effectfence

effectfence

MCP server that stops agents double-firing side effects. 1,000 racing duplicates, exactly one execution — proven on every commit.

npx effectfence

No Rust toolchain. No build step. No install-time downloads.

Add it to your MCP client

{
  "mcpServers": {
    "effectfence": {
      "command": "npx",
      "args": ["-y", "effectfence"]
    }
  }
}

What it does

An agent that retries a tool call after a timeout does not know whether the first attempt landed. The request reached the server, the work happened, the response never came back. Nothing failed loudly — it succeeded twice.

EffectFence sits in front of those effects. Identical intents are admitted once; every duplicate is fenced, replayed from the original result, or refused. The guarantee is enforced under real contention, not assumed:

• 1,000 racing duplicates → exactly one execution. Run cargo run --release --example storm against the source and watch it yourself.
• CI runs that storm on a fresh runner with no cache on every commit.
• Simultaneity is forced with a real barrier, not produced by spawning threads quickly and hoping the scheduler cooperates.

Platforms

Platform	Included
macOS Apple silicon (darwin-arm64)	yes
Linux x64 (linux-x64)	yes
Linux arm64 (linux-arm64)	yes
Windows x64 (win32-x64)	yes
macOS Intel (darwin-x64)	no — see below

Every binary was built on its own native runner and made to answer an MCP initialize on that platform before being published. None were cross-compiled.

Intel macOS is deliberately absent. GitHub retired the Intel runners, so the only way to produce that binary would be cross-compiling it on Apple silicon — shipping something that has never once executed. A missing download costs less trust than a broken one. If you need it: cargo install effectfence.

Why this package is ~8 MB

It bundles all four platform binaries rather than downloading the right one after install. That is deliberate:

• No postinstall script. A tool whose entire job is guarding side effects should not fetch and execute code from the network while being installed.
• Works with --ignore-scripts, which is increasingly the default in CI. A download-on-install package silently produces a broken install there.
• Works offline and air-gapped.

The cost is that you download three binaries you will not run. A future release will split these into per-platform packages selected by optionalDependencies, so you fetch only yours. Correctness first, then size.

Also available

• Rust crate — effectfence to embed the fence directly.
• once-kernel — the same exactly-once guarantee as a TypeScript library, zero dependencies, for when you want it inside your own code rather than as a separate MCP server.
• once-kernel on PyPI — the Python kernel. It shares a payload hash with the TypeScript one (RFC 8785), so a Python service and a Node service agree about whether an operation already ran.

Licence

MIT — same as the Rust crate this packages.