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When developing directly in the Rust Core (specifically within the wasm-filters crate), you are writing the exact logic that an Agent will inject into remote Data Nodes. Because this logic must be completely OS-agnostic and strictly sandboxed, you cannot compile it targeting standard architectures like x86_64 or arm64. It must be compiled to WebAssembly, explicitly targeting the WASI (WebAssembly System Interface) standard.

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Setting up the Compiler Target

Ensure your Rust toolchain has the WASI standard library installed. 
rustup target add wasm32-wasi
NMP currently targets WASI Preview 1 (wasm32-wasi). We are actively monitoring wasm32-wasip2 (The Component Model) and will migrate the Wasmtime engine to support strongly-typed complex objects crossing the engine boundary in our next release.

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Writing a Filter

A typical NMP Filter is standard Rust code, with the caveat that network and threading operations will panic if invoked, due to the Sandboxing constraints on the destination server.
src/main.rs
use std::env;
use std::fs;

fn main() {
    // 1. Receive arguments passed by the Data Node runtime
    let args: Vec<String> = env::args().collect();
    let query = args.get(1).expect("Missing query parameter");

    // 2. Access the virtualized file system
    // /logs is mapped by the Server, not the physical root!
    if let Ok(content) = fs::read_to_string("/logs/system.log") {
        for line in content.lines() {
            if line.contains(query) {
                // 3. Output is trapped by WASI and streamed via gRPC
                println!("{}", line);
            }
        }
    }
}

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Compilation

To compile your logic into the .wasm injection payload, run Cargo targeting the WASI architecture: 
cargo build --target wasm32-wasi --release
The resulting microscopic file (e.g., target/wasm32-wasi/release/filter.wasm) is what the nmp-client loads into memory and streams to the peer over the network.

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Testing Locally

Before deploying your filter to the production mesh, you can test its behavior locally using the standalone Wasmtime CLI. Provide the capability mappings (like pre-opened directories) explicitly: 
wasmtime run --dir /local/physical/path::/logs filter.wasm "CRITICAL"