SnapStack Server

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<p align="center"> <img src="assets/demo.gif" alt="SnapStack demo — capture a browser tab, your AI reads the screenshots over MCP" width="900"> </p>

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 ─►  ~/.snapstack/   (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

Run it once in the foreground:

npx -y snapstack-server@latest        # → SnapStack server listening on http://127.0.0.1:4123

For start-at-login + crash-restart + self-update, install the auto-start unit (launchd on macOS, systemd --user on Linux, a logon scheduled task on Windows):

npx -y snapstack-server@latest install     # register auto-start; uninstall with `… uninstall`

The unit runs a best-effort npm install --prefix <appDir> snapstack-server@latest then launches the locally installed copy — so the server self-updates on each (re)start, and still starts offline once installed. No git involved.

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. To run a tool without a per-call confirmation, add its identifier to your client's allow-list (for Claude Code: mcp__snapstack__<tool> in permissions.allow).

Token cost: get_screenshots returns only the manifest, so it stays cheap whatever the stack size — the client then reads just the files it needs. WebP + downscaling keep those reads light.

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" (in the extension): start the server (npm start) or check the auto-start. Test: curl http://127.0.0.1:4123/health.
• Port already in use (EADDRINUSE): set SNAPSTACK_PORT to another value.
• 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).

License

MIT — see LICENSE.