code: claude mcp list should show sidecar as connected.

• VS Code Copilot: Command Palette → "MCP: List Servers".

• Codex CLI: codex mcp list.

• Cursor / Zed: check the MCP panel in settings.

Other clients (VS Code Copilot, Codex CLI, Cursor, Zed, …)

sidecar-mcp speaks plain MCP stdio — the same primitive every MCP client consumes. The claude mcp add commands above are just Claude Code's CLI wrapper for the same config. See Wire into any MCP client below for the canonical shape and where each client stores it.

Related MCP server: MCP Codex Worker

Configuration

sidecar-mcp reads everything from env vars. No config file. No CLI flags (MCP stdio can't pass them).

Default models:

• ollama → llama3.1:8b

• openai → gpt-4o-mini

• anthropic → claude-3-5-haiku-latest

Per-backend quick config

Ollama (local, free, no API key)

ollama serve &
ollama pull llama3.1:8b
export SIDECAR_BACKEND=ollama

OpenAI

export SIDECAR_BACKEND=openai
export SIDECAR_OPENAI_KEY=sk-...

Anthropic (or any Anthropic-compatible provider)

export SIDECAR_BACKEND=anthropic
export SIDECAR_ANTHROPIC_KEY=sk-ant-...
# Optional — point at a proxy that speaks the Anthropic Messages API:
# export SIDECAR_ANTHROPIC_URL=https://your-anthropic-compatible-host

Wire into any MCP client

The MCP spec is the same everywhere — sidecar-mcp is a subprocess with a command and some env vars. Every MCP client wraps that primitive in its own config syntax, but the primitive itself doesn't change.

Canonical shape (the only thing you actually need to know):

{
  "command": "npx -y sidecar-mcp",
  "env": { "SIDECAR_BACKEND": "ollama" }
}

For a from-source install, swap npx -y sidecar-mcp for node /absolute/path/to/sidecar-mcp/dist/index.js.

Where each client stores it:

Example — VS Code Copilot (.vscode/settings.json):

{
  "github.copilot.chat.mcp.servers": {
    "sidecar": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "sidecar-mcp"],
      "env": { "SIDECAR_BACKEND": "ollama" }
    }
  }
}

Example — Codex CLI (~/.codex/config.toml):

[mcp_servers.sidecar]
command = "npx"
args = ["-y", "sidecar-mcp"]

[mcp_servers.sidecar.env]
SIDECAR_BACKEND = "ollama"

The command/args split varies by client (some take a single string, some take an array); the primitive above is what every client is configuring.

Tool reference

bulk_read

Returns: the worker's text summary. The full file content never appears in the caller's context.

Response footer: every bulk_read reply ends with a one-line footer for cost spot-checks:

---
sidecar: model=<model>, backend=<backend>, tokens=<in> in / <out> out

(or tokens: n/a if the backend didn't report usage). Footer is included automatically; no flag to disable in MVP.

Example call:

{
  "paths": ["src/Service.java", "src/Handler.java"],
  "question": "What does this service do and what are its key methods?"
}

Worker prompt shape (for debugging):

<files>
<file path="src/Service.java">…</file>
<file path="src/Handler.java">…</file>
<!-- unreadable: path/to/binary.bin — binary file -->
</files>

<question>
What does this service do?
</question>

How it works

main agent (Claude Sonnet — expensive)
   │
   │  MCP stdio JSON-RPC:
   │  {"method":"tools/call","params":{"name":"bulk_read", ...}}
   ▼
sidecar-mcp subprocess
   │
   │  reads files, wraps in XML, POSTs to:
   ▼
worker model (Ollama 8B / GPT-4o-mini / Claude Haiku — cheap)
   │
   │  returns ~600 token summary
   ▼
back to main agent as tool result

The full file content stays between sidecar-mcp and the worker. The main agent only ever sees the summary.

Limitations

• Non-streaming. Worker replies are returned as one text blob.

• No code_write. Bulk boilerplate generation is out of scope for MVP. If you need it, build a separate tool.

• Claude Code hook layer is opt-in. Auto-redirect of large Read/Bash cat|head|tail calls to bulk_read lives in extras/claude-hooks/ and is not installed by default. See extras/claude-hooks/README.md to wire it into ~/.claude/settings.json.

• SIDECAR_ALLOW_ROOTS defaults to cwd. Files outside are skipped with a warning. Set explicitly for stricter scoping.

Development

git clone https://github.com/dEMonaRE/sidecar-mcp.git
cd sidecar-mcp
pnpm install
pnpm dev          # run with tsx watch
pnpm test         # vitest
pnpm demo         # offline self-check (fake backend, prints prompt + reply)
pnpm build        # tsc → dist/
pnpm pack         # build a tarball to verify before publishing

Manual smoke test with real Ollama:

ollama serve &
ollama pull llama3.1:8b
SIDECAR_BACKEND=ollama sidecar-mcp &   # in one terminal
# Wire into your MCP client (Claude Code, Copilot, Codex) and call bulk_read.

License

MIT — see LICENSE.