MCP server SnapStack

The SnapStack server is a single always-on Node process: it receives browser captures from the extension, stacks them on disk, and serves them to any MCP-capable LLM client over Streamable HTTP. It listens only on 127.0.0.1 — nothing ever leaves your machine.

New here? The full install + usage guide lives in the extension README: snapstack-extension. This page is the technical reference.

Architecture

One always-on process serves both the extension (capture) and your MCP client, decoupled by a folder on disk.

[MV3 extension]  --POST /push (bytes) ┐
                                      ▼
                              [SnapStack server - 127.0.0.1:4123]   
                                 ├─ writes           →  stack on disk
                                 └─ MCP /mcp (HTTP)  ←  MCP client

• Capture — the extension encodes the shot as WebP (PNG fallback), downscales it, and POSTs it here.
• Stack — one image file (.webp/.png) plus a twin .json (url, title, timestamp, dimensions) per capture, named NN <timestamp>: a stable two-digit number (assigned in capture order, restarts at 01 when the stack empties) plus a timestamp, under ~/.snapstack/.
• Retrieval — get_screenshots returns a JSON manifest (number, absolute path, dimensions, metadata — no image bytes); the client reads only the files it needs, by path. Deletion is a separate, explicit clear_screenshots step. Retrieval never deletes.

Requirements

• Node.js ≥ 18 (tested on Node 20). No git needed at runtime.
• An MCP-capable LLM client speaking the HTTP (Streamable HTTP) or stdio transport.
• The snapstack-extension loaded in your browser.

Install & run

On Windows, use an Administrator terminal, otherwise the global npm install and the scheduled-task registration may get rejected.

The server ships on npm and installation is straightforward on macOS, Linux and Windows:

1. Install globally: npm i -g snapstack-server
2. Enable background service: snapstack enable

SnapStack auto-starts on login, restarts on crash, and updates itself on each launch.
To check its status or if an update is available, simply run snapstack in your terminal.

Available commands:

snapstack                            # status report: service + server health, update check
snapstack start | stop | restart     # control the running service (this session)
snapstack update                     # update the CLI (npm i -g) + restart the server on the latest
snapstack run                        # run the daemon in the foreground (no auto-start)

The daemon self-updates on each (re)start/login; the global CLI (the snapstack command) does not. Run snapstack update to bring both to the latest in one go.

The full end-to-end walkthrough (idiomatic install paths, MCP client registration, the extension) is in the extension README.

MCP

SnapStack speaks two MCP transports over the same on-disk stack — pick whichever your client supports:

// HTTP (server already running) — register http://127.0.0.1:4123/mcp; copy deploy/mcp.json
{ "type": "http", "url": "http://127.0.0.1:4123/mcp" }

// stdio (the client spawns the process)
{ "command": "npx", "args": ["-y", "-p", "snapstack-server", "snapstack", "mcp"] }

The HTTP /mcp endpoint is stateless (a fresh server + transport per request); the stdio front-end (snapstack mcp) is spawned on demand and reads the same ~/.snapstack stack.
Capture intake (/push) always stays in the running server, independent of either MCP front-end.

Exposed tools

Tool	Description
get_screenshots	Lists pending captures as a JSON manifest (stable number, absolute path, dimensions, metadata) — no image bytes, no deletion. Pass numbers (e.g. [1,3]) to list only those.
clear_screenshots	Deletes captures. Pass numbers to delete specific ones; omit to clear the whole stack. Numbering restarts at 01 once empty.
count_screenshots	Number of pending captures, without retrieving them.

get_screenshots and count_screenshots are read-only; only clear_screenshots is destructive.

Configuration

Environment variables (infrastructure)

Variable	Default	Purpose
SNAPSTACK_DIR	~/.snapstack	Stack folder.
SNAPSTACK_PORT	4123	Listening port (always on 127.0.0.1).

Capture policy (shared across your browsers)

The encoding/capture settings are owned by the server and stored in ~/.snapstack/config.json, so a single edit applies to every browser running the extension. They are edited from the extension's options page — not an environment variable — and fetched by the extension before each capture.

Key	Default	Meaning
format	webp	Image format: webp, png or jpg.
quality	0.85	Lossy quality (0–1; the extension UI shows it as a percentage).
maxWidth	1568	Downscale captures wider than this to this width in px (0 = no resize).
maxSlices	50	Full-page capture: hard cap on stitched slices.

Two endpoints back it: GET /config returns the effective policy; POST /config validates and replaces it (host- + CORS-guarded like every capture route). The file is a non-image, so a stack clear never touches it; deleting it just restores the defaults above.

Troubleshooting

• Capture server not started message in the extension: run snapstack start (or snapstack run in the foreground), or check the auto-start with snapstack. Test: curl http://127.0.0.1:4123/health.
• Port already in use (EADDRINUSE): set SNAPSTACK_PORT to another value.
• snapstack: command not found after switching Node version (nvm, fnm, volta, Laravel Herd, nvm-windows): npm i -g drops the snapstack command in the global bin of the Node version that was active at install time only, so switching versions hides it. This is how npm globals work, not a SnapStack bug — the background service is unaffected and keeps capturing; only the CLI command disappears. Fix: re-run npm i -g snapstack-server under the current Node version (or switch back to the one used at install).
• The client doesn't see the tools: the server must run before the MCP client starts; check the config (type: "http", correct URL). Direct test: curl http://127.0.0.1:4123/count.
• Inspect the stack: ls ~/.snapstack (image files + human-readable .json).

Support

• A question or an idea? → GitHub Discussions
• Found a bug? → open an issue

License

MIT — see LICENSE.